[0001] The present invention provides a method for the selective recovery of tin from a tin-containing material, preferably electronic scrap, particularly printed-circuit boards, the method comprising pretreatment of the tin-containing material, selective solubilization of the pretreated material to precipitate selectively the tin compounds comprising exposing the pretreated tin-containing material to a washing bath containing nitric acid and isolation of tin from the washing bath as tin oxide. The present invention provides also a reactor useful for the selective solubilization of the tin-containing material. The invention provides also a method for the purification of tin oxide contaminated with lead oxides to give metal lic tin by thermal treatment conducted in the presence of a novel fluxing agent and use of a novel fluxing agent as an environment for conducting thermal reduction of tin dioxide to metallic tin. The present invention finds application in a utilization process of electronic scrap and recovering from said scrap a valuable raw material comprising a tin material from electronic scrap.

Field of the invention

[0002] Tin occurs in three known allotropic forms with different densities, crystallo graphic structures, and in consequence also physical forms. The most abundant tin form is a beta variety/form which is stable at the room temperature. This is a form of a silver-grey metal with a density of 7,29 g/cm ³ i and a melting point of 23 l,88°C, characterized by its high forgeability and ductility. However, the beta form stored for a longer period at tem peratures below l3°C display a tendency to convert into grey (alpha) tin, present as a dark grey powder. In practice, said process takes place with perceptible speed not before -40°C. This effect, denominated a„tin pest”, is responsible for progressive destruction of objects made of this material. Metallic tin heated to the high temperature and rapidly cooled to the low temperature is converted into a gamma form characterized by high fragility.

[0003] As a chemical element, tin finds use in processes for the preparation of various types of chemical compounds. Organometallic compounds of tin are used as pharmaceuti cals, plant protection agents, stabilizers or catalysts for the production of fertilizers. Tin is a„p” block element and form oxides at the oxidation state of +2 and +4. Tin oxides have an amphoteric character and consequently they can form a broad spectrum of various chemical compounds in reaction with both acids and bases. [0004] Both natural and primary industrial source of tin is cassiterite, which is its diox- ide (Sn0 ₂ ). Deposits of this mineral created by hydrothermal processes are of the principal industrial importance. Cassiterite, as a mineral resistant to weathering, very rarely passes to mechanogenic deposits and consequently forms breccia deposits. The second useful tin ore is stannite, which is chemically copper (II), iron (II) and tin (II) sulphide. This is a mineral occurring much scarcely than cassiterite mentioned before. However, cassiterite, deposits of which provide about 70% of the global metallic tin production, is of the highest indus- trial importance. Generally, the lower limit of cost-effectiveness of mining the deposits of this kind is a tin content level not lower than 0,1%. [0005] The other form of tin-containing deposits is a hydrothermal cassiterite-sulphide formation connected mainly to shallow magmatic centres. A characteristic feature of thid kind of deposits is also, apart of Sn0 ₂ , the presence of stannite, tin and lead sulphides, mainly in a form of galena. These are generally vein deposits of diversified thickness of from several metres and diversified lengths of the formed veins. Such deposits constitute the third successive industrially important group of tin ores. They take the shape of quartz veins and cassiterite-impregnated zones. Average tin content in such deposits ranges from 0,2 to 0,3%.

[0006] The main producers of tin are: Malaysia, Indonesia, Bolivia and China. The highly industrialized countries have no significant own deposits of this raw material. They are forced to cover their needs for tin by import. In Poland, industrially significant tin de- posits are located in Lower Silesia around Gierczyn. In the zone of micaceous chlorite schists, inserts of biotite, garnet and various quartz forms are also found. Within said in serts, some aggregates and sputters of sulphides and scattered microcrystalline cassiterite are encountered. [0007] Metallic tin is generally obtained in the process of thermal reduction of densified cassiterite conducted at l200-l400°C in the presence of carbon. While very easily con ducted, this method suffers many disadvantages in practice. The most important of them is a relatively high temperature of the process, involving indirectly use of high amounts of fuels necessary to reach the smelting temperature. Moreover, during the process, carbon is oxidised to CO to a high degree in consequence of controlling air access in the reaction zone. Incomplete carbon combustion connected with the lack of precise control of recircu lation of the formed CO is a serious disadvantage of this method. It should be additionally noted that in the case of employing coal, which regardless of its form is not free of impuri ties such as, for example, metal oxides, a problem appears of generating high amounts of ashes. Due to the presence of non-reduced heavy metal compounds within they are treated as hazardous wastes and require proper dumping forms. The above-presented disad- vantages of the basic process for the production of metallic tin form its oxide form force the search of other alternative methods of acquisition of tin requested in many branches of industry.

[0008] One proposed method of acquisition of tin for subsequent uses is recovery there- of from waste materials. In particular, a substantial source of tin could now be electronic scrap, the quantity of which grows continuously along with continuously growing number of electronic device items present in both industry and household, and printed circuits in- eluded in the e-scrap in particular. The obligation of recycling such an equipment is at the present imposed on member states of the Community by the Directive of the European Parliament and Council 2012/19/EU of 4 July 2012 concerning waste electrical and elec- tronic equipment (WEEE), according to which in the majority of member states of the Eu- ropean ETnion producers of electronic devices are required to take back the equipment after its service period and pass it to further obligatory processing. The regulations concern in particular the producers of industrial electronic equipment. Thus management of electronic waste, and particularly such methods which allow to recover at least part of raw materials, including much desired tin, are one of the significant areas where the present research is conducted. [0009] More and more of this research is focused on recovery of selected metals from electronic scrap elements, and printed-circuit boards in particular. However, many of thee known and commercially used treatment processes of printed-circuit boards are not envi- ronment-friendly, as they include, for example, heating the scrap in furnaces for pyrolysing organic compounds and melting metals, potentially detrimental vapours being liberated during this step. Alternative known processes comprise selective removal of more valuable components, such as golden edge connectors and microprocessor integrated circuits, fol- lowed by scrapping the remainder of the printed-circuit board e.g. on a landfill. However, such handling of the electronic scrap is highly uneconomical.

[0010] During the manufacture of printed-circuit boards, tin or tin-lead alloy is used for forming lines and overlays on boards. Therefore, a very important step in the process of recycling the printed-circuit board scrap is selective solubilization of tin or tin-lead alloy to isolate them, for example, from a copper substrate. There is thus a need for a method for the treatment of such electronic scrap elements such as printed-circuit boards, the method allowing to recover the desired metal, such as tin, with the high yield and high purity. Prior art

[0011] Limited amount of natural tin resources and small tin content in its deposits posi- tively raises costs of exploitation of such sources, contributing lately to the growing ten dency to seek alternative (secondary) sources of tin. Novel methods are needed for acquisi- tion of tin in a free state or in the form of alloys mainly with lead, bismuth or silver from waste recycled materials containing said metal. Such procedure is more preferable, since it does not require use of initial flotation step, and tin concentration in the processed material is relatively high. Tin occurs mainly in the form of alloys, including, in particular, tin-lead alloys present in the broadly comprehended electronic scrap. However, according to the regulation ROHS of the European Union of 2002, use of a lead-containing solder in elec tronic appliances, except in the defence, medicine and aviation uses, is forbidden. Thus, in the case of equipment manufactured after the regulation came into force, a lead-containing solder is not employed. However, due to the presence of the older equipment in the market the problem of selective separation of tin from other metals, including in particular the problem of management and processing of the electronic waste containing tin-lead solders, is still meaningful. The solutions proposed to this end and known in the art employ hydro- metallurgic methods, pyrometallurgic methods or combinations of the same to maximise acquisition yield of tin from the processed material.

[0012] Numerous methods of management and recovery of metals used are based on thermal processes, wherein electronic waste is introduced into furnaces for subsequent melting. The metallic material obtained, which comprise a mixture of various metals, should be subsequently separated. The separation is conducted in metallurgical processes. Consequently the methods are very expensive, as they are characterized by high energy consumption. Further, they comprise a heavy burden to the environment due to emissions of harmful gases. For example, extraction of tin and lead conducted by melting a metallic granulate containing tin and lead alloys with solid NaOH at 600°C was described in the paper by P. Kaewboonthong, P. Bunnaul, K. Kooptarnond, T. Plookphol„Recovery of metals from Sn-Ag-Cu Solder Alloy Dross”, Proceedings of the International Conference on Mining, Material and Metallurgical Engineering, Prague, Czech Republic, 2014 Paper No. 79. Such an approach could lead to obtaining metals in a pure form, but a large scale implementation thereof requires implementation of sequential separation of other metallic components and is connected with employing equipment resistant to the molten leaching ingredients such as alkaline reagents. [0013] It should be emphasized here that the tin content in e-scrap is not high and aver ages usually to several percent. Therefore, use of the thermal methods in the case of e- scrap is highly non-effective. Further, the metal is largely formed as films on elements of the processed electronic boards. For that reason it is a more effective approach to initially comminute the scrap to a granulated form with relatively unified composition and use a bath with suitably selected physical-chemical parameters to selectively wash out the tin solder from the board surfaces, followed by selective separation of the refined desired ma terial such as tin or a tin compound.

[0014] A broad group of solutions known from the prior art for the recovery of tin, ei ther metallic tin or an oxide thereof, are the methods comprising the following steps:

• granulating the scrap and separating the fraction comprised of plastics from the ground metallic fraction;

• solubilizing the solders comprising mainly tin and lead optionally omitting solubilization of other metals (such as copper);

• selectively separating tin and lead compounds contained in the solution

• reducing the tin and lead compounds to the metallic form (by electrolytic or thermal means)

[0015] In specific solutions various different reagents and operating conditions of par ticular chemical reactions are used. In the process of solubilizing tin alloys, either very aggressive reagents, or highly specific reagents are used, which are often at the same time dangerous substances. Use of specific chemical reagents raises the cost of tin recovery from recycled materials. A very important factor is a price of the reagent used and possibil ity of its regeneration.

[0016] The most representative chemical reagent used in acquisition of tin from elec- tronic layers of materials is concentrated hydrochloric acid (HC1). Hydrochloric acid is a hydracid, which enables selective solubilization of tin and lead (solder alloy components) without simultaneous disturbing other elements made of metals of higher (positive) stand- ard potential, such as for example copper. This allows to selective transfer both metals into the solution in the cation form simultaneously separating them from the matrix containing more noble metals. Tin (II) chloride obtained, in contrast to lead (II) chloride, is well solu- ble in solutions acidified by an excess of hydrochloric acid. Lead (II) chloride is relatively well soluble i aqueous solutions at significantly higher temperature. Cooling the solution will lead thus to precipitation of lead chloride as a white deposit. [0017] To selectively separate tin and lead compounds contained in the solution it is also proposed, apart of using specific reagents, use of electrolytic methods. For example, such methods, which allow relatively simple separation of tin from lead, are disclosed in the patent specifications PL-49664“Process for the production of secondary tin from met- allurgical waste” and US 3394061 A“Tin recovery” USA, 1968 r. and US 2319887 A “Hydrometallurgical process for the recovery of tin”, 1943 r. According to this disclosure, the metallic form of tin is obtained by electrolysis of the concentrated solution thereof treated by an excess of HC1 to avoid uncontrolled increase of pH caused by the progressing electrolysis process. In the tin (II) chloride solutions in the presence of a great excess of hydrochloric acid, chloride complexes of tin (II) are formed according to the following chemical reactions:

Sn + 2HC1 SnCh + ¾ at insufficient HC1 and

SnCh + 2HC1 H ₂ SnCl ₄ at HC1 excess

[0018] However, the important problem encountered during electrolysis in such condi- tions is evolving toxic chlorine gas. Further, costs of such an approach a raised by necessi- ty of using a high molar excess od a strong inorganic acid, namely hydrochloric acid. At a concentration of the acid of 36% it results unavoidably to the need of separating tin and lead form excessive volumes of solutions obtained after the solubilization. [0019] Some improvement of the above-described method is treating the e-scrap mate- rial in a bath containing, apart of hydrochloric acid, an oxidising agent (M. A. Barakat and K. Koike“Acid Leaching of Indium-Lead-Tin Alloy Wire Scrap”, Journal of the Society of Materials Engineering for Resources of Japan, 10 (1997) 36-44; L.A. Castro and A. H.Martins“Recovery of Tin and Copper by Recycling of Printed-circuit boards from Ob- solete Computers” Brazilian Journal of Chemical Engineering, 26 (2009) 649-657). The additives for such compositions most frequently suggested in the references are: hydrogen peroxide, sulphuric acid (VI) and nitric acid (V). In the case of a hydrochloric ac- id/hydrogen peroxide mixture tin is solubilized more efficiently, to pass into the solution in the form of well soluble hexachlorostannates. However, such mixture is also able to easily solubilize other metals contained in the processed material, including those with more positive standard potentials. Similar oxidising properties possesses a mixture comprised of nitric acid (V) and hydrochloric acid. Extremely strong oxidising properties of such a bath is a factor which significantly restricts possibility of their use at the industrial scale. [0020] In the electrolytic recovery processes of metals such as tin, bases are also used, inorganic bases in particular, which play a role of a metal solubilizing agent. For example, for the selective solubilization of tin and lead sodium or potassium hydroxide may be used. Due to amphoteric character of tin and lead oxides, said metals are prone to solubilize in concentrated alkalies by passing into the solution as stannates or plumbates. The process proceeds according to the following chemical equation:

Me + 2MOH + 2H ₂ 0 M ₂ [Me(OH) ₄ ] + H ₂ where

M = potassium / sodium

Me = tin / lead

The obtained solutions are subsequently subjected to electrolysis to release a metallic form of tin. A big advantage of this method is the lack of evolving toxic gases during the elec- trolysis. It is worth mentioning that the proper operation of the process of selective solubil- ization of tin/lead, a significant excess of a concentrated alkali, which is a highly danger- ous reagent due to its highly caustic properties, is needed. That is why use of this solution at an industrial scale has only a historical meaning (US 1811142 A“Process for the recov- ery of tin from alkaline stannate solutions“, 1931 r.; US 73777534 A“Method for recovery of tin and lead from scrap mixtures containing same”, 1935 r.) or is left in the area of use in laboratory scale industrial research. [0021] The common feature of the approaches described above, which utilize washing- out of metals, tin/lead in particular, by the use of concentrated hydrochloric acid (with pos sible assistance of added highly oxidising components) or highly concentrated strong bases is a step of electrolytic isolation of metallic forms of the recovered metals. This step can be replaced with chemical reduction. This step can, in particular, be replaced by reduction implemented by metallic zinc. However, this is connected with consumption of the reduc ing agent, regeneration of which would entail involvement of the electrolytic process. Such an aspect lowers significantly cost-effectiveness of the whole process.

[0022] To avoid emissions of toxic gases in the process of electrolytic isolation of met als from their baths obtained by selective solubilization of tin and lead in the presence od copper, also use of other reagents based on oxyacids of diverse structures was proposed. One of them is trichloroacetic acid (US 4004956 A.“Selectively stripping tin or tin-lead alloys from copper substrates”, 1977 r.). Due to its relatively high strength, comparable to sulphuric acid (VI), trichloroacetic acid is able to solubilize components of tin solders to provide, in appropriate conditions, their well soluble salts. The disadvantage of this rea- gent, being an organic chloro derivative, is its high toxicity against living organisms and natural environment. Use of trichloroacetic acid requires thus employment of strictly closed circulation during the tin recovery process. Further, a significant disadvantage of using trichloroacetic acid is also the high price of the reagent.

[0023] Newer approaches for conducting a method of such isolation embrace those, where milder organic acids are proposed for solubilizing, as well as conducting electroly- sis, instead of aggressive inorganic acid. According to the improved method of„washing out” the tin-lead solders from the electronic scrap disclosed in the specification of US 6 641 712, they are subjected to the treatment with a high valence metal salt able to get reduced in the specific conditions together with simultaneous oxidation of the solder com ponents. Practical implementation of this concept is the use titanium salt at the oxidation state of +4 (US 6641712 Bl,“Process for the recovery of tin, tin alloys or lead alloys from printed-circuit boards”, 2003). In this solution the comminuted boards of electronic inte- grated circuits free from plastics are subjected to treatment with a titanium (IV) salt in the highly acidic environment. In such conditions both titanium and tin present in the solution are not able to hydrolyse resulting in appearance of sparingly soluble hydroxysalt precipi tates. The system containing the cations listed is subjected to hydrolysis in the final step, to isolate from it tin and lead as separate phases. Titanium (III) cations left in the solution can be oxidised in the highly acidic environment to titanium (IV) cations by activity of oxidis ing agents. In the specification of the quoted patent other potential reagents capable of se lectively solubilize tin and lead were also indicated. They include the following: fluorobo- ric, fluorosilicic, hexafluorophosphoric and acrylsulphonic acids.

[0024] Development of these concepts are reflected by solutions suggested by EcoBack company. For example, the patent application PL-408674 discloses a method for the re covery of tin material from the electronic scrap, according to which the electronic scrap containing tin material is provided, then the tin material from the electronic scrap is solu bilized by a solubilizing solution comprising the organic acid such as alkanosulphonic acid to give a solution for the electrolysis and the electronic scrap after the solubilization essen- tially free from the tin material, followed by passing the electronic scrap to the further pro cessing and subjecting the solution for the electrolysis to the electrolysis to provide the tin material and the spent solution after the electrolysis. The tin material obtained according to this electrolytic process and the spent electrolysis solution are passed to further processing. According to the preferred embodiment the solubilization of tin-lead solders contained in the waste electronic materials is conducted according to this description in the solution of methanesulphonic acid containing additionally potassium nitrate as an oxidising agent. Such processing results in obtaining well soluble methanesulphonate salts of lead (II) and tin (II). Isolation of the tin from lead salt is carried out by precipitating out a sparingly sol- uble tin salt with oxalic acid. When added in a free state to the highly acidic solution, oxal- ic acid causes precipitation of the tin salt only due to very strongly reversed dissociation. As a result of this, the reaction environment contains very small quantity of free oxalate ions, and consequently the solubility product of oxalate tin only is exceeded. In the case of the lead (II) salt said product is much higher and not exceeded for the obtained concentra- tions of the lead cation and oxalate anion. Further processing embraces electrolytic isola- tion of lead as a metal from the solution. The isolated tin oxalate is calcined above 300°C, to obtain its thermal decomposition to the SnO oxide. A reduction of the obtained oxide with carbon is carried out at relatively low temperatures below 600°C. The presented solu- tion is actually well developed and leads to good final outcome. A disadvantages of this procedure is its multistep character and costs, since use of oxalic acid, which is not sub- jected to regeneration, raises the price of the recovered tin.

[0025] However, for their conducting, electrolytic processes require not only significant power input, but also constant monitoring of process conditions, including the electrode poisoning level. Therefore, there is still significant interest in processes, in which metal recovery, and metallic tin in particular, is provided by use of suitable components of solu- bilizing solutions and parameters such as pH of the solution.

[0026] A component commonly used in such solutions is nitric acid. As indicate in the publication of WO 2009/129271, nitric acid has a number of advantages as a solubilizing agent for processing the e-waste. First of all, many, if not all, metal nitrates prepared by dissolving metals in nitric acid are soluble in the same acid and do not form precipitates. Secondly, on dissolving metals in nitric acid, any undesired releasing of hydrogen is ob- served (apart of a few exceptions comprising some transition metals 3d, 4d and 5d and lanthanides), and thereby no depletion of the acid takes place and there is no need to make it up continuously (as it is the case for mineral acids such as sulphuric, phosphoric or al- ready mentioned hydrochloric acid. The publication WO 2009/129271 discloses a method for the recovery of a metal, such as tin, in the form of its oxide or nitrate utilizing as a sol- ubilizing solution for metals a nitric acid bath, where precipitation is conducted by raising pH of the bath by adding a base such as potassium hydroxide, ammonia, potassium car bonate or ammonium carbonate, or else by changing the temperature of the solution. Ac- cording to the preferred embodiment, the method pertains to recovery of a metal from the scrap, such as the electronic scrap, which is subjected to comminution in the particularly preferred embodiment.

[0027] In accordance with another suggested approach to recovering tin from tinned surfaces such as, beside the printed-circuit boards, tinned objects of daily use, a method is proposed, as described in the application WO2013/104895, providing use of a regenerative solution with a particular composition, such as a solution comprising nitric acid and iron (III) chloride. Initially, low pH of the solution is maintained by adding a strong inorganic acid, such as hydrochloric acid. Metals, particularly ones present in the alloys used for the manufacture of the printed-circuit boards, are separated by gradual, stepwise raising pH of the regenerative solution. However, the method is relatively inefficient, with about 30-40% of metals being left in a sludge. Moreover, due to low selectivity of the recovery method according to this method, tin is contaminated with other elements such as iron.

[0028] According to the description of US 2016/0222487, a better yield was observed in a method analogous to the above-described by using as a tin recovery solution a mixture of nitric acid with iron (III) nitrate and precipitating at least one of metals by gradually raising pH in analogy to the description above. However, the tin recovery yield obtained by this method still seems to be unsatisfactory, and the reagents used, despite heavy level of use thereof, cannot be recovered. [0029] Another alternative solutions for recovering tin from electronic scrap elements utilize pyrometallurgical methods. Such methods (not necessarily expressed in patent pub- lications or claims) allow to recover tin of purity in the range of 50-70% as a tin solder. They use thermal separation (melting of tin-lead films by directly subjecting the electronic boards to suitably raised ambient temperature) and mechanical separation of a tin-rich ma- terial with vibrating tables. The first approach is an environmental nuisance, as it requires initial degassing of varnishes and a part of plastics, which is associated with emissions of gaseous decomposition products. The other is efficient in the case of fine initial comminu tion of the electronic material and leads to satisfactory results in the case of significant differences of specific weights of the separated metallic particles. [0030] Unfortunately, cheap and easily implemented methods are generally insufficient ly selective. The final product often comprises lead-contaminated tin. The more complicat ed and hard to implement in practice, such as for example hydrometallurgical methods, allow to obtain tin of relatively high purity, but are not devoid of a number of disad- vantages. First of all, the implementation thereof require significant amount of labour, in cluding use of toxic or expensive chemical compounds. Mostly, such as for example in processes utilizing electrolytic methods, they require significant amount of power, and thus they are less competitive source of metals such as tin. They are often multistep processes and very often a nuisance for the environment.

[0031] Thus there is still the unmet need for more environmentally friendly methods of selective recovery of metals, including tin from the electronic scrap in particular, which methods utilize non-toxic and non-concentrated, and therefore safe reagents, without a loss of selectivity of the method. A method is required for the treatment of the electronic scrap in particular, which enables selective solubilization of metals and isolation thereof either in a metallic form of in a form of their compounds characterized by a high level of purity. Further it is required for the proposed method for selective recovery of a metal needed due to its scarce natural abundance, such as tin, to be economically beneficial, and thus not requiring significant amounts of power. Moreover, it is highly desirable for the method to enable also recovery of the reagents used. Another desired advantage of such a method is simplicity of its implementation and the lack of need to control all steps of the method, including, for example, controlling pH variations.

[0032] The present inventors discovered unexpectedly that due to adequate selection of a washing bath based on environmentally safe diluted nitric acid, it is possible in a safe, environmentally friendly and economical way to isolate selectively tin as tin oxide of a high level of purity from tin-containing materials such as the electronic scrap. Moreover, due to a proposed specific environment for conducting thermal decomposition of tin oxide to metallic tin which allows the temperature of such decomposition to be lowered, the method is much safer and energy efficient. Object of the invention

[0033] The present invention provides a method for the selective recovery of tin from a tin-containing material, comprising: pretreatment of the tin-containing material comprising its comminution and removal of ferromagnetic grains and non-ferromagnetic metals, solu bilization of the pretreated material comprising exposing the treated tin-containing material to the operation of a washing bath containing nitric acid, and isolation of tin from the washing bath in the form of tin dioxide and optionally further steps of converting tin diox ide to metallic tin wherein in the method: a) the tin-containing material is subjected to initial selection and comminuted into a granulate, which finally in its largest dimension does not exceed 3 mm;

b) the material comminuted into the granulate is subjected to magnetic separation to remove ferromagnetic grains;

c) the granulate comminuted and free from ferromagnetic grains is subjected to elec trodynamic separation to remove non-ferromagnetic metals;

d) the material comminuted into the granulate is purified by rinsing with an organic solvent selected from common alcohols, inorganic solvents such as bases and their aqueous solutions to remove varnish films present on the tin-containing material; e) the rinsed and stripped from outer varnish films, free from ferromagnetic grains and non-ferromagnetic metals tin-containing comminuted material is solubilized by ex posing it to the operation of a washing bath containing diluted nitric (V) acid for the selective solubilization of tin and isolating it in the form of tin dioxide; followed by optionally purifying the obtained tin dioxide; and

f) the purified tin dioxide obtained in step e) is smelted to give metallic tin; and g) water comprising an environment for the purification and solubilization steps is re generated by recycling it into the solution to dilute concentrated nitric (V) acid to prepare the diluted nitric acid solution as the washing bath.

[0034] The tin-containing material comprises preferably the electronic scrap, and ele ments of the electronic scrap such as printed-circuit boards. According to the preferred embodiment of the method according to the invention the tin-containing material is com minuted by means of a hammer and knife mill.

[0035] In one preferred embodiment of the method according to the invention the sol- vent used for purifying in step d) the granulate of the tin-containing material before sub- jecting to solubilization in step e) is a common alcohol selected from methanol, ethanol, mixtures and aqueous solutions thereof. In another preferred embodiment of the method according to the invention the solvent is a base selected from aqueous solutions of NaOH, KOH and mixtures thereof, preferably comprising an aqueous NaOH solution.

[0036] Preferably, as a washing bath for the selective solubilization of tin from a tin- containing material the aqueous solution of nitric (V) acid of the concentration of 1-5% by volume, preferably of the concentration of 2% by volume is used.

[0037] Preferably, the magnetic separation of the tin-containing material is conducted by using one or an assembly of permanent magnets. Also preferably, the electrodynamic separation is conducted by using a separator with magnetoelectric induction. [0038] According to the preferred embodiment of the invention, before step d) of puri fying the granulate comminuted in step a), the granulate free from the ferromagnetic grains and non-ferromagnetic metals removed in steps b)-c) is subjected to assessment of its chemical composition to demonstrate absence in the granulate of ferromagnetic metals and aluminium. Especially preferably, when the assessment of the chemical composition of the granulate indicates the presence of ferromagnetic metals and aluminium, the granulate is again subjected to the magnetic and electrodynamic separation defined in steps b) and c) of the method.

[0039] In the preferred embodiment of the invention the solubilization step e) is con- ducted at the temperature not higher than the room temperature to restrict conversion lead (V) nitrate to lead oxide and lead dioxide. The room temperature according to the present invention is a temperature in the range of l5-25°C, and particularly in the range of 21- 23°C. Preferably, in the method according to the invention, after the solubilization step e) the solubilized granulate is rinsed with water to wash out from its surface the remainders of the oxide phase. Also preferably, after the solubilization step e) the solubilized granulate is subjected to assessment of its chemical composition.

[0040] According to the preferred embodiment of the invention tin dioxide obtained in step e) is purified by washing the suspension of tin dioxide at the room or elevated temper ature with the 5-15%, preferably 10% aqueous acetic acid solution for no longer than one hour, followed by the determination of a contamination level of tin oxide (IV) with lead compounds, and in the case of the impurity level higher than 2% by weight, the suspension of tin dioxide is washed again. Especially preferably, washing the suspension of tin oxide with the acetic acid solution is conducted at the temperature not exceeding 85°C, prefera bly at the temperature in the range of l8-25°C, most preferably at the temperature about 20°C. According to the especially preferred embodiment of the invention, in the case of the presence of significant contamination with lead oxides the solid oxide fraction is calcined additionally at the temperature of 500-800°C in the presence of carbon such as the wood charcoal.

[0041] Preferably, the purified tin dioxide obtained in step f) is smelted to give metallic tin. According to invention the smelting of tin oxide is conducted in the molten sodium carbonate at the temperature 1050-1 l50°C in the presence of carbon. Preferably, the mol ten sodium carbonate is used in at least onefold weight excess of the smelted tin dioxide, at the ratio to the smelted tin dioxide from 1 : 1,9 to 1 :2,1. Preferably, carbon is the wood charcoal. [0042] According to the preferred embodiment, in the method of the invention the solu- tions remaining after conducting the steps of the selective tin acquisition and purification of the tin-compound containing oxide fraction e) are subjected to the selective precipitation by the precipitating agent to precipitate the lead ions present in the solutions. In the pre- ferred embodiment, the lead ions are precipitated as lead carbonate by adding calcium bi- carbonate prepared by means of saturating the suspension of calcium carbonate with car bon dioxide prepared during the thermal reduction of tin (IV) oxide to metallic tin. Prefer ably, the copper (II) cation-containing aqueous solution forming after the precipitation of the lead salt is recycled as the nitric (V) acid diluent used in step e) as the washing bath for the selective solubilization of tin from a tin-containing material, comprising the regenerat ed washing bath supplied, if necessary, with concentrated nitric acid in an amount suffi cient to obtain the desired concentration of the nitric acid in the range of from 1 to 5% by volume. In still more preferable embodiment the regenerated washing bath contains an agent enhancing the oxidising efficiency of the bath in the form of the copper cations. [0043] Another object of the present invention is a method for the purification of tin dioxide contaminated with lead oxides, wherein tin dioxide contaminated with lead oxides is smelted in the molten sodium carbonate at the temperature of 1050-1 l50°C in the pres ence of carbon to give metallic tin. Preferably, the molten sodium carbonate is used in at least single weight excess of the smelted tin dioxide, at the ratio to the smelted tin dioxide from 1 : 1,9 to 1 :2,1. According to the preferred embodiment, carbon is the wood charcoal.

[0044] The object of the present invention is also use of the diluted nitric (V) acid o of the concentration not exceeding 5% by volume, preferably of the concentration of 1-5% by volume, most preferably of the concentration of 2% by volume as the washing bath for the selective washing out tin from a tin-containing material. Preferably, the tin-containing ma- terial is an electronic scrap, and especially preferably elements of the electronic scrap such as printed-circuit boards.

[0045] Yet another object of the present invention is use od the molten sodium car bonate as a fluxing agent comprising the environment for conducting the thermal reduction of tin dioxide to metallic tin. Preferably, the thermal reduction is conducted at the tempera- ture of 1050-1 l50°C. Especially preferably, the amount of the fluxing agent for smelted tin oxide is from 1 : 1,9 to 1 :2,1.

[0046] The present invention provides also a reactor (1) for selective washing of the oxide suspension in the process of the selective separation of tin from a tin-containing ma- terial in the granulate form, preferably the electronic scrap, comprising a reaction vessel (2) including:

a reaction chamber (4) for conducting washing out metallic tin from the granulate of the electrochemical scrap by means of the bath and

a baffle (6) for holding the granulate of the electrochemical scrap during washing by means of the bath, comprising openings (7) to ensure the frre flow of the bath and the oxide suspension from the granulate of the electrochemical scrap to the reaction chamber below (4) the baffle (6), which baffle (6) is adapted for removing from the reactor (1) the granulate of the electrochemical scrap after washing metallic tin. Preferably, the baffle (6) comprises a moving screen, more preferably the baffle (6) is located in a basket for the granulate of the electrochemical scrap. According to the further preferred embodiment of the invention, the reactor for the selective collection of the oxide suspension according to the invention is connected to a centrifuge to facilitate quantitative separation of the oxide suspension. Preferably, the baffle (6) is located in the reaction vessel (2) in parallel to the base (3). In the particular embodiments of the invention the base (3) has a circular shape, and side walls (4) form one cylinder-shaped side wall.

Short description of the drawings

[0047] These and other aspects of the present invention will be apparent for the person skilled in the art on the basis of the detailed description of the embodiments of the method according to the invention disclosed below, which is illustrated on the drawing, wherein: Fig. 1 shows a scheme of the reactor for the selective washing out tin;

Fig. 2 shows a diagram of the processing of the electronic scrap granulate, wherein the final products of the method were marked grey;

Fig. 3 shows photographs of the granulate (Fig. 3a) and e-scrap elements (Fig. 3b) respec- tively, before the treatment for the solubilization and selective acquisition of metallic tin and photographs of the granulate (Fig. 3c) of e-scrap elements (Fig. 3d) respectively after the treatment with the solubilization and selective acquisition of metallic tin.

Detailed description of the invention

[0048] The present invention provides a method for the recovery of metallic tin from the electronic scrap, in particular the electronic scrap elements such as printed-circuit boards or metal elements, the method comprising the following steps:

1) preparing the electronic scrap / waste material granulate comprising purification of the granulate in organic solvents such as common alcohols, in particular denatured alcohol or other inorganic solvents suitable for removing fats, such as the solutions of bases, particularly aqueous NaOH and KOH solutions or mixtures thereof for re- moving the varnish films present on the electronic scrap elements;

2) selectively washing out the tin or tin alloy films from lead or silver or bismuth;

3) purifying the solubilization product containing the oxide form of tin from small amounts of other admixtures;

4) smalting tin by means of suitably selected composition of flux agents and reducing agents;

5) regenerating water comprising the environment for conducting the processes of the solubilization and purification.

[0049] Described below are the steps listed above in more detail in reference to the preferred practical embodiments of the present invention

Examples of practical embodiments of the invention

[0050] The above-described method illustrated at the scheme depicted on Fig. 2 was im plemented according to the detailed description presented below and the disclosed working examples of the invention denoted as Examples 1 and 2.

Step 1

[0051] Before proceeding to the further, principal steps of the method for the selective recovery of tin, the chemically heterogenous mass of the waste materials should be sub- jected to properly configured pretreatment. The pretreatment comprises several basic oper- ations: preselection and classification of the scrap;

comminution by means of hammer mills, followed by knife mills with suitably se- lected screens and with use of filters to gather finely grained metallic dust;

magnetic separation by means of one or a assembly of permanent magnets of suita- bly selected characteristics of the magnetic field and the size of the slit between a magnetic separator and a belt, for separating the ferromagnetic grains from the granulate;

electrodynamic separation by means of a separator employing the magnetoelectric induction, to enable separation of non-ferromagnetic metals, which basically corresponds the separation of fragments of pure aluminium and copper from the fraction containing a mixture of the remaining metallic components, including tin alloys as films deposited on copper or plastic (laminate) supports; and

purification of the granulate by rinsing with an organic solvent such as common alcohols, in particular denatured alcohol or other inorganic solvents suitable for removing fats, such as the solutions of bases, particularly aqueous NaOH and KOH solutions or mix tures thereof for removing varnish films present on the electronic scrap elements.

[0052] The end product in this step is the electronic scrap granulate of the grain size not exceeding 3 mm and a definite (repeatable) chemical composition confirmed by means of independent chemical analyses. The granulate is deemed to be ready for the further pro- cessing steps, if the chemical composition of the inorganic (metallic) part does not indicate the presence of ferromagnetic metals and aluminium, and the major metallic components are: Cu, Sn, Pb and to the smaller extent Bi and Ag.

Step 2 [0053] The second step of the method for the selective recovery of tin comprises prep- aration of the chemical bath capable for the selective solubilization of metallic tin often comprising galvanically applied external layer of metal or solder elements on the electronic boards. The components of the chemical bath used, known here also as the washing bath, are characterized by their oxidative activity. Their selection is made with respect to the differences in reactivity between tin and other metals present in the processed material. The obvious advantage of such a bath is providing the selective method for the acquisition of tin which does not demand running specific, laborious operations of separating tin from the remaining metals (mainly Cu, Pb, Ag et al.).

[0054] The suggested agent providing selective solubilization of films made of tin-lead solder with leaving out in a significant extent the copper support is diluted nitric (V) acid of the concentration in the range of 1 - 5% by weight, and particularly of the concentration of 2%. Under the action of diluted nitric (V) acid tin and lead contained in the granulate and, to a very small extent, copper, undergo oxidation. The solubilization processes could be illustrated by the following chemical equations: 3Sn + 4HN0 ₃ ^® 3 Sn0 ₂ + 4NO + 2H ₂ 0

3Pb + 8HNO3 ^® 3Pb(N0 ₃ ) ₂ + 2NO + 4H ₂ 0

3Cu + 8HNO ₃ ^® 3Cu(NO,) ₂ + 2NO + 4H ₂ 0 (to a very small extent)

[0055] Raising the temperature of the solubilization step favours conversion of lead (V) nitrate to the oxide, and subsequently to lead dioxide. Nitric (II) oxide which evolves in small amounts is absorbed in water scrubbers, recycled and used for the secondary gen eration of nitric acid. Use of more concentrated nitric (V) acid, with concentrations greater than 5% by volume, or excessive raise of the washing step temperature to above 85°C can lead to intensification of decomposition of lead (V) nitrate to their undesirable, water- insoluble oxide forms.

[0056] At the acid concentration used, i.e using diluted nitric acid, the remaining met- als present optionally in the granulate such as copper, silver, bismuth react to a very mi- nute, simply negligible degree. In consequence of this the suggested method of„washing” the solder from the electronic scrap has a very selective character.

[0057] Thus, the major product of the above-indicated reaction will be tin (IV) oxide, Sn0 ₂ , in a fine-grained form, contaminated to a small extent with lead and copper com pounds. After initial separation of the fine-grained form of tin dioxide from the granulate comprising a„scrap” fraction and the solution, the separated metal-electronic elements are additionally rinsed with water to wash out from their surfaces the remainders of the oxide phase. Finally, the reaction system comprises three phases: two solid phases, i.e. the elec tronic scrap granulate depleted in tin (enriched in copper) and the oxide fraction, and the liquid phase - the solution. Due to formation of the additional fine-grained phase in the reaction mixture, the method according to the invention should be conducted in a special reactor dedicated for conducting phase separation which allows to selectively collect the oxide suspension separated from electronic parts even in the course of washing out tin. The design of such a reactor dedicated to the separation of the forming phases is shown sche matically in Fig. 1. As it is shown on the drawing, the reactor (1) which allows to selective- ly collect the oxide suspension has a shape of a reaction vessel (2) open at one side, com prising a base (3) and side walls (4), which in the case of the base (3’) having a circular shape, form one cylinder-shaped side wall (4’), and which reaction vessel (2) comprises a reaction chamber (5) for introducing a chemical bath capable for the selective solubiliza tion of metallic tin for conducting washing out metallic tin from the electronic scrap granu- late by means of the bath, divided by a baffle (6) located within the reaction vessel (2) for holding the electronic scrap granulate during washing out metallic tin by means of the bath, preferably a movable one, moving along the side walls (4) of the reaction vessel (2), in the direction essentially perpendicular to the base (3), which baffle (6) is provided with openings (7) of a diameter smaller than the size of the granulate located in the moving bas- ket (6) and subjected to washing, which provide free flow of the solution and suspension forming during washing out the oxide suspension from the electronic scrap granulate to the reaction chamber below (5) to result in collecting the oxide precipitate at the bottom (8) of the reaction vessel (2) defined by the base (3) of the reaction vessel (2) from its inner side, being the side the chemical bath is introduced from. The baffle (6) is adapted for removing from the reactor (1) the electronic scrap granulate after washing out metallic tin. In the preferred embodiment of the reactor, due to movable arrangement of the moving basket (6) simple separation of the washed granulate from the oxide precipitate and isolation of the granulate located in the moving basket (6) performing the function of a screen is possible. Preferably, the moving basket (6) is located in the reaction vessel (2) in parallel to the base

(3). Especially preferably, the reactor according to the invention is connected to a centri fuge to facilitate quantitative separation of the oxide suspension.

[0058] Average contents of metals in the obtained oxide fraction are respectively: Sn 68 - 70%, Pb 12 - 16%, Cu 0,5 - 1,0%, Ag 0,3 - 0,6%, Bi < 0,2%. [0059] The suggested approach allows to obtain minimum 60% efficiency of solubiliz ing tin and tin alloy layers to the fine-grained fraction form enriched in tin (IV) oxide. The washing efficiency is determined on the basis of a specification of results of chemical analyses concerning the averaged contents of metallic tin present before the washing in the granulate and the averaged contents of metallic tin present after the washing in the fine- grained amorphous oxide fraction.

Step 3

[0060] The third step of the method according to the invention embraces the entirety of operations resolving itself into the purification of the fine-grained oxide fraction separated from the aqueous suspension. In this step the co-precipitated and occluded small amounts of compounds of other metals, mainly lead present as the nitrate (V) and oxide and copper present as the nitrate (V) are passed into the solution and the tin (IV) oxide form as pure as possible is obtained.

[0061] The purification process comprises rinsing at the temperature not higher than 85°C the suspension of tin oxide with 10% acetic acid (CH ₃ COOH) solution for a specified period, not longer than for one hour (1 h). After that period, the level of tin (IV) oxide im purities in the isolated oxide fraction is determined. In the case of unsatifactory results, if the impurity level is determined as higher than above 2% by weight, the above-indicated purification operation of the tin oxide suspension should be repeated. Lead (V) nitrate formed in step 2, that is the step of solubilizing the tin-lead films, is water soluble at the significant level, which enables its separation from the solid oxide fraction. As indicated above, use of more concentrated nitric (V) acid (at the concentration higher than 5% by volume) or excessive elevation of the washing process temperature above 85°C can lead to the decomposition of lead (V) nitrate to its oxide forms. Their removal from the precipitate is implemented by means of washing out with 10% by volume acetic acid. In this reaction the very well soluble lead salt -lead (II) acetate is formed. With the increase in the concen tration (above 10% by volume) of the nitric acid solution used for solubilizing the solders, apart from a perceptible digestion of copper, the process of lead dioxide formation is also intensified. The visual effect of this process is the darker shade of the tin oxide fraction. In the case of the presence of relatively high levels of Pb0 ₂ the solid oxide fraction is cal- cined at the temperature of about 500-800°C with the addition of the wood charcoal. In the described conditions Pb0 ₂ is reduced selectively to the PbO oxide, which could be passed into the solution by means of the nitric (V) acid or acetic acid solution. The process of se- lective reduction of lead (IV) oxide to PbO by means of the wood charcoal proceeds ac- cording to the following reaction:

2Pb0 ₂ + C 2PbO + C0 ₂

[0062] The process of dissolving the oxide form of lead in the acetic acid environment id described by the reaction equation: PbO + 2CH ₃ COOH Pb(CH ₃ COO) ₂ + H ₂ 0

[0063] The purification process of the oxide tin fraction can be deemed completed, when contents of lead and copper does not exceed respectively 2% and 0,5% by weight. The remaining impurities in the silver oxide and bismuth forms do not exceed levels for silver and bismuth 0,8% and 0,2% by weight, respectively. [0064] During the purification process the soluble copper and lead compounds and, though to positively lower degree, the silver compounds are concentrated. The essential element of the purification process which positively lowers environmental nuisance of the proposed solution providing the method for the selective recovery of tin, is separation of these metals and regeneration of water employed in this process. The obtained purified tin (IV) oxide will be subsequently subjected to reduction resulting in smelting of metallic tin.

[0065] The main purpose of step 3 is providing tin oxide (IV) of a high level of purity (at least 95%), which is a valuable commercial product and which could be commercially utilized without any need for further treatment. However, due to a part of practical applica- tions which require use of purified metallic tin, tin dioxide obtained in step 3 of the method of the present invention is subjected to further treatment - a reduction process - which leads to obtaining the desired product in the form of metallic tin. It should be emphasized here that purity of tin oxide (IV) determines purity of metallic tin obtained in the subse- quent step. It should be noted that the presence of lead in the oxide form of tin at a level higher than 2% by weight does not put any obstacle in conduction the process of thermal reduction of tin oxide.

Step 4 [0066] The subsequent step of the discussed method for the selective recovery of tin from the electronic waste is conducting thermal reduction of tin (IV) oxide to metallic tin in the case of a sufficiently purified fraction of tin dioxide or to the tin and lead alloy in the case of a more contaminated fraction of tin dioxide. Preparation of the feed for suitably designed furnace consists in selection of: a) suitable fluxing agents and reducing agents;

b) a proper weight ratio of all components comprising: purified tin dioxide, all flux agents and a reductant;

c) optimal configuration of melting as a sequential combination of oxide activation and proper reduction steps while employing the lowest possible temperatures.

[0067] According to a method of the present invention the step of thermal reduction of the purified tin (IV) oxide is conducted in the presence of an excess (at least a onefold ex- cess by weight) of the molten sodium carbonate in the presence of carbon (for example, carbon in the form of the wood charcoal). Sodium carbonate employed in this step pro- vides alkaline environment of the reaction, in which a more easily reducible form of tin oxide is formed. The melting point of sodium carbonate is 85l°C. Above this temperature, decomposition of sodium carbonate to sodium oxide increases. For that reason it is indis- pensable to use a significant excess of this reagent in relation to the amount resulting from stoichiometry of the reaction.

[0068] To smelt metallic tin according to the suggested method according to the inven- tion, a portion of the solid, purified or contaminated Sn0 ₂ is solubilized in the bulk of mol- ten sodium carbonate at the temperature of 1050-1 l50°C. The process of tin (IV) oxide reduction proceeds in two steps, according to the method of the invention. In the first step, sodium stannate (IV) is obtained, which is then subjected to reduction by means of carbon or carbon monoxide formed in the reaction of carbon dioxide with carbon. The described process could be presented by means of the following equations of chemical reactions:

Sn0 ₂ + Na ₂ C0 ₃ ^® Na ₂ Sn0 ₃ + C0 ₂

Na ₂ Sn0 ₃ +C ^® Na ₂ 0 + C0 ₂ + Sn

Na ₂ 0 +C0 ₂ ^® Na ₂ C0 ₃ (fluxing agent regeneration) [0069] The content of oxide forms of lead above the assumed (2%) level leads to a slight decrease in yield of the metallic tin reduction due to increased participation of the competitive reaction of reduction of lead oxide to metallic lead. Considering the fact that lead has a more metallic character than tin, its oxide form is directly reduced to lead by means of carbon or carbon monoxide. Such a reaction directly decreases the amount of the reducing agent available for the process of tin (IV) oxide reduction.

[0070] The process of the thermal reduction of tin (IV) oxide conducted at the tem perature up to H50°C in aluminosilicate or graphite crucibles proceeds with high yield of 70% - 90%, the value of which depends slightly of the level lead oxide impurities present in the smelted sample. The yield of smelting is determined on the basis of a specification of results of chemical analyses of the contents of metallic tin after the melt and the contents of tin in finely-grained tin (IV) oxide.

Step 5

[0071] The important element of the technology under development jest selective sepa- ration of lead from the solution obtained after purification of tin oxide fraction. A suggest- ed selective agent for precipitation of lead ions from the solution containing also copper cations is calcium bicarbonate prepared by saturating the suspension of calcium carbonate with carbon dioxide obtained in high amounts in the method of the invention in step 4 - during the thermal reduction of tin (IV) oxide. Solubility of lead carbonate is about one order of magnitude lower as compare to the solubility of copper carbonate. Lead carbonate obtained can be an additional commercial product of the processing line. The copper (II) cation-containing aqueous solution obtained after the separation of a lead salt is recycled as a dilution medium for nitric (V) acid used in step 2 as the washing bath for the solubiliza tion of tin solders. An additional factor which enhances effectiveness of oxidative activity of such a regenerated bath is the presence of copper cations in the recycled solution.

Summary

[0072] The final effect of the suggested method for the recovery of tin, apart of the high purity of metallic tin and, alternatively, tin/lead alloy, selectively solubilized from the tin-containing material, is homogenization in respect of chemistry of the electronic scrap granulate (most frequently in the form of substantial increase of copper contents). Sche matically, the course of method according to the invention is presented in Fig. 2, where reaction modules and products obtained according to the invention are shown, which can comprise the final products by presenting them on the grey background. [0073] The suggested technology of processing the electronic scrap to acquire tin is characterized by relative simplicity of proceeding, is environmentally friendly (closed re- cycle of the reagents) and leads to obtaining metallic tin of the high purity and other com mercial products. In contrast to the method discussed previously, the suggested technology employs reagents which are non-toxic, inexpensive and widely used in numerous industrial processes. Any of the steps of the suggested technology of obtaining metallic tin generates additional by-products which could be toxic materials. The experimental studied conducted to the present day demonstrated that the technology allows to recycle Sn/Pb with the high yield reaching 85-90%.

[0074] The presented technology employs a number of innovative solutions as com pared with the existing ones. In particular they embrace:

[0075] The selective washing out Sn/Pb alloys from the surfaces of printed electronic integrated circuits by means of diluted (1 - 5%) nitric (V) acid, which is an inexpensive, widely available and easily regenerated reagent. The use of the acid with such a low con- centration allows to maintain copper substrates included in electronic materials in practi cally intact state.

[0076] The method of smelting tin from its dioxide by means of molten sodium car bonate used in a weight excess. Such a solution allows to conduct the process in a continu ous mode. The additional advantage of such procedure is practically complete regeneration of the initially used sodium carbonate, which results in practical elimination of ashes and slag forming in the melt.

[0077] The regeneration of water (used as a medium for diluting nitric (V) acid to sol ubilize tin alloys and to purify oxide tin fraction obtained from the solubilization of a tin alloy) by isolating the lead carbonate precipitate by means of calcium bicarbonate obtained by saturating the suspension of calcium carbonate with carbon dioxide (generated during the thermal reduction of tin (IV) oxide).

Examples

Example 1— selective recovery of tin from e-scrap

[0078] Tests of the developed method was conducted with 10 kg of the granulate from the class C printed-circuit boards free from ferromagnetic elements, as shown in Fig. 3a. The process of comminuting the board to form the granulate was conducted with initial use of the hammer mill to remove ceramic parts of the boards, and then the knife mill with suitably selected set of screens with mesh diameter of from 8 to 3 mm to provide the gran ulate with its largest dimension not exceeding 3 mm, and removing additionally varnish films having been applied onto connections and tracks covered by the tin solder. Then the washed granulate was cleaned in a denatured alcohol to provide the purified granulate with exposed metallic elements and solders. In the case of insufficient washing of the outer films the operation of diluted nitric (V) acid does not provide the expected effect of the selective solubilization of tin present in the granulated material.

[0079] The powdered sample of granulate for washing was subjected to chemical anal- ysis by XRF. The following results were obtained (in weight percent) concerning the gran ulate composition: Cu 23%, Sn 8%, Pb 3%, Ag < 0.05%.

[0080] Then the provided granulate was introduced into 10 litres of 2% nitric (V) acid. The contents of the reactor was vigorously stirred all the time until voluminous white pre- cipitate of the oxide tin fraction was formed. After 4 hours of pickling the aqueous suspen sion of tin (IV) oxide was separated from the metallic residue enriched with copper, and depleted in tin and lead. The weight of the residue containing copper in the first place was 8,5 kg. Tin oxide contaminated with insoluble lead compounds (mainly in the oxide form) was separated by decanting from the aqueous solution comprising lead as soluble nitrate (V). The separated oxide tin fraction contaminated with compounds of lead was added to

2 1 of pure water at 80°C. Successively, the reaction system was intensively stirred for about 2 hours. After this period, the obtained suspension was centrifuged. The separated solution was combined with solution obtained before by decantation process, and the solid part was dried for 3 hours at the temperature of H0°C. The dried oxide fraction weighing 0,98 kg and comprising 69% Sn, 16% Pb and 0,7% Cu and 0,7% Ag was combined with 1 kg of sodium carbonate and 0,30 kg of the wood charcoal. The whole was homogenized and added the cast-iron crucible preheated to H00°C. The furnace feed was calcined for 4 hours it the constant temperature of the furnace. The crucible contents was transferred to a metallic form to obtain 0,64 kg of the Sn/Pb alloy containing 73% of tin, 23% of lead, 2% of copper, 0.8% of silver. The residue weighing 0,31 kg as a slag comprised a mixture of sodium oxide/sodium carbonate, compounds of tin and a negligible level of lead (below 1%).

[0081] Then the solution containing lead (II) cations (from the steps of acid washing and water rinsing) was subjected to calcium bicarbonate. The lead (II) carbonate was ob- tained slightly (5%) contaminated with copper carbonate.

[0082] Average efficiency of the tin recovery obtained in the present method was 60%.

[0083] The granulate obtained after the selective tin removal is shown in Fig. 3c. As it is visible on the photographs made before the solubilization of the granulate for the selec tive washing of tin, it was characterized by silver and metallic colour specific for tin. Washing tin from the granulate exposed deeper layers rich in elements such as copper, evidenced by brown-coppery colour of the granulate after the selective washing of tin.

Example 2— thermal reduction of pure tin (IV) oxide in the molten sodium carbonate with addition of carbon [0084] The sample of tin (IV) oxide of the purity of 98% and the weight of 50 g was mixed with 150 g of sodium carbonate and 25 g of the wood charcoal. The mixture was homogenised and added to a graphite crucible, which was then inserted into a resistance furnace at the temperature of l050°C. After the contents of the crucible was melted, the whole was held at the indicated temperature for about 40 minutes. After this period the contents of the crucible was transferred to a steel mould, the smelted metal was weighed and reduction yield determined. Average yield determined for 4 analogous tests was 80 - 95%. For each consecutive test the yield decreased slightly for about 5%. Due to corrosion cause by the action of emerging gases which penetrated easily the porous structure of the crucible material, the graphite crucible used was being consumed.