Description

TECHICAL FIELD

[0001] The present invention relates to a method for extraction of iron from industrial waste, e.g. middling slime (waste) which is a refuse of the flotation processing of copper and other ore concentrates with a subsequent obtaining of iron oxide compounds - haematite, magnetite (iron(II, III) oxide), hydrated ferric oxide (ferrihydrite) and ferric hydrooxide using leaching with oxalic acid.

PRIOR ART

[0002] Actually the use of oxalic acid for extraction of metals from ores is known. In the Chinese Patent CN 1962 437 A is disclosed a method for leaching with oxalic acid of alluvial materials and clays containing iron and in another Chinese patent CN 102 674 643 A the leaching is used for the extraction of iron from a silicone sand. The method of leaching with oxalic acid is applied also in the case of such difficult for processing materials as ceramic powders with the purpose to reduce the iron content because its tendency to enter into chemical reaction with the ceramic glaze, EP 0 514 577 Al. In Greek patent GR 1 002 723 B the oxalic acid is used for the extraction of iron from industrial waste and ores.

[0003] However all these patented technologic approaches have significant common disadvantages. On the first hand they allow the treating only of such materials in which the iron content is below several per cents. In higher iron content the selected conditions of these methods lead also to the dissolution of significant quantities of the silicate backbone and the obtained iron containing products are polluted with a colloidal silicone dioxide.

[0004] Another common characteristic of these methods is the fact that during the leaching of the iron with oxalic acid is applied chemical or photochemical reduction of its trivalent forms to the divalent forms using the huge difference in the solubility of both oxalates. The divalent oxalate is practically insoluble and so the effectiveness of the leaching is increased but in the presence of alkali earth metals which always are present in the industrial waste from the ore output the oxalate precipitate is accompanied also by calcium and magnesium oxalates which have a commensurable solubility.

[0005] Another drawback of the existing methods is the fact that at the stage of separation of the precipitates of the oxalate solution are obtained mixed phases, without any possibility of their separation unless a reverse dissolution of the iron oxalate is applied followed by its secondary sedimentation.

[0006] In the Bulgarian Patent BG 66201 Bl is implemented a preliminary separation of the silicate phase by a high temperature dissolution of the silicone matrix with alkali reagents whereupon it passes into the soluble forms of alkali silicates with a different modulus. However a drawback of the method is the fact that at these conditions copper compounds are transformed into copper oxide which remains as impurity in the iron containing product and this renders the process not useful for the iron oxide pigments industry.

[0007] In EPA 15000806 is disclosed a method of processing industrial waste including remnants of the flotation processing of ore concentrates - fayalite, the methods using leaching with oxalic acid, treatment with a aqueous alkali hydroxide and addition of an aqueous solution of hydrogen peroxide. However in this method is used a preliminary magnetic separation of the reaction mixture and the used temperature regimes, sequence of added reagents and duration of their stay in the reactor have nothing in common with the stages of the proposed method according to the present invention. Besides, the method of the European patent application is not quite suitable for processing of waste with content of heavy and rare earth metals.

DISCLOSURE OF THE INVENTION

[0008] The purpose of the invented method is to extract iron from middling slime which is a remnant of the flotation treatment of copper or other ore concentrates with a subsequent obtaining of iron oxide compounds (haematite, ferrihydrite or iron hydroxide). The middling waste is characterized as iron containing silicate in which the prevailing phases are fayalite and magnetite but which contains also remnant quantities of other metal compounds in the form of their oxides or silicates as well as a free silicone dioxide.

[0009] The method of extraction according to the present invention represents a treatment of the middling slime with aqueous solution of sodium hydroxide during which are created defects in the silicate cover of the particles of the ore concentrate with the subsequent leaching of the iron in the form of a complex compound and its sedimentation to hydroxide forms. As a complex forming agent is used oxalic acid which has a low acid aggressiveness and during its use no harmful gases are eliminated into the environment.

[0010] The present method avoids the drawbacks of the technological methods reflecting the state of the art in the leaching with oxalic acid of iron containing industrial waste. In this method is applied a novel approach which activates the properties of the system oxalic acid-iron containing phases and in this manner the leaching is promoted and the purity of the obtained products is increased. Besides the method according to the present invention allows the separation of the impurities of heavy and rare earth metals which is a technological difficulty and is not proposed adequately by the prior art.

[0011] In the first place the method works in an especially created oxidative medium which is a significant difference from the existing methods which apply reduction of the trivalent iron to bivalent iron.

[0012] It is known that the oxalic acid has a reducing activity which impedes the depth of leaching of the iron. In the proposed method is applied a solution which reduces significantly the reducing activity of the oxalic acid by providing conditions for formation of a stable iron-oxalate complex with an alkali metal which is a novelty in the existing practice. In this manner are preserved simultaneously its properties as a strong complex-forming agent and its reducing properties are subdued.

[0013] The proposed method is directed mainly to processing of flotation refuses with high content of fayalite but it is suitable for other iron containing residual products of industrial activity. The starting material is industrial waste containing as major components fayalite, magnetite, free silicone dioxide in amorphous and crystalline forms as well as other metals in lower concentrations with regard to the iron. Accompanying elements usually are copper, calcium, magnesium, zinc, molybdenum and aluminum, and arsenic and lead are present at trace levels. In the industrial waste may be present impurities of other heavy and rare earth metals as well.

[0014] The method according to the invention comprises also a technological novelty allowing the processing of fayalite with content of heavy and rare earth metals, e.g. molybdenum, causing the separation of such metals from iron using the different solubility of their compounds in the conditions of their common treatment, in contrast to the existing methods of leaching wherein iron and molybdenum are present simultaneously and their separation represents a significant difficulty. [0015] The method according to the invention is applicable for all types of fayalite irrespective of the presence of other elements. It affects the silicate matrix and the reaction of the oxalic acid with elements other than iron is a secondary reaction. The method according to the invention is a method for extracting the iron from industrial waste by means of the transformation of its silicate and an additional interaction of accompanying iron oxide compounds to oxalates. The formation of oxalates of other elements contained in the industrial waste is a consequence of their affinity to the oxalic acid as a strong complex- forming agent and the proposed method makes use of the fact that almost all oxalates of the heavy and rare earth metals are insoluble. This guarantees their separation from the iron because according to the method of the invention the iron is converted to a soluble complex salt such as the other elements do not form.

[0016] The substance of the method consists in the following:

Step 1: The iron containing pulp is mixed with 25-40 % of aqueous sodium hydroxide solution and heated at stirring to 35-80°C

Step 2: To the obtained pulp is added continuously an oxalic acid solution saturated at a different temperature until an equivalent molar proportion of Na/Fe/C ₂ 0 ₄ = 3/1/3 is achieved. The concentration of the oxalic acid is from 118 g/1 at 20°C to 36 % by weight at 60°C measured as a dehydrate. During the addition of the oxalic acid and within the predetermined temperature interval a 0.5 - 10% of hydrogen peroxide solution is added continuously so that an oxidative medium is maintained. Upon addition of the oxalic acid begins the formation of the highly soluble sodium-iron oxalate complex which passes into the solution and does not cause a diffusion control of the reaction, and maintaining of low temperature and equimolar proportion of the oxalic acid and the sodium hydroxide don't create kinetic conditions for dissolving of the silicone dioxide from the fayalite matrix. Step 3: The oxalic acid is added in excess of 5-10% with regard to the quantity which is necessary for the equimolar proportion and is heated at 25 to 80°C for 1 hour and no more hydrogen peroxide is added during the heating.

Step 4: The iron containing pulp which is a suspension of the solution of the complex salt and residual solid phase is filtered. The solid phase is separated and the filtrate is precipitated at alkali pH with a 5 to 15% sodium hydroxide solution in a precipitator, at temperature of 60-70°C.

Step 5: The obtained precipitate is washed with deionized water to neutral pH and is dried at temperature of 115-160°C.

Step 6: The solid phase obtained after the filtration of the sodium ferric oxalate contains residual iron in the form of magnetite but its reactivity is raised because of the destruction of the silicate matrix during the previous steps. This phase is treated with 15-30% sulphuric acid solution in order to extract at maximum the iron therefrom which is transformed into iron sulphate and is processed according to known methods to obtain pure iron.

In one embodiment the method comprises an additional Step 7. This step is applied in case of molybdenum content of the fayalite and carried out after the filtering of the sodium ferric oxalate.

Step 7: The obtained solid phase is treated with 11-15% sodium hydroxide solution which leads to the transformation of the molybdenum present in the phase as a highly insoluble molybdenum oxalate into the soluble sodium molybdate and the dissolving of the iron is impeded by the alkali medium. The obtained sodium molybdate solution is processed according to known methods to extract the molybdenum in a pure form.

EXAMPLES [0017] Hereinafter the invention will be explained with reference to examples in which are used samples with the following content:

Sample 1: Content of total iron expressed as Fe - 29%. Content of free silicone dioxide 14%. Proportion of magnetite to fayalite 47:53.

Sample 2: Content of total iron expressed as Fe - 48%. Content of free silicone dioxide 28 %. Proportion of magnetite to fayalite 78:22.

Sample 3: Content of total iron expressed as Fe - 36 %. Content of free silicone dioxide 26 %. Proportion of magnetite to fayalite 67:33 and molybdenum content of 4%.

[0018] All temperatures in the examples are given in Celsius grades.

Example 1

[0024] 10.0 g of fayalite waste (Sample 1) containing iron in bivalent and trivalent form is put in a reactor and at continuous stirring is treated with 25% aqueous solution of sodium hydroxide, the quantity thereof with regard to the iron being 3:1 at a temperature of 35-40°C, and for the duration of 30 minutes. Thereafter an aqueous oxalic acid solution is added to the suspension in an equivalent quantity with regard to the iron and at concentration of 118 g/1 so that the proportion Na/Fe/C ₂ 0 ₄ = 3/1/3 is guaranteed in the reactor. The obtained suspension is heated to 65°C with stirring for 1 hour. During the heating a 5% aqueous solution of hydrogen peroxide is added in a weight proportion of H ₂ 0 ₂ /Fe = 1:0.3. After the complex salt sodium-iron oxalate has been formed the suspension is filtered. The filtrate is precipitated in an alkali medium at pH 12 and 60°C, the obtained iron hydroxide is filtered, washed and dried at 160°C. The analysis by means of Mossbauer spectroscopy and X-ray phase analysis reveal the presence of ferrihydrite as the sole phase.

[0020] The solid silicate phase obtained after the filtration of the complex salt reveals the content of 14% of total iron.

Example 2

[0021] 10.0 g of fayalite (Sample 2) is put in a reactor and is treated with sodium hydroxide under the conditions of Example 1 but the concentration of the oxalic acid is 36% by weight. The quantity of the sodium hydroxide is in an equivalent proportion of 3:1 with regard to the iron and the oxalic acid is in a molar proportion with regard to the sodium hydroxide of 1:1.15. The reaction temperature is raised to 80°C and maintained for 1.5 hours adding 1,5-% of hydrogen peroxide during the treatment, in a weight proportion of 0.5 with regard to the iron.

[0022] Upon concluding the reaction during this time the suspension is filtered and the precipitation of the iron hydroxide from the complex salt is carried out at pH 8 and temperature of 30-40°C. The hydroxide obtained after drying at 120°C represents goethite with a partial crystallization of ferrihydrite.

[0023] The total iron content in the solid phase is 6.2%.

[0024] The solid phase is put in a reactor and is treated with a 20% aqueous sulphuric acid at temperature of 80°C for 1 hour with continuous stirring and equivalent proportion of l.T.lof the sulphuric acid to the iron. After cooling to the temperature of the environment the reaction mixture is filtered and the filtrate is evaporated until a crystallization is started at 95°C. Upon cooling crystals of ferric sulphate with iron content of 21% are formed. [0025] The iron content in the residual silicate phase is 0.85%.

Example 3

[0026] 10.0 g of fayalite (Sample 3) is treated with sodium hydroxide and oxalic acid identically to the methodology of the conditions and manner of Example 2 but without adding a hydrogen peroxide solution during the reaction. After the pulp is cooled to the environment temperature, the adding of 1.5% hydrogen peroxide solution is started, controlling the temperature not to be raised above 25-27°C. The weight proportion of peroxide to the iron is 1:0.3.

[0027] Upon filtering a solid phase with iron content of 7.5% is obtained.

[0028] The filtrate is precipitated at pH 13 and temperature of 60°C to iron hydroxide and after the second filtration and washing the oxalate content expressed as oxalic acid is analyzed. The degree of regeneration of the oxalic acid is 90 molar % and under the conditions of Example 2 this proportion is 86 molar %.

Example 4

[0029] 10.0 g of Sample 3 are put in a reactor and treated with 30% sodium hydroxide solution, the quantity of the oxide in an equivalent proportion to the iron being 3:1. After treating at 80°C for 1 hour a saturated solution of oxalic acid is added in a molar ratio with regard to the sodium hydroxide of 1.15:1. The reaction temperature is 70°C for the duration of 1.54 hours. After cooling the suspension is filtered and the filtrate is precipitated at pH 13 and 60°C to give iron hydroxide. After washing and drying at 160°C ferrihydrite with iron content of 51% is obtained.

[0030] The solid phase containing molybdenum oxalate is treated with 15% sodium hydroxide solution for 30 minutes at temperature of 80°C.

[0031] After cooling the reaction mixture is filtered and washed, the filtrate is concentrated with an intermediate filtration from the crystalizing sodium oxalate and the obtained concentrate of sodium molybdate is subjected to purifying according to known methods. The yield of sodium molybdate dehydrate is 0.85 g which is 65% of the theoretic yield.

[0032] The present method does not create new chemical reactions but it uses known chemical interactions applying these purposefully in a non-traditional manner overcoming the drawbacks of the known practice and it has the following differences:

1) Flotation raw material with magnetite and fayalite content in the proportion from 47:53 to 78:22 and containing other metals is subjected to the processing.

2) Treatment of the magnetite-fayalite raw material with a sodium hydroxide solution from 25 to 40 % by weight at a low temperature from 35 to 40°C for a partial leaching of the silicone dioxide (biting).

3) Treatment of the raw material in the simultaneous presence of an alkali hydroxide and oxalic acid, the iron containing pulp from item 2) being treated by a gradual addition of an oxalic acid solution saturated at temperature from 20 to 60°C and the temperature of the reaction medium being from 40 to 70°C.

4) The addition of oxalic acid and sodium hydroxide is in an equivalent proportion with regard to the iron (Na/Fe/C ₂ 0 ₄ = 3/1/3) so that the formation of a stable sodium-iron oxalate complex is guaranteed. 5) In order to maintain the iron in its trivalent form during the process a hydrogen peroxide solution at a concentration of 1.5 to 5% is added continuously so that a constant oxidative medium is maintained.

6) The solution of the obtained sodium-iron oxalate complex is precipitated at pH 7.0 - 13.0 to obtain iron hydroxides (goethite and ferrihydrite).

7) The silicate phase obtained after the separation of the solution with the stable complex is treated with a 20-25% sulphuric acid solution extracting the residual iron under the form of Fe ₂ (S0 ) ₃ .

8) The impurities from heavy and rare earth metals are separated and extracted as insoluble oxalate salts, which allows the processing of industrial waste - middling slime containing iron and impurities of heavy and rare earth metals.