A method for precipitating metal oxides from an aqueous waste solution

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for treating aqueous waste streams containing metal oxides. More specifically, the present invention relates to a method for treating aqueous waste streams to recover reusable reactants and possibly metals, such as copper, tin, lead and iron from the solution. A specific application is treating spent tin or tin/lead stripping solution used in the electronic industry, particularly in the manufacture of printed circuit boards.

BACKGROUND OF THE INVENTION

Along with expanding applications and market of different electrical devices, the printed circuit board (PCB) production is growing strongly. At the moment, an average PCB manufacturing plant generates tons of heavy metals containing effluent. This hazardous waste is usually transported elsewhere for treatment, which treatment actually does not recycle the components but only diminishes the volume before burial of the waste.

The nature of tin or tin/lead stripping solution

The most popular method of processing outerlayer printed circuit boards (PCB) in the United States and Europe is by pattern plating. In this common PCB manufacturing process, tin or tin/lead is usually electroplated onto a copper layer to serve as an etch-resistant layer. After the non-circuitry regions of the copper layer are etched away, the next step is to use nitric acid based solution for stripping the electroplated tin or tin/lead layer to expose the copper circuitry.

The nitric acid based fresh stripping solutions comprise nitric acid and ferric nitrate. Typical composition is (w/w percentages) 10-40 % nitric acid, 1-10 % ferric ion, Fe ³⁺ , <1 to 2 % anti-tarnish and <1 to 3 % suspending agent. When the concentration of free nitric acid is lower than 3,6-4,2 N or the concentration of tin is higher than 50-100 g/l during stripping depending on the process conditions of the tin or tin/lead stripping module used, it is necessary to replace the spent solution with fresh stripping solution.

Normally, the spent stripping solution comprises about 5-30 % (w/w) nitric acid, 1- 10 % (w/w) ferric ion, Fe ³⁺ or Fe ²⁺ , and minor amounts, <1 to 2 % (w/w) anti- tarnish and <1 to 2,5% suspending agent, 2-20 g/l copper ion Cu ²⁺ , about 10-200 g/l Sn ⁴⁺ , and in some cases about 0-5 g/L Pb ⁴⁺ . Minor amounts of Sn ²⁺ or Pb ²⁺ could be present if the oxidation has been incomplete as well as other minor components.

During the stripping reaction tin is oxidized to Sn ⁴⁺ that forms tin oxides or hydroxides. Thus, tin is present in the waste streams mainly as Snθ2. It is known that most stannic salts are insoluble in water or even in any concentration of nitric acid. As the metal loading of the stripping solution increases, the metal tends to precipitate out of solution as sludge. In spray applications, the sludge can cause clogging in the spray nozzles. A suspending agent is used to prevent this. The suspending agent keeps the stannic oxide in solution by creating a fine dispersion of stannic oxides within the stripper solution. These suspending agents are typically organic or inorganic acids as described by e.g. Kawanabe et al. in their patent "Stripping solution for tin or tin alloys" (U.S. Pat. No. 4,374,744).

When a certain metal loading level is reached, the stripping solution often becomes unstable, creating a potential for an exothermic condition, which is an instant release of a massive amount of heat. Typically, large amounts of toxic NO _x gas are released during an exothermic condition, and the stripping solution foams excessively. This condition possesses a safety risk at the plant and is most undesirable. Obviously, the occurrence of exothermic conditions can damage the operating equipment. Another component in the fresh stripping solution is used to solve this problem, here referred to as the anti-tarnish agent or nitric acid stabilizer. These are often organic compounds free of sulfur but containing a nitrogen atom, reported for example in US patents 4,374,744, 5,911 ,907, 5,512,201 and 5,244,539. In addition to acting as nitric acid stabilizers, these organic additives are believed to have interaction or form complexes with metal oxides and salts thus leading to varying and very complicated the chemical compositions.

Treatment of spent tin or tin/lead stripping solution

One conventional method for treating spent stripping solution is neutralization, for example by adding sodium hydroxide to neutralize the free nitric acid. When the pH value is adjusted to 8-13, most metallic cations are converted to metal oxide or hydroxide precipitates. After filtration they can be further processed to recover the

metallic tin, iron, copper or lead. The resulting filtrate is then vaporized to produce sodium nitrate crystals. Although the metals can be satisfactorily recovered, the recycling possibility of the solution is lost. The method requires use of large amounts of sodium hydroxide, and the resulting sodium nitrate crystals do not possess commercial value.

As an example of neutralization, abstract of the Taiwanese patent publication 177,911 , titled "A method for recovering metallic tin from spent tin stripping solutions," teaches use of a neutralizer, a precipitant and a reducing agent to make the treated effluent meet environmental requirements and to recover metallic tin from the spent stripping solution. As the process requires use of large amounts of neutralizer and reducing agent it certainly has economic disadvantages.

Another approach is presented in the abstract of a Taiwanese patent publication 258,758, titled "A method and an apparatus for regeneration of tin-electroplating solution," which describes a method for separating Fe, Cr and Sn ions from the electroplating solution by using ion exchange resins to regenerate tin flux and to recover metallic tin. However, the method set forth by these inventors involves complex procedures so the method is not favorable for treating spent tin/lead stripping solution. Yet another abstract of a Taiwanese patent publication 553,906 titled "A method for the treatment of waste tin-lead stripping solution" teaches a multistep method wherein stannic and plumbous components of the spent stripping solution are first oxidized, precipitate removed and dissolved in alkali or a strong acid and then reduced electrolytically. This method is complex and requires many steps and several unit processes. Besides, additional chemicals must be introduced during various steps leading to elevated costs and weakened profitability.

In all these methods, the main emphasis is on recovering the metals.

Therefore, there is a need for a method for treating spent tin and tin/lead stripping solutions and recovering metal oxides in a environmentally sustainable, simple and efficient, cost-effective manner and especially retaining the active ingredients of the liquid phase to enable recycling.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a method for treating spent tin or tin/lead stripping streams to recover the valuable chemicals for reuse. More specifically, the desired component for recycling is the aqueous reactive acid, which may contain other agents acceptable in fresh solution. This is achieved by conversion of the organic compound present in the waste by heat treatment, which also leads to precipitation of the heavy metal oxides. The remaining aqueous phase then comprises nitric acid, Fe and Cu ions, but inconsequential amounts of Sn or Pb and thus is suitable for recycling with or without further processing.

Another aspect of the invention is to treat spent tin or tin/lead stripping solutions to recover the reusable nitric acid from the waste.

Another aspect of the invention is to treat spent tin or tin/lead stripping solutions to recover metals, such as copper, tin, lead or iron from the waste.

Another objective of the present invention is that the solution obtained after heat treatment according to the method can be recycled back to tin or tin/lead stripping process.

Yet another objective is to substantially decrease the volume of hazardous waste formation during PCB-production and therefore reduction of the environmental burden.

A significant benefit is also that during the process there is no requirement for adding chemicals different from those already present.

The present invention is based on the unexpected discovery that valuable metal oxides can be precipitated and recovered from a spent tin or tin/lead stripping solution by treating said aqueous solution at a temperature to initiate the conversion of said organic additive into one or more of its precipitating derivatives and simultaneous precipitation of said metal oxide(s).

DETAILED DESCRIPTION OF THE INVENTION

The present specification describes a method for precipitating metal oxide(s) from a spent tin or tin/lead stripping solution further containing inorganic acid(s), metal salts and/or complexes, and one or more organic additive(s), comprising treating of said aqueous solution at a temperature and for a period of time to effect the

conversion of said organic additive into its precipitating derivative and simultaneous precipitation of said metal oxide(s).

The organic additive of which the conversion is the key to clearance of the waste solution comprises an organic amine. Remaining in the aqueous solution, the organic amine is in the form of its salt, preferably halogenide and more preferably its chloride salt. The treating of said aqueous solution at a temperature, or optionally further intensified by addition of a volume of approximately 100 % nitric acid, initiates the said organic amine salt to convert into a compound which is less soluble under reaction conditions and thus precipitate. Simultaneously, also the said metal oxides, which preferably comprise stannous and/or plumbous oxides, precipitate to give the desired effect.

The amount of the organic amine is in the range of 0,2 to 5% (w/w), preferably 0,5 to 2% (w/w) and most preferably 0,5 to 1 % (w/w) of the total spent tin or tin/lead stripping solution weight. The amount halogen, preferably in the form of its hydrogen halogenide is in the range of 0,2 - 2,0 % (v/v), preferably 0,5 - 1 ,0 % (v/v) of total waste solution. The amount of the metal oxides is in the range of 1 to 300 g/l, preferably 10 to 120 g/l and most preferably 50 to 120 g/l in comparison to the spent tin or tin/lead stripping solution total volume. Conditions to effect the desired reactions comprise elevated temperature in the range of 55-21O ⁰ C, preferably in the range of 60-150 ⁰ C and most preferably in the range of 60-80 ⁰ C.

If the optional procedure to intensify the conversion of said organic additive into its precipitating derivative by addition of nitric acid, is taken the amount of 100% nitric acid added is 2-20 % (v/v), preferably 5-10% (v/v) and most preferably 4,5-5,5% (v/v). Another means for initiating the reaction is elevated pressure during the treatment at a closed system. When addition of nitric acid is applied, the reaction temperatures range from 68°C to 100°C, preferably from 70°C to 90°C and most preferably from 70°C to 85 ⁰ C.

Another option to intensify the treatment to effect the conversion of said organic additive into its precipitating derivative is to implement the treatment under pressure. When pressure is elevated, it ranges from 1 to 16 bars during the treatment at a closed system. Then the temperatures range between 70-210 ⁰ C, preferably from 100°C to 160 ⁰ C.

Here the expression "a closed system" refers to a reactor, for which exchange of material and energy with its environment is controlled. When needed the inventors here used an autoclave from which the gases forming during the reaction cannot escape the system and as a consequence the pressure is simultaneously elevated

as the temperature rises. However, the experiments conducted below temperature of 100 ⁰ C could also be carried out in an open system.

In one preferred embodiment, the aqueous waste solution is spent tin or tin/lead stripping solution. The method could further comprise recycling the recovered liquid phase back to tin or tin/lead stripping process. Optionally, the copper level in the liquid phase is lowered by the means of extraction, ion exchange, chemical reduction, eutectic crystallization or electrowinning or the liquid phase could be further processed to recover nitric acid, copper compounds or iron compounds.

Preferably the solid phase contains over 80 % of the initial total amount of the tin in the spent tin or tin/lead stripping solution and the volume of tin containing fraction is reduced to less than quarter of the initial volume.

The period of time needed for the desired reactions varies between 1 to 30 hours, preferably from 50 min to 4 hours.

Under these conditions, at elevated temperature and optionally elevated pressure, the waste stream suspension clears out. The composition, which under stripping conditions remains solution-like, separates into solid and aqueous phases. Phenomena during autoclave treatment comprise conversion of the organic amine, its precipitation, release of the stannous oxide as a precipitate from the solution and settling of these precipitates. The temperature and pressure can be gradually elevated. After cooling the mixture, the two phases can easily be separated by decanting, filtering, centrifuging or using any conventional techniques. The solid phase has high heavy metal content. In one experiment, described in detail below, the solid phase contained 70% SnO2 which corresponds to tin content of 54,8%. The expression "gradually elevate" refers to starting the reaction at the temperature of the waste solution in the closed system and then bringing energy from outside to heat up the solution. The pressure increases synchronically with the temperature.

Additional steps, aim at recovering the stripping chemicals for reuse. As described, the main object is recycling the recovered liquid phase back to tin or tin/lead stripping process. Optionally, this liquid phase can be further processed to recover nitric acid, copper compounds or iron compounds. A man skilled in art could find many ways for these separations. For example, the copper level in the liquid phase can be diminished for example, but not restricted to, by the means of extraction, ion exchange, chemical reduction, eutectic crystallization, electrowinning or any combination of these.

As used here, the "a spent tin or tin/lead stripping solution further containing inorganic acid(s), metal salts and/or complexes, and one or more organic additive(s)," refers to complex side flux to be treated. It can for example, but not restricted to, originate from chemical industry, metallurgy or electronics industry. An example from electronics industry is spent tin or tin/lead stripping solution containing nitric acid, ferrous, cuprous, stannous and/or plumbous oxides and salts and organic stabilizer for nitric acid. To be precise, the word solution is commonly used for tin or tin/lead stripping solution, although the Snθ2 is actually dispersed to the liquid phase. As the expression is widespread in the art, this expression is used here.

Inorganic acids are used here to refer to inorganic acids present in the waste solution. In the case of spent tin or tin/lead stripping solution the major acid is nitric acid and the other would be suspending agents to keep the metal oxides from precipitating but rather staying in the aqueous phase. Suspending agents are typically inorganic acids, such as hydrochloric acid, nitric acid, sulfuric acid, borofluoric acid, boric acid or chloric acid.

The Kawanabe patent (U.S. Pat. No. 4,374,744) taught that also organic acids could be used for this purpose. Examples of these are oxalic acid, acetic acid, propionic acid, gluconic acid, tartaric acid or formic acid. Here organic additive compounds include these and also nitric acid stabilizers as described below.

The nitric acid stabilizer here is used in the meaning taught by Campbell in US 5,911 ,907. When reporting the composition of fresh or spent tin or tin/lead stripping solution, these are referred as "anti-tarnish agents". These compounds are added to inhibit the critical problems such as expenditure of HNO3 during the treatment and the creation of exothermic conditions. Exothermic conditions include emission of toxic NO _x gas from the nitric acid while in the presence of tin and copper ions, and excessive copper attack. The stabilizer concentration should be in the range of about 5-30 g/l. As a general characterization Campbell presented "an organic additive containing nitrogen but free of sulfur". More specifically, the stabilizer is an organic amine, preferably cyclic. Suitable nitric acid stabilizers include amino-triazole (preferably 4-amino-1 ,2,4-triazole) and amino- isoxazole (preferably 3-amino~5-methylisoxazole. Other stabilizers provided by patent literature are (US patents 4374744, 5512201 and 5911907) 4-amino-1 ,2,4- triazole, 1 , 1 , 1", 1" tetrakishydroxyethylenediamine, 1 ,2,3-triazole, 1 ,4- diazabicyclo 2.2.2 octane, 2,4-diamino-6-hydroxypyrimidine, 3,5-diamino-1 ,2,4, triazole, 3-amino-5-methylisoxazole, 4-amino-antipyrine, 5-amino-3- methylisoxazole, alanine, benzotriazole, cyclohexylamine, diethanolamine,

diethylenetriamine, ethylenediamine, glycine, hexamethylenetetramine, imidazole, indazole, indole, indolylacetic acid, monoethanolamine, naphthotriazole, N-ethyl- substituted imidazole, N-methyl-substituted imidazole, propanolamine, propylenediamine, pyrazole hydrochloride, pyrazole, pyrazolecarboxylic acid, pyrrole, pyrrolecarboxylic acid, triazole hydrochloride, triazole, triethanolamine, triethylenepentamine and urocanic acid. Other additives containing both sulphur and nitrogen, such as 2-aminobenzenesulfonamide, 3-sulfamoyl-L-alanine, 4- (aminomethyl)benzenesulfonamide, 4-amino-6-chlor-1 ,3 benzenedisulfonamide, 4-carboxybenzene sulfonamide, amino-methane sulfonic acid, ammonium sulfamate, N-(2-thiazolyI)sulfanilamide, sulfamethazine, sulfamide, methanesulfonamide, sulfaniliamide, sulfisomidine, sulfisoxazole and sulfadiazine.

The "conversion" of the organic compound implies to any reactions subjected to the organic component present in the waste solution, which occur during heating and consequential changes on solubility. These reactions comprise polymerization, degradation, oxidation, salt formation or other reactions typical for the compound. The reaction product(s) can either remain in the solution or transfer from the liquid phase to the precipitate depending on the solubility of the reaction product(s). Here settling of the precipitates indicates, how by the treatment of the invention, both the above mentioned organic precipitate and the SnO2 loose the properties that kept them dispersed or solubilised, and they both sink to the reactor bottom.

The objective and advantage of the present invention will be described in more detail by way of the non-limiting examples with reference to the drawing.

EXAMPLES

Two sets of examples were conducted the first set represents experiments at normal atmosphere and the second set represents experiments conducted under pressure.

EXAMPLE 1

A spent tin stripper was analyzed to contain 4 % Sn, 24,4 % NO ₃ , 0,9 % organic C, 0,4 % Cu, and 1 ,6 % Fe was used in these tests.

Effect of addition of 5 weight- % 100% HNO ₃ at 60 and 85 ⁰ C

To a sample of this spent tin stripper 5 w-% of 100 % HNO3 was added and stirred for 1 hour in a 1 -liter glass reactor at 1 bar pressure and at 60 ⁰ C. In a settling test no clear layer was observed. When the heating at 60 ⁰ C was continued for 2 more hours, the suspension settled out during 1 day to give a 79 vol-% clear layer and 21 vol-% of settled SnO ₂ .

To a sample of the spent tin stripper 5 w-% of 100 % HNO3 was added and stirred for 1 hour in a 1 -liter glass reactor at 1 bar pressure and at 85 ⁰ C. Some NO _x - gases were observed during the reaction. In a settling test the Snθ2-suspension settled out during 1 day to give a 88 vol-% clear layer and 12 vol-% of settled SnO ₂ .

EXAMPLE 2. Effect of higher nitric acid additions at 80 ⁰ C

To a sample of the spent tin stripper a) 0, b) 10, c) 20 and d) 30 w-% of 100 % HNO3 was added and stirred for 1 hour in a 1 liter glass reactor at 1 bar pressure and at 80 ⁰ C. The corresponding clear layers after 1 day in a settling test were a) 80 vol-%, b) 80 vol-%, c) 78 vol-% and d) 73,5 vol-%. Change to a lighter color and NOχ-gases were observed with the 10-30 w-% addition of 100 % HNO3.

EXAMPLE 3. Effect of temperature 60-67 ⁰ C and 75 ⁰ C on original spent tin stripper

The original spent tin stripper sample, without nitric acid addition, was stirred in a 1 -liter glass reactor at 1 bar pressure and at 60 ⁰ C for 1 hour and for an additional 1 ,5 hours at 67 °C. In a settling test the SnO ₂ remained as a suspension and did not settle out. The original spent tin stripper sample, without nitric acid addition, was stirred for 1 hour in a 1 -liter glass reactor at 1 bar pressure and at 75 ⁰ C. In a settling test the SnO ₂ -suspension settled out during 1 day to give 81 vol-% clear layer and 19 vol- % of settled SnO ₂ .

EXAMPLE 4. Temperature 20-50 ⁰ C and 2-20 % HNO ₃ addition gives no effect

When 2 or 5 w-% of 100 % HNO3 was added to a sample of the spent tin stripper and stirred for 1 hour in a 1 liter glass reactor at 1 bar pressure and at 20 ⁰ C, no settling occurred in the settling test. With addition of 20 % of 100 % HNO ₃ and

stirring for 1 hour in a 1 -liter glass reactor at 1 bar pressure and at 28 or 50 ⁰ C no settling was found in the settling test.

Conclusions on experiments with 1 bar pressure: The reaction leading to the settling of Snθ2 starts at ca. 70 ⁰ C for the spent tin stripper as such and at ca. 60 ⁰ C when 5 w-% of 100 % HNO3 was added. No reaction at 50 ⁰ C even with 20 % HNO3 addition was observed. Higher levels of nitric acid additions create more NO _x -gases during the reaction. The calculated range of nitrate in these tests is from 24,4 w-% NO341 ,8 w-% NO3. A combined liquid sample from all of the tests showed, compared to the original sample, an increased nitrate level due to the nitric acid additions, similar levels of copper, iron and organic carbon and a much lower level of tin. Tin is in the form of Snθ2 both in the original suspension and the settled precipitates.

EXPERIMENTS CONDUCTED UNDER PRESSURE; EXAMPLE 5

These experiments were conducted in a Parr titanium autoclave model 4563 with a maximum volume of about 1000 ml. The reactor was equipped with a double 4 bladed impeller, connected with a motor by a magnet coupling. The stirring rate was 300 rpm. In order to protect autoclave walls, a glass liner was used. The pressure was measured with a manometer. An electric mantle was used to heat up the autoclave. At the end of the experiment, a cooling coil connected around the impellor cooled the liquid. For temperature measurements, two k-type thermocouples were placed through a special opening in the autoclave. One thermocouple was connected to a controlling device (heater), which controlled the electric mantle. The set point temperature could be programmed on this control device. The second thermocouple was connected to a portable digital reader for reference. After the reaction, the autoclave was quenched with water. The acidity and the concentration of the final solution were measured after pouring out the contents of the autoclave. The autoclave was filled about 70 % (v/v). Before the reaction took place, the reactor was flushed with nitrogen gas to remove all gaseous oxygen left in the system.

Spent tin stripper solution (700 ml), received from a PCB producer, was placed in the reactor after mentioned preparations. The composition of the solution was approximately 12-15 % (w/w) nitric acid, 5 % (w/w) ferric ion, Fe ³⁺ Or Fe ²⁺ , and

minor amounts, <1 % (w/w) anti-tarnish and <1 % suspending agent, 2-15 g/l copper ion Cu ²⁺ and about 10-120 g/l Sn ⁴⁺ . Here the anti-tarnish agent used was benzotriazole.

The experiment was conducted in a way described above. The temperature was initially 20 ⁰ C and gradually rose during a 5-hour reaction time to 210 ⁰ C. Simultaneously the pressure rose from 1 bar to 15,2 bar.

After the autoclave treatment, the spent tin stripper solution split into two parts: a clear solution and a precipitate that settled at the bottom of the autoclave. The metal derivatives divided so that tin mainly (98 %, as a result of an ICP analysis) precipitated and less than 2% remained in solution whereas the copper and iron derivatives remained in solution.

As an important finding, on elevated pressure system, the emission of NO _x gases was negligible. Compared to the prior art, this is a considerable advantage.