ENVIRONMENTALLY FRIENDLY PROCESS TO OPTIMIZE COPPER DISSOLUTION AND RECOVER COPPER AND GOLD FROM ELECTRONIC WASTE

Abstract

The present invention is related generally to recovering metals from waste electronics, and more particularly to a process to recover copper and gold commonly found in waste printed circuit boards using a lixiviant containing a weak acid such as citric acid or acetic acid, a particular concentration of table salt and an oxidizer. By using this lixiviant, the copper found in the printed circuit board reacts to form copper salts and gold becomes detached. Importantly this recovery method of copper and gold found in waste PCBs is fast, does not pose environmental hazards and is economically feasible.

Claims

1. A single lixiviant solution used to recover copper and gold from electronic waste comprising a weak acid; an oxidizer; and a salt wherein a concentration of the salt is less than 30% by weight of the single lixiviant solution and the weak acid to the salt ratio in the single lixiviant solution is 1 grams to 15 grams and has pH less than 2.3.

2. The single lixiviant solution of claim 1, wherein the weak acid is acetic acid.

3. The single lixiviant solution of claim 1, wherein the salt is table salt (NaCl).

4. The single lixiviant solution of claim 1, wherein the oxidizer is hydrogen peroxide.

5. The single lixiviant solution of claim 3, wherein the concentration of table salt (NaCl) is between 2% to and 10% by weight of the lixiviant solution.

6. The single lixiviant solution of claim 1, wherein the ratio of the weak acid to the salt in the lixiviant solution is 2 grams to 13 grams.

7. The single lixiviant solution of claim 1, wherein the pH of the single lixiviant solution is between 0.6 to 2.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a graph showing the effect on the pH of the lixiviant solution containing a weak acid and salt concentration.

[0012] FIG. 2 is a graph showing the effect on the pH of the lixiviant solution containing a weak acid and salt concentration.

[0013] FIG. 3 is a UV-VIS chart showing the progress of the copper dissolution.

DETAILED DESCRIPTION

[0014] Typical hydrometallurgical processes of recovering precious metals from e-waste involves leaching the copper from the e-waste by using strong acids and/or base in a leaching/lixiviant solution. Such chemicals generate hazardous waste and pose safety hazards. As pointed out above, copper is extensively used in PCBs because of its excellent electrical conductivity. However, copper can easily be oxidized, leading to the loss of electrical contacts. To prevent this loss of conductivity, copper contacts in PCBs are usually coated with gold to maintain good connections. Nickel or tin are also used but do not have good electrical conduction.

[0015] Weak acids like acetic acid and citric acid do not attack the copper contacts found in PCBs but these weak acids can dissolve copper oxide. It is known that these weak acids are often used to dissolve and remove dull oxide layer from copper artifacts. Theoretically, it is possible to dissolve an all-copper penny by repeated dipping in a solution containing these acids. After the copper oxide layer has been dissolved, the copper exposed to air will regenerate a fresh copper oxide layer which can then be dissolved by dipping in the weak acid solution again. If such a process is repeated many times, it is possible to dissolve the entire penny.

[0016] One method of supplying oxygen for the oxidation of copper element is either by the addition of hydrogen peroxide or by bubbling air/oxygen through the weak acid solution containing copper artifacts.

[0017] The inventors of the present inventive process have found that this copper oxide reaction could be further enhanced and sped up by using a lixiviant solution containing a weak acid, such as citric acid, acetic acid or a combination of both acids, and common table salt (NaCl) in a particular concentration with any choice of oxygen introduction. The salt concentration is less than 30% weight of the lixiviant solution, preferably, 2% to 10% by weight. Additionally, the weak acid to salt ratio in the lixiviant solution in the range of 1 to 15, preferably, 2 to 13.

[0018] The acetic acid in vinegar has the following chemical structure:

##STR00001##

[0019] The copper oxide reacts with the weak acetic acid to from copper acetate which is soluble in water.

CuO+2CH.sub.3COOH.fwdarw.Cu (CH.sub.3COO).sub.2+H.sub.2O  (2)

[0020] Citric Acid has the following chemical structure

##STR00002##

[0021] There are several possible reactions of citric acid with copper and copper oxide that have been reported in the prior art:

2CuO+C6H8O7=Cu2C6H4O7+2H2O  (4)

[0022] The exact mechanism for what happens when common table salt (NaCl) is added to the weak acid lixiviant solution in not totally clear. There are several outcomes: (1) Salt could act as an electrolyte and promote a redox process. (2) Salt could act as a catalyst. (3) Na+ ions enhance the dissolution of the weak acid in water, pushing reaction to the right. (4) Chloride can penetrate the copper oxide layer and promote dissolution of copper.

[0023] The inventors discovered that the addition of common table salt to the lixiviant solution containing only weak acid and an oxidizer surprisingly accelerated the copper dissolution. The salt concentration is less than 30% weight of the lixiviant solution, preferably, 2% to 10% by weight. Additionally, the weak acid to salt ratio in the lixiviant solution in the range of 1 to 15, preferably, 2 to 13.

[0024] The pH of the acetic and citric acids is also different and the pH changes with the concentration of the weak acid used for copper dissolution. Moreover, as shown in FIG. 1, when common salt is added to any of these acid solutions, the pH drops further by about 0.4 pH units for every 10% addition of salt. It is unclear if some small quantity of HCl is being formed but appears unlikely. A fixed drop of pH at all acid concentrations argues against that some quantity of HCl is being formed as well. The inventors believe that acid dissolution is being promoted by the salt presence in the lixiviant solution. Moreover, as shown in FIG. 2, the addition of salt to the acetic acid solution also lowers the pH.

[0025] The data in FIGS. 1 and 2 suggest that by controlling the amount of citric or acetic acid as well as the salt concentration in the lixiviant solution, the effective optimum pH could be achieved for copper dissolution in waste PCBs and well as the aiding in the delamination of the gold electroplated on top of the copper contacts. The effective pH of the lixiviant solution is less than 2.3, preferably in the range 0.6 to 2.2.

[0026] The progress of the copper dissolution reaction was followed using temperature data, pH data and UV-VIS absorption. Different concentrations of acetic acid, citric acid and salt were tested to determine most efficient process for copper dissolution. In Experiment 1, a lixiviant solution having the salt (NaCl) and Citric acid (CA) amounts in grams as listed in Table 1 below and mixed with 100mls of 3% hydrogen peroxide and 50 grams of washed and shredded PCBs.

TABLE-US-00001 TABLE 1 CA(g) NaCl(g) Sample 1 20  5 Sample 2 20 10 Sample 3 20 20 Sample 4 10 10 Sample 5 40 10

[0027] In Experiment 2, following ratios of citric acid and salt listed in Table 2 were used in the lixiviant solution mixed with 100mls of 3% hydrogen peroxide and 50 grams of washed and unshredded PCBs.

TABLE-US-00002 TABLE 2 CA(g) NaCl Sample A 10 10 Sample B 20 20 Sample C 30 30 Sample D 40 10

[0028] In Experiment 3 the ratio of salt was increased in the lixiviant solution to ensure that salt to citric acid was stoichiometrically matched. In Experiment 3, a lixiviant solution having the salt (NaCl) and Citric acid (CA) amounts in grams as listed in Table 3 below and mixed with 100mls of 3% hydrogen peroxide and 50 grams of washed and shredded PCBs.

TABLE-US-00003 TABLE 3 CA(g) NaCl Sample A 10 12 Sample B 20 24 Sample C 30 36 Sample D 40 24

[0029] The reactions in Experiments 1, 2 and 3 were followed by measuring the pH and temperature. UV-VIS reaction progress was monitored by removing 1 ml of lixiviant solution and diluting the 1 ml of lixiviant solution with 2 mls of DI water. UV-VIS scans were performed from 325 to 1100 nm using a Thermo Fisher GYNESYS 50 UV-VIS spectrophotometer. End of the reactions in Experiments 1, 2 and 3 was assumed when the peak of the UV-VIS spectra has attained maximum. Similarly, the completion of reaction could be indicated by measuring the pH throughout the reaction. As the weak citric acid is being used to remove copper in the unshredded PCBs, the pH of the lixiviant solution rises. When pH ceases to rise any further or the change in pH slows down significantly, it suggests the end of the desired reaction.

[0030] In Experiment 4 both acetic and citric acids were compared at various salt concentrations as shown in the Table 4 below. The objective of Experiment 4 was to ascertain the effect of the acid type used in the lixiviant solution, preferable acid concentration and preferable acid to salt ratio to obtain optimum gold recovery.

TABLE-US-00004 TABLE 4 Reaction Materials 4-A 4-B 4-C 4-D 4-E 4-F 4-G 4-H Acetic Acid, g 12.5 12.5 25 25 Citric Acid, g 12.5 12.5 25 25 H.sub.2O.sub.2, g 10 10 10 10 10 10 10 10 H.sub.2o, g 100 100 100 100 100 100 100 100 NaCl, g 2 5 2 5 2 5 2 5 Chip, g 50 50 50 50 50 50 50 50 Acid/H.sub.2O.sub.2 1.25 1.25 2.5 2.5 1.25 1.25 2.5 2.5 Acid/Chips 0.25 0.25 0.5 0.5 0.25 0.25 0.5 0.5 Acid/NaCl 6.25 2.5 12.5 5 6.25 2.5 12.5 5

[0031] In reviewing FIG. 3, the UV-VIS results suggest that higher acid concentration and salt content were critical in speeding the reaction rate. Hourly samples from Experiments 4A-4H were taken and measured by UV-VIS in a standard cuvette (hour 1-black, hour 2-orange, hour 3-gray, hour 4-yellow, hour 5-blue). In general, the completion of reaction was achieved faster with citric acid the reaction was further enhanced by increased salt concentration in the lixiviant solution. The optimum concentration of the salt in the lixiviant solution is less than 30% by weight in the lixiviant solution. However, acetic acid is an acceptable alternative to using citric acid in the lixiviant solution in combination with an optimum salt concentration of less than 30% by weight of the lixiviant solution.

[0032] In various other experiments involving acetic or citric acid and salt, following experimental plans were used. The lixiviant solution consisted of different amounts of acid %; salt %; hydrogen peroxide %; acid/chips ratios; and acid/salt % ratio as set forth in Table 5 below. The stripping or delamination time (copper becomes copper salt and dissolves in the lixiviant solution after reacting with the weak acid and the electroplated gold becomes separated or delaminated from the chips) for the gold is also set forth in Table 5.

TABLE-US-00005 TABLE 5 CONCENTRATION.sup.1 RATIO UNSHREDDED ACID/ ACID/ STRIPPING TIME EXPT# CHIPS, g ACID ACID % NaCl % H.sub.2O.sub.2 % CHIPS NaCl % FOR GOLD, hrs. 1 500 acetic 8.33 1.2 1.7 0.6 12 24 2 500 citric 8.33 1.2 1.7 0.6 12 24 3 1000 acetic 11.3 1.1 1.5 0.3 12 24 4 1000 acetic 17.6 1.1 1.5 0.5 20 24 5 500 citric 5.2 2.7 2 0.23 2 6 6 500 citric 9.9 2.6 2 0.46 4 6 7 500 citric 5.3 1.4 1.9 0.23 4 24 8 500 citric 10 1.3 1.8 0.46 8 24 9 500 citric 5.1 4 1.9 0.23 1.3 5 10 500 citric 5.1 5.3 1.8 0.23 1 5 11 500 acetic 5.1 4 1.9 0.23 1.3 n/a 12 500 acetic 5.1 5.3 1.8 0.23 1 n/a 13 500 citric 2.38 5.3 1.9 0.2 0.8 5 14 500 citric 2.38 6.6 1.8 0.2 0.7 24 15 500 acetic 8.6 3.9 1.8 0.4 2.2 24 16 500 acetic 8.1 3.6 3.4 0.4 2.2 24

[0033] In all the experiments described in Table 5, gold flakes were clearly isolated and identified by EDS and ICP testing results. When a metal washer was added to the lixiviant solution in several experiments listed in Table 4, it was found that overnight a copper coating had covered the washer, confirming the presence of copper ions. This copper ion recovery could be sped up by setting up a electrical cell and by applying some desired potential (i.e. by electrowinning). Also, when the lixiviant solution was allowed to dry slowly, aqua-green crystals of copper acetate or copper citrate were recovered.