Recovery of gold from gold-containing waste liquid

According to the chemical composition, it can be divided into the waste effluent containing gold cyanide, chloride waste, various gold washing waste liquid and aqua regia. Among them, gold-plated waste liquid (general acid gold plating liquid contains gold 4～12g/L, neutral gold plating liquid 4g/L, alkaline up to 20g/L, but often contains cyanide), aqua regia in the production of electronic components or The iodine corrosion solution is the main gold-containing waste liquid.

1. Recovery of gold from gold-plated waste liquid

Gold-plated waste liquids generally contain cyanide, and the treatment methods include electrolysis, displacement, adsorption, and the like. The tail liquid after recovery of gold should also be treated for regeneration, and it can be discharged when it is non-toxic.

(1) Electrolysis

Under the action of direct current, the gold plating ions migrate to the cathode and deposit and precipitate on the cathode. Electrolytic equipment can be electrolyzed in a slotted or closed tank.

1. Slotted electrolysis

The waste liquid in the container is heated to about 90 ° C, and the stainless steel plate is used as an electrode, and the cell voltage is controlled at 5 to 6 V for electrolysis until the gold content in the solution is reduced to a certain level, and then the new waste liquid is replaced. When the gold on the cathode is deposited to a certain thickness, it is brushed off and the ingot is smelted.

2, closed cell electrolysis

When the method is operated, the gold-containing waste liquid is first circulated in the equipment for 10 minutes, and the silicon rectifier is adjusted to make the cell voltage electrolyze at 2.5V. Until the gold content in the waste liquid is reduced to a certain value, replace it with a new gold-containing waste liquid and continue to electrolyze until a certain thickness of gold is deposited on the cathode. Open the gold extraction electrolysis device, take out the cathode and brush out the gold mud, dry and melt cast into ingots.

In addition, some studies have pointed out that the use of lead as an insoluble anode for secondary electrolysis can reduce the gold content of the waste liquid to less than 1 mg/L without stirring or using a separator. The primary electrolysis is about 5 to 10 h, the current density is 1.5 to 3 A/dm ² , the secondary electrolysis is about 25 to 50 h, and the current density is 0.5 A/dm ² . The electrolysis power consumption is 15-20 kW·h/kg.

(2) Replacement method

The displacement method is suitable for treating gold-containing waste plating bath and rinse water. The gold-containing waste liquid needs to be acidified and controlled to have a pH of 1 to 2. It was then diluted 5 times with distilled water, and the rinse water was no longer diluted. It should be carried out in a fume hood during acidification because it is acidic and easily releases HCN gas. Replace with zinc , zinc or zinc wire until the reaction is complete. After the black gold powder that has sunk into the bottom of the tank is concentrated and filtered, the excess zinc is leached with a sulfuric acid solution, and then washed, dried, and smelted into the ingot to obtain a crude gold.

(3) Activated carbon adsorption method

Activated carbon has a high adsorption capacity for gold cyanide complex. When using activated carbon to treat waste liquid, it is generally considered that the adsorption of activated carbon in NaAu(CN) ₂ is a physical adsorption process. The porosity of activated carbon directly affects the activity of the activated carbon. The stronger the activity of carbon, the greater the adsorption capacity of gold. Commonly used activated carbon has a particle size of -10 to +20 mesh and -20 to +40 mesh. The adsorption process of activated carbon includes adsorption, desorption, backwash regeneration of activated carbon, and gold extraction from the backwash. The desorption is carried out under the condition of warming and pressing with a mixture of 10% NaCN and 1% NaOH, and then the gold ions can be washed off from the activated carbon with ion-free water, and the activated carbon can be reused due to regeneration. The adsorption capacity of activated carbon for gold is 29.74g/kg, and the adsorption rate of gold is 97%.

The South African patent believes that the treatment of waste cyanide solution with ozone, air or oxygen, and then adsorption with activated carbon can achieve better results. Further, the desorbent may be selected from water-soluble alcohols and aqueous solutions thereof, and ketones and aqueous solutions which are soluble in strong alkali liquids may also be used. The composition of such a desorbent; H ₂ O, 0% to 60% (by volume); CH ₃ OH or CH ₃ CH ₂ OH, 40% to 100%, NaOH ≥ 0.11 g / L. Or CH ₃ OH, 75% to 100%, H ₂ O 0% to 25%, and NaOH 20.1 g/L.

The former Soviet patent proposed the exchange of gold from the cyanide waste liquid with an ion exchange resin, followed by desorption with thiourea. In China, anion exchange resin (717) has also been used to exchange gold from cyanide waste liquid and to extract gold with hydrochloric acid acetone solution. Of course, solvent extraction is also possible.

2. Recovery of gold from the corrosive liquid containing gold waste water

In the crystal production of electronic components, the thickness is 0.04mm sheet of gold-antimony (AuSb _0.7) which is common to aqua regia etching to 0.03mm, are examples of such scrap gold in aqua regia etching solution, a method of selectively recovered from gold.

(a) reduction method

1, ferrous sulfate reduction

Reduction of gold with ferrous sulfate (3FeSO ₄ +HAuCl ₄ =HCl + FeCl ₃ +Fe ₂ (SO ₄ ) ₃ +Au↓) has a relatively small reducing power, and it is difficult to reduce it by other metals other than precious metals, so even if it is treated with a large amount of ruthenium-containing metal The gold-containing waste liquid, the gold produced by its reduction, can also reach more than 98%. However, this method is slow, the end point is not easy to judge, and the gold is not easy to be completely reduced, and it is still necessary to further treat the tail liquid with zinc replacement.

2, sodium sulfite and acid are easy to produce gaseous sulfur dioxide, so the essence of sodium sulfite reduction is sulfur dioxide as a reducing agent or directly with sulfur dioxide can reduce gold chloride ions to produce metal gold, the reaction is:

Na ₂ SO ₃ +2HCl=SO ₂ ↑+2NaCl+H ₂ O

2HAuCl ₄ +3SO ₂ +6H ₂ O=2Au↓+8HCl+3H ₂ SO ₄

In order to prevent the reduction product from being re-dissolved by aqua regia, it is required that the waste aqua regia be heated and boiled before reduction, and the free nitric acid and nitrate are exhausted. Heating the solution properly during the reduction is advantageous for producing large particles of yellow sponge gold. It has been proposed in the United States to add a small amount of polyvinyl alcohol as a coagulant to the solution, which is beneficial to the sedimentation of the floating gold powder, and the amount of polyvinyl alcohol added is about 0.3 to 30 g/L.

3, sodium bisulfite method

The sodium hydrogen sulfite (NaHSO ₃ ) method is a US patent. First adjust the pH of the gold-containing waste aqua regia with a solution of an alkali metal or alkaline earth metal hydroxide (for example, containing 25% to 60% NaOH or KOH) or carbonate, and then heat it to 50 ° C. After maintaining for a while, add sodium bisulfite to precipitate gold. In order to accelerate the sedimentation and sedimentation of the precipitate, butyl succinate should also be added as a coagulant. This method is particularly suitable for the treatment of waste aqua regia with less gold, because it does not need to be washed.

Gold is recovered from the corrosive liquid containing gold waste aqua regia, and oxalic acid or formic acid reduction method can also be used.

(2) Replacement method

Similar to the replacement method of gold plating, zinc can also reduce gold chloride ions. When zinc powder is used for replacement, the liquid is required to drive the nitrate to increase the direct yield of gold. Controlling pH=1～2 during the replacement process can prevent the hydrolysis of zinc salt, which is beneficial to product clarification filtration. The zinc precipitate contained in the metal precipitate produced by the displacement can be dissolved by an acid. When hydrochloric acid is used for dissolution, the precipitate should not contain nitrate. Except silver , lead and mercury , the others are easily dissolved by hydrochloric acid, and the powdery product is easily washed. When dissolved in nitric acid, it dissolves almost all common metal impurities. In order to prevent gold from re-dissolving, it is required that the precipitate does not contain chloride ions, and after washing the precipitate dissolved in nitric acid, the sponge gold color is bright yellow and the agglomeration is good. In addition, sulfuric acid can also be used to dissolve zinc and other impurities, and the precipitated gold is not easily re-dissolved, but calcium and lead ions cannot be separated from the precipitate, and the product tends to be black. The black powdery product gold must also be treated with nitric acid, and the color of the cleaned product gold returns to normal yellow.

3. Recovery of gold from gold-containing waste iodine corrosion solution

When producing electronic components, the iodine solution is used to corrode gold and produce gold-containing iodine etching solution. These gold-containing etching solutions can recover the gold from the sodium sulfite. When the saturated sodium sulfite solution is added to the liquid, the iodine liquid changes from a purple-red color to a pale yellow color, and is naturally clarified and filtered to obtain a crude gold.

4. Recovery of gold from laboratory waste liquid

The collected gold-containing test waste liquid is concentrated and evaporated to near dryness, and aqua regia is added to dissolve the gold; the evaporation is continued, and an appropriate amount of hydrochloric acid is added to steam the NO ₃ ^- and NO ₂ to just dry; and 5% hydrochloric acid water is added to boil. Dip gold, filter, and wash the residue with 5% hydrochloric acid water until colorless, then add appropriate amount of 10% hydroquinone solution to the solution to reduce the monomer gold (2H[AuCl ₄ ]+3C ₆ H ₆ O ₂ = 3C ₆ H ₄ O ₂ + 8HCl + 2Au ↓). According to the theory, 1mg hydroquinone is enough to reduce 1mg gold, but it must be considered that during the reduction process, some substances may consume hydroquinone, so hydroquinone must be added in excess to ensure complete reduction.

After adding hydroquinone, heat to 70-80 ° C, filter with slow filter paper, wash the beaker with warm water, wash the precipitate 2 to 3 times, transfer the precipitate together with the filter paper to the porcelain crucible, low temperature carbonization, high temperature ashing That is, a soft sponge-like pure gold is obtained, and then the sponge gold is wrapped with a lead sheet to carry out ash blowing to obtain a glittering gold ingot, and the gold content is above 99.9%.

Various liquid after Tail whether the recovery of gold recovered fully, the following methods can be determined :( a) is determined by the end of fluid color, if the end of a colorless liquid, the precipitated gold is fully extracted; (ii) an acidic solution of stannous chloride inspection When there is gold, the colloidal fine gold is suspended in the solution to make the solution purple-red, otherwise, the tail liquid has been completely extracted.