Treatment of mercury-containing wastewater

Gold mine production of contact with the mercury amalgamation mainly job amalgam washing, filtration and distillation. Among them, mercury amalgamation, mercury scraping, mercury amalgamation, mercury embedding, etc. are still all manual operations. People have frequent contact with mercury, and the mercury in the plant is lost to the wastewater, which causes serious air pollution. Many mercury-containing wastewater treatment method, the effect of various treatment methods depending on the presence of Mercury [Hg (mercury), mercury oxide, mercury (corrosive sublimate) chloride, mercurous chloride (calomel), mercury sulfide ( See Table 1 for the initial concentration of Cinnabar), the coexisting ions in the wastewater, and the standards for the water quality to be discharged. The methods of mercury treatment are:

Table 1 National emission standards and water quality standards for mercury and mercury compounds

1. Reduction of mercury-containing wastewater

(A) sodium perborate (NaBH ₄₎ reduction

Chemical principle: non-metallic reducing agent - sodium borate, which reacts with mercury at pH 9-11 to produce mercury and boric acid, releasing hydrogen.

The redox half-reaction is:

Hg ^{2 +} +2e=Hg↓

B ^{5 -} =B ^{3 +} +8e

6H ⁺ +6e=3H ₂

That is, a non-metallic reducing agent sodium borate with a concentration of 12% is added to the mercury-containing wastewater, and then mixed with a base (sewage pH=9 to 11) to form mercury particles (about 10 μm in diameter), which are separated and recovered by a hydrocyclone. The mercury remaining in the overflow was separated by water and gas and trapped with a filter having a pore size of 5 μm. The mercury vapor in the exhaust gas is washed with dilute nitric acid and returned to the original wastewater tank for recycling. This method has been used in industrial production in the United States, and the amount of residual mercury in the discharged wastewater is less than 0.01 mg/L.

(2) Metal reduction method

The redox potential of some metals is substituted for Hg ^{2 +} metals, such as Cu, Zn, Fe, Mn, Mg, Al, etc., and their corresponding metal chips are charged into the packed column to displace the Hg ^{2 +} ions in the wastewater. For example, the removal of mercury by Fe:

Fe+Hg ^{2 +} =Fe ^{2 +} +Hg↓

Couple Fe ^{2 +} / Fe is ^{E 0 = -0.44V, Hg 2 +} / Hg of E ⁰ = -0.854V, and therefore, the above reaction can be carried out. The replacement rate is related to factors such as pH, temperature, metal purity, and contact area.

Metal reduction process may also be combined with other mercury removal methods, such as filter cloth and lye in a joint replacement an aluminum powder mercury purified water. Copper, for example, a domestic gold ore (amalgamation - flotation process) copper concentrate clarified water tests show that clarified water mercury 7.28mg / L, filter cloth mercury removal was 81.51%, the total mercury removal rate was 97.64% . However, organic tributaries cannot be treated by direct metal reduction. Usually, they are first destroyed by an oxidizing agent (such as chlorine), converted into inorganic mercury, and then replaced with a metal.

Second, the treatment of mercury-containing wastewater by vulcanization

The method is to add sodium sulfide to the mercury-containing wastewater with a pH of 9 to 10, and combine the sulfur ions with the mercury ions in the wastewater to form a mercury sulfide precipitate having a very low solubility:

The mercurous sulfide formed in the reaction is unstable and is easily decomposed into mercury sulfide and mercury. The resulting sulfide has a small solubility product, so that even under acidic conditions (pH ≥ 1), precipitation of HgS can be precipitated. Because small amounts of mercury in waste water, waste water was added sodium sulfide resulting in an excess of S ^{2 -} ions, in order to avoid the adverse consequences of this time, may be supplemented with ferrous sulfate iron sulfide precipitate. The addition of a part of Fe ^{2 +} can also combine with OH ^- ions in wastewater to form Fe(OH) ₂ and Fe(OH) ₃ , and co-precipitate and coagulate sedimentation of a small amount of small HgS aerosols. The addition of FeSO ₄ does not affect the preferential precipitation of HgS. Since the solubility product of the generated FeS (K _sp = 3.7 × 10 ^{- 19} ) is hundreds of millions of times larger than the solubility product of HgS (K _sp = 4 × 10 ^{- 53} ), after the addition of FeSO ₄ , the sulfur in the solution must first Hg ₂ S (K _sp = 1.0 × 10 ^{- 45} ) is formed with mercury, and then a FeS precipitate is formed.

For example, if a plant is treating 5mg/L of acidic wastewater containing mercury, the actual operation is to adjust the pH to 8~9 with lime, the wastewater is alkaline, and then add sodium sulfide (30mg/L) and FeSO ₄ (60mg/L). Mercury removal by sodium sulfide precipitation reduces the amount of mercury in the wastewater to 1 to 0.1 mg/L. If iron filings, activated carbon adsorption, coagulant precipitation, etc. are used, the mercury content in the wastewater can be reduced to less than 0.05-0.01 mg/L.

For organic mercury, it is first oxidized to inorganic mercury and then treated as described above.

3. Static adsorption method for treating mercury-containing wastewater

Activated carbon is used as an adsorbent or an adsorbent made of kaolin . The specific method is to adopt the static adsorption method, that is, to precipitate and then adsorb. First, sodium sulfide is used to precipitate mercury ions as mercury sulfide. At the same time, suspended solids such as mud and sand in the wastewater are removed, pH is adjusted with calcium hydroxide, ferrous sulfate is used as a coagulant, and then activated carbon is used to adsorb the leaked metal mercury and mercury. The compound, the treated cleaning solution contains residual mercury to meet emission standards.

Foreign agricultural and sideline products containing tannin are used as adsorbents in foreign countries. Such as walnut chips, peanut soft skin, peanut shell, straw, bagasse, olive core, etc., also used treated clay . This type of adsorbent containing tannins. When other metals are present in the mercury-containing wastewater, the adsorption effect on mercury is not affected, and the adsorption capacity exceeds 130% of the activated carbon.

4. Solvent extraction method for treating mercury-containing wastewater

At present, some foreign countries use triisooctylamine/xylene to extract mercury-containing wastewater. After extraction, the residual mercury in the purified liquid is below 0.01mg/L. The extractant after extraction of mercury is extracted by non-acid salt. To recover mercury.

V. Condensation precipitation method for treatment of mercury-containing wastewater

The coagulant uses lime. Lime is introduced into the mercury-containing wastewater to form Ca(OH) ₂ , and Ca(OH) ₂ has a cohesive adsorption effect on mercury. When there is trivalent iron ion, the effect is better. When the coagulant is made of aluminum sulfate, the treatment of mercury-containing wastewater has a good effect. After the coagulation and sedimentation, the mercury content of the effluent water drops below 0.05 mg/L.

6. Other methods for treating wastewater containing mercury

It is reported that foreign countries also use microbial recovery of mercury, electrolytic recovery of mercury, ferrite precipitation method for mercury removal, and sulfide precipitation-flotation separation. In addition, mercury is removed by ion exchange in China, and mercury removal by conversion method and mercury removal by humic acid-containing coal adsorption are being studied.

At present, gold mines use gold-mixing operations to raise gold. For the mercury that is lost to the discharged wastewater, no treatment measures are taken. After the natural sedimentation by the gravity of mercury itself, the mercury contained in the wastewater discharged from the outlet of the tailings pond can basically Meet the standard of ground water quality.