Containing mercury vapor, fumes and industrial sewage must undergo purification treatment. Direct discharge without purification will greatly poison the air and rivers and lakes, destroy aquatic resources and endanger the health of the population. The consequences are extremely serious.

Shuiyu Town, Kumamoto Prefecture, Japan is a bay where people live on seafood. In 1950, some cats in the town suddenly got strange diseases. Not long after, some residents also got symptoms similar to cats. First, they were slurred, their faces were expressionless, they walked eastward, and then their muscles groaned. The body bent like a bow, yelling and yelling. Die under extreme pain. This is the world famous Minamata disease. After nearly two decades of research, it was not until 1969 that the occurrence of mink disease was caused by mercury-containing sewage from a chemical plant in the town that used mercury as a catalyst.

Today, many of the major waters of the industrialized countries of the world, such as the five major lakes in the United States, the St. Kia River, and the Detroit River, are also heavily polluted by the large amount of mercury-containing sewage discharged from chemical plants.

According to research scientists in Sweden, waters of mercury poisoning is due to inorganic mercury compounds in waste water into the water, the oxygen is reduced to water of bacteria suspected to metallic mercury, metallic mercury and then into methylmercury caused due methylmercury Soluble in water, after absorption by floating animals and plants, through the accumulation of "food chain" of small fish and small shrimps to eat plankton and big fish to eat small fish, the concentration of mercury in the fish body is higher than the concentration of mercury in the water. It is 10,000 times higher, even 100,000 times and millions of times. Humans eat fish and shellfish containing high concentrations of mercury, and are poisoned by accumulation.

Direct mercury in mercury-containing flue gas and amalgamation operations in metallurgical plants is mercury vapour and its compounds. Its performance is also mainly chronic poisoning. Since mercury has a strong volatility at normal temperature, according to the measurement of a factory, in the normal amalgam production, when the room temperature is at 27 ° C, the mercury vapor concentration measured at a distance of 10 cm from the head end, the middle and the tail of the mercury plate. They were 94, 87, and 51 μg/m ³ , respectively. Mercury plate aisle breathing zone has a slightly lower mercury concentration than the plate surface. When the ventilation is poor, the mercury concentration in the working space is generally evenly distributed. When the mercury paste and mercury are scraped, the concentration of mercury at a distance of 50 cm from the surface of the plate is as high as 200 to 250 μg ∕m ³ . When the amalgamated slurry is subjected to flotation, the foam surface of the flotation machine contains 10 μg of ∕m ^{3 of} mercury, and the clarified water of flotation concentrate and tailings contains 5 and 11 μg/L of mercury, respectively.

As for the hazards of inorganic mercury and other compounds in the mercury-mixed plant sewage and abandoned tailings, no systematic report has been reported.

To prevent mercury damage, many countries have enacted environmental protection regulations. The UK requires that the mercury contained in the effluent should be less than 1 mg/L. The limits of mercury content in drinking water in the former Soviet Union and Finland are 0.005 mg ∕L. The United States and Canada stipulate that fish containing more than 0.5 mg ∕ kg of mercury (1 mg/kg in Sweden) is banned. China's "Integrated Wastewater Discharge Standard (GB8978-88)" stipulates that the maximum allowable emission concentration of mercury in sewage is 0.05mg∕L. The maximum concentration of mercury in the flue gas is 0.01-0.02 mg/m ³ .

A large number of experimental studies have been carried out on the purification of mercury-containing sewage and flue gas, and some achievements have been made. For example, sodium sulfide co-precipitation and activated carbon adsorption have been widely used in the production to purify industrial wastewater containing inorganic mercury. However, the sodium sulfide co-precipitation method is difficult to reduce the mercury content in the sewage to less than 1 mg ∕L; the activated carbon adsorption filtration method is generally only suitable for treating sewage containing less than 5 mg ∕L of mercury. In addition, the water purification of water hyacinth has a certain effect. Others such as reduction, oxidative decomposition, ion exchange resin adsorption, microbial and radioactive decomposition are too costly and economically uneconomical. As for the purification of organic mercury-containing wastewater, it is still in the research stage. Today, the most important methods for removing mercury from mercury-containing flue gas are: the sulfuric acid washing method of Outokumpu, Finland, the mercury chloride method of the Norwegian zinc company [also known as the Odda method], and the Swedish company Politon selenium, carbon filtration, the Japanese company Toho potassium iodide is added and the precipitation of the iodine complex development method and the like.

In view of the advantages and disadvantages of various mercury-containing wastewater treatment methods, Japan Electrolytic Plant uses a sodium sulfide co-precipitation method and activated carbon adsorption filtration process to treat 200 tons of wastewater containing 5-10 mg of hydrazine per day discharged from the plant. That is, 90% of the mercury precipitate is removed by the sodium sulfide co-precipitation method, and the supernatant containing 0.1 to 1 mg of hydrazine of mercury is obtained, and then it is passed through the granular activated carbon tank at a flow rate of 3 to 4 m∕h to remove the residual mercury. %. The treated wastewater contains mercury and can be reduced to 0.01-0.05 mg ∕L. The liquid is then pumped to a comprehensive wastewater treatment plant, which is retreated and diluted 10 to 20 times and discharged. The precipitate obtained during the operation is recovered by calcination.

The main purification methods for mercury vapor, flue gas and industrial wastewater are described below.

1. Purification of mercury-containing waste gas in amalgamation operations

The method of purifying an exhaust gas containing mercury amalgamation of many jobs, is currently the most commonly used chloride - activated carbon adsorption and absorption soft manganese ore and the like.

Chlorination-activated carbon adsorption method is to make the mercury ions in the exhaust gas react with chlorine to form precipitated mercurous chloride.

Hg+Cl ₂ HgCl ₂ ↓

The remaining mercury is then recovered by adsorption of activated carbon.

Mercury removal by chlorination-activated carbon adsorption method, the adsorption rate of mercury can reach 99.9%.

The pyrolusite absorption method is to wash the mercury-containing waste gas with a dilute sulfuric acid solution containing pyrolusite to form a mercury sulfate recovery.

2Hg+MnO ₂ HgMnO ₂

Hg ₂ MnO ₂ +4H ₂ SO ₄ +MnO ₂ 2HgSO ₄ +2MnSO ₄ +4H ₂ O

The operation of the pyrolusite absorption method is to introduce mercury vapor or waste gas containing liquid fine mercury beads into a washing tower with bricks, and send a dilute sulfuric acid solution containing finely divided pyrolusite to the tower for scrubbing, mercury and The pyrolusite is absorbed after contact and produces mercuric sulfate. In the circulation the washing liquid column, when the sulfite concentration of mercury in the washing liquid enriched to about 200g / m ^3, that is discharged from the tower, with iron chips or substitutions to precipitate copper scrap recovery of mercury. The mercury absorption efficiency of this method can reach 95% to 99%.

Second, the filtration of mercury mixed industrial wastewater, aluminum powder replacement purification

Mercury-containing industrial waste water can be filtered and combined an aluminum powder is added in substitution for the alkaline solution purification cloth. After amalgamation of a gold-copper mine ore in China, the copper concentrate produced contains 7.28mg of mercury in the clarified water. First, filter with a filter cloth to remove 81.51% mercury; then add aluminum powder to replace the solution under alkaline conditions. The total removal rate of mercury was 97.64%.

3. Sodium sulfide co-precipitation method for purification of industrial wastewater containing mercury

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

2Hg ^{2 +} +S ^{2 -} Hg ₂ S↓ HgS↓+Hg

The mercurous sulfide formed in the reaction is unstable and is easily decomposed into mercury sulfide and mercury. The solubility of mercury sulphide is 1.4 × 10 ^-24 g ∕L. Since the amount of mercury contained in the wastewater is small, the mercury sulphide formed is mostly fine particles, and most of the suspended mercury is not easily precipitated in water. For this purpose, the coprecipitation agent and the fractional precipitation method are often used. That is, a slight excess of sodium sulfide is first introduced, mercury sulfide is formed with mercury, and an appropriate amount of ferrous sulfate is added to cause excess sulfur in the liquid to form iron sulfide precipitate. At this time, a part of the ferrous iron may also form a Fe(OH) ₂ precipitate with the hydroxide ions in the water.

Since the solubility product constants of mercury sulfide and iron sulfide in water at 18 ° C are 4 × 10 ^-53 and 6 × 10 ^-18 , respectively, the difference between them is hundreds of millions of times. After the addition of FeSO ₄ , the sulfur in the solution must first form Hg ₂ S with mercury before the FeS precipitate is formed. Therefore, the input of ferrous sulfate not only can quickly remove excess sulfur ions in the wastewater, but more importantly, as a coprecipitation carrier, the Hg ₂ S of the particles can be adsorbed on the surface of the floc of FeS to form a complete precipitate. The purpose of precipitating mercury.

In a chemical plant in China, the process and conditions for treating 5 mg ∕L of acidic wastewater from acetaldehyde workshop by sodium sulfide co-precipitation method are as follows:

(1) Mixed neutralization

The acidic wastewater is pumped to the mixer, and at the same time, the lime milk (or calcium carbide slurry) in the stirring tank is pumped into the mixer, and when the wastewater has reached half of the batch throughput, the supply of the lime milk (or calcium carbide slurry) is stopped. . The mixture is placed in a wastewater treatment tank and the other half of the wastewater is pumped into the tank. The mixed wastewater in the wastewater treatment tank is thoroughly mixed and stirred under the stirring of the stirring blade, and when the pH value is determined to be 9, the neutralization operation is completed.

(two) co-precipitation

First, add 3% sodium sulfide solution to the wastewater, stir for about 10 minutes, add 6% ferrous sulfate solution, stir for about 15 minutes, let stand for half an hour, and take the supernatant to analyze the mercury content to meet the requirements. Dehydrated by a centrifuge. The separated mercury slag is concentrated. The supernatant is diluted with water and discharged.

The acidic wastewater containing 5 mg/L of mercury was treated by the above procedure, and the dosage was: sodium sulfide 30 mg ∕L, ferrous sulfate 60 mg ∕L. When the pH of the neutralized solution is in the range of 8 to 10, the method can completely remove the inorganic mercury in the wastewater.

In addition to the sodium sulfide co-precipitation method, the treatment of mercury-containing wastewater by sodium sulfide co-precipitation flotation has also been successful in Japan. This method is divided into three methods of adding trivalent iron salt flotation, adding sodium sulfide flotation and adding sodium sulfide and adding trivalent iron salt flotation. When the pH of the waste liquid is about 9, Fe ^{3 +} is added, and three-stage flotation is carried out with sodium oleate; or according to the total content of mercury in the waste water, it is effective to add a stoichiometric amount of sodium sulfide flotation twice. When the waste liquid has a pH of 6.5 to 9.5, it is effective to add Fe ^{3 +} 40 mg of ∕L and sodium sulfide having the same total amount of mercury, even if only one stage of flotation is carried out with sodium oleate. The waste water purified by this method was found to have no mercury.

4. Activated carbon filtration and adsorption method for purification of industrial wastewater containing mercury

There are two kinds of operations for purifying industrial wastewater containing mercury by the granular activated carbon adsorption method recommended by a factory in China.

(1) Granular activated carbon column filtration

The test uses strip activated carbon, beacon charcoal and self-made activated charcoal produced by activated carbon plant, and is crushed and sieved to make 2.262～1.397mm (8～12 mesh). Sample 1 is industrial wastewater containing 0.94 to 2.8 mg of mercury, and the other is simulated industrial wastewater containing 1 to 6 mg of mercury. The carbon particles are packed in columns, and the waste liquid passes through the single-tube, double-tube and three-tube series activated carbon columns at a rate of 1 m∕h, and the adsorption rate of mercury is above 98% on average. The mercury-absorbing charcoal is returned to the use after distilling mercury, but the mercury absorption rate is slightly reduced (over 96%) when used again.

(2) Activated carbon powder adsorption method

The mercury-containing wastewater treatment tank established according to the small test results is divided into three layers. The lower layer of the pool is below the surface of the earth and functions as catchment and sedimentation. After the wastewater is filled with the lower pool, the pump automatically draws the water into the upper tank to which the activated carbon powder is added. After the water is full, the compressed air tube at the bottom of the pool is automatically compressed and stirred for half an hour. After the sedimentation sample test, the supernatant is discharged after the mercury content meets the discharge standard; if it is not required, it is placed in the middle pool for repeated treatment. The secondary treated wastewater is discharged by a water pump through a microporous plastic filter tube. The wastewater purified by this method has clear water and no carbon powder. The treatment operation is intermittent, and each batch of mercury-containing wastewater is treated for 40 tons, and the process takes about 2 to 3 hours.

When the treated wastewater contains 1 mg/L of mercury, 50 g of waste activated carbon powder is added per liter of the pharmaceutical plant. After treatment, the mercury content of the wastewater decreased to 0.0215 mg∕L, and the mercury removal rate was over 97%. The mercury-carrying carbon powder is recovered by distillation.

5. Iodine law purifies mercury-containing flue gas

The iodine complex method developed in China for treating mercury-containing flue gas from the desulfurization of zinc concentrate is to pass the flue gas containing mercury and sulfur dioxide into the absorption tower filled with the porcelain ring through the bottom of the absorption tower. Mercury is absorbed by spraying the absorption liquid containing iodized salt from the top of the tower.

The rich liquid that circulates and absorbs mercury is quantitatively and partially extracted for electrolytic mercury removal to produce metallic mercury.

The iodine complex is used to treat flue gas containing mercury and sulfur dioxide. The mercury removal rate is 99.5%, the tail gas contains less than 0.05mg ∕m ³ , and the mercury produced by the flue gas removal mercury contains less than one part per million. Recycling 1t of mercury consumes 200kg of iodized salt and consumes 56,000kW·h.

The iodine complex treatment of flue gas containing both mercury and sulfur dioxide, compared with the mercury chloride method, does not contain secondary pollution of mercury chloride poisoning, has a short process, and is suitable for mercury removal of high concentration sulfur dioxide flue gas.

Sixth, sulfuric acid washing method to purify mercury-containing flue gas

The Outokumpu plant in Finland removes mercury from flue gas by sulfuric acid from the flue gas of the calcined zinc sulfide concentrate. This method is also known as Outokumpu mercury removal. When the concentrate is calcined at around 950 °C, all of the mercury in the ore is volatilized. After the dust is removed by the dust collector, a part of the mercury is removed, and about half of the mercury enters the washing tower, and the residual mercury enters the sulfuric acid product.

The flue gas is first removed by a high temperature electric precipitator and then washed with 85% to 93% concentrated sulfuric acid in a washing tower equipped with a packing. The washed acid is maintained at the desired temperature by the heat exchanger. As the acid reacts with the mercury vapor, the resulting precipitate settles in the tank. The precipitate was washed with water and filtered for distillation. The condensed metallic mercury is filtered to remove solid impurities with a purity of 99.99%. The recovery of mercury in the precipitate is 96% to 99%.

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