Mercury gas treatment and recycling

First, the pollution of mercury gas to the environment

(1) Properties of mercury and its compounds

Mercury is the only liquid metal, but also a highly toxic substance. Metallic mercury is almost insoluble in water. The solubility of mercury is only 20μg/L at 20°C, and the density of mercury is 13.55g/cm ³ . The divalent mercury ion has strong oxidizing property, and the mercury is highly volatile at normal temperature. Mercury vapor is present in a single molecule state, and the saturation concentration of mercury in air is relatively large. The mercury vapor partial pressure is 0.04 to 0.37 Pa at 5 to 30 ° C, and the saturation concentration is 3.52 to 29.5 mg/m ³ . The heat of vaporization is 271.7 J/g.

Mercury compounds are generally toxic, and inorganic mercury such as mercury cyanide, mercury nitrate, and mercury chloride are extremely toxic. Organic mercury is highly toxic, such as methylmercury, ethylmercury, methylmercury chloride, and alkylmercury.

(2) Pollution and harm of mercury gas to the environment

In the process of producing gold by amalgamation method, around mercury, washing, mercury, mercury, mercury, smelting, etc., due to the high volatility of mercury itself, the probability of mercury exposure to the air is high. Mercury vapor pollution exists in different environments around the site. Due to the physical and chemical properties of mercury, the degree of contamination of mercury vapor and mercury in water is different. It can be seen from Table 1 that mercury vapor in the air is highly diffusible and highly fat-soluble. When inhaled by humans, it can be completely absorbed by the alveoli and transport the whole body through the blood circulation. Therefore, long-term work in the mercury vapor environment will cause Mercury poisoning.

Table 1 Partial mine monitoring results

Second, mercury vapor treatment and recovery methods

(1) Iodine-based purification of mercury-containing flue gas

The iodine complex method for treating mercury-containing flue gas for desulfurization of zinc concentrate is to pass the flue gas of mercury-containing SO ₂ into the absorption tower filled with the porcelain ring through the bottom of the absorption tower, and spray the iodine containing the top of the tower. The absorption of salt absorbs mercury. The rich liquid that circulates and absorbs mercury is quantitatively partially extracted for electrolytic mercury removal to produce metallic mercury.

The iodine complex method is used to treat the flue gas containing mercury and SO ₂ , the mercury removal rate is 99.5%, the tail gas contains less than 0.05 mg/m ^{3 of} mercury, and the mercury content of the flue gas after mercury removal is less than 1×10 ^{- 6} . Recycling 1t of mercury consumes 200kg of iodized salt and consumes 56kW·h of electricity. This method is suitable for mercury removal from high concentration SO ₂ flue gas.

(2) Purification of mercury-containing flue gas by sulfuric acid washing

In the Outokumpu plant in Finland, when sulfuric acid is produced from the flue gas of the calcined zinc sulfide concentrate, the mercury in the flue gas is removed by a sulfuric acid washing method. The flue gas is first removed by high temperature electric dust removal, and then washed with 85% to 93% concentrated sulfuric acid in a washing tower equipped with a filler. Since the washed acid reacts with the mercury vapor, the formed precipitate settles in the tank, and the precipitate is washed by water and filtered to be distilled. The condensed metal mercury is filtered to remove solid impurities with a purity of 99.999%. The recovery of mercury in the precipitate is 96% to 99%.

(3) Chlorine-containing activated carbon purification method

Activated carbon is used to adsorb mercury-containing air, and chlorine and mercury act to form mercury chloride. The purification efficiency of this method can reach 99.9%.

(4) Manganese dioxide absorption method

Natural pyrolusite can strongly absorb mercury vapor, and can also absorb fine mercury beads (MnO ₂ + 2Hg = Hg ₂ MnO ₂ ) in the whole liquid.

When there is sulfuric acid, Hg ₂ MnO ₂ can form mercury sulfate (Hg ₂ MnO ₂ + 4H ₂ SO ₄ + MnO ₂ = 2HgSO ₄ + 2MnSO ₄ + 4H ₂ O).

The absorption efficiency of pyrolusite can reach 95% to 99%. In addition, the mercury-containing waste gas can be purified by a water rinsing method.

(5) Recovery of mercury gas by potassium permanganate absorption method

The exhaust gas containing mercury vapor is circulated and absorbed by the potassium permanganate solution in the inclined plate absorption tower, and the gas is exhausted after purification. Potassium permanganate is continuously added to the absorption liquid to maintain the potassium permanganate solution concentration. The oxidized mercury and mercury-manganese complexes produced after absorption can be sedimented and separated by flocculation precipitation. Its chemical principle:

2KMnO ₄ +3Hg+H ₂ O→2KOH+2MnO ₂ +3HgO

MnO ₂ +2Hg→Hg ₂ MnO ₂

The method is simple in equipment and high in purification efficiency. Suitable for high mercury vapor concentration applications.

(6) Treatment and recovery of mercury gas by blowing replacement method

For small and medium-sized mines, mercury can be recovered by the blow-off method to purify mercury vapor. The specific method is: in the steaming operation, the inlet pipe with a valve is installed on the first end of the steaming tank or the tank door, and a fan is connected by a hose. The mercury is vaporized and the fans and inlet valves are completely closed. At the end of the steaming of mercury, the exhaust pipe inserted into the water tank escapes without bubbles, and when no mercury beads are precipitated, the blower is turned on, and the inlet pipe valve is opened for 1 min, and when no air bubbles escape in the water tank, the wind is blown again. Three times in a row, the mercury vapor in the tank can be basically replaced. The whole steamed mercury and air blower replaced the mercury vapor in the tank, and the furnace temperature only increased and could not be reduced. In this way, the three processes of removing mercury from mercury, recovering mercury from steaming mercury, and purifying mercury vapor are simultaneously carried out. This method can get better indicators.

Third, the protection of mercury vapor

The workplace where ore is mixed with mercury and mercury is distilled, which is a place where poisoning is likely to occur. According to the general requirements, the limit concentration of mercury in the air in the production plant is <10μg/m ³ , in order to ensure personal health and achieve safe production. To this end, the production plant should be ventilated, and the extracted air can be discharged after being purified. It is necessary to ensure that the equipment seal of the steaming operation is perfect.

For the mercury plate gas collection method, firstly, the gas collection efficiency is high, so that the mercury-containing vapor gas does not leak, and when the worker performs the mercury plate operation, the mercury vapor-containing gas does not pass through the worker's breathing zone, and the worker is protected from mercury. harm. There are two ways of collecting gas: (1) for the entry and exit of the mercury plate. The gas collecting method is such that the clean air enters the gas collecting hood from both sides of the upper part, and enters the outer row of the exhaust pipe from the middle of the middle part of the hood to form a gas flow, and a vortex area is arranged in the middle and lower part of the hood, and the mercury-containing vapor is stagnate in the middle and lower parts of the hood. The operator's breathing belt is always in clean air. (2) For the mercury plate, only one side is in and out. The gas collecting method is such that the upper part of the clean air and the side of the collecting hood are discharged from the lower side through the exhaust pipe, and a vortex area is provided, and the operator does not inhale the air containing the mercury vapor. Both of the above gas collection methods require continuous operation of the fan.

Mercury extrusion chamber: The suction port of the mobile trap adsorption device can be directly sucked at the amalgam extrusion, and the air containing mercury vapor is directly discharged into the atmosphere after being treated by the adsorber.

During alchemy: The exhaust pipe of the mobile trap adsorption device can be inserted into the water to cool to prevent the escape of mercury vapor. After the end of the gold smelting, the suction port of the mobile trapping adsorption device can be directly sucked into the mercury vapor generating source to capture the mercury vapor generated during the gold smelting, and then discharged through the adsorber.