Distillation of mercury paste and smelting of distillation slag

The distillation separation of gold and mercury in a hard mercury paste is based on the boiling point of mercury (357 ° C) which is much lower than the melting point of gold (1064 ° C) and the boiling point (2860 ° C). The mercury amalgam produced by amalgamation by mercury filtration still contains 60% or more of mercury. Cast iron was charged to the pot (cylinder) and placed in a heating furnace coke, coal gas or electric furnace slowly warmed. When the temperature exceeds the boiling point of mercury, the mercury is sublimated from the mercury paste. In order to completely separate the mercury, many plants control the temperature of the evaporated mercury to 400-450 ° C and heat it to 750-800 ° C for 30 min. The sublimated mercury is condensed by a steel pipe equipped with a condenser, and the spherical liquid droplets are dropped into a container containing water to be recovered. The time for steaming is about 5 to 6 hours or longer. Typically, the recovery of mercury from steaming operations is greater than 99%.

In small mines, use a distillation tank (Figure 1) to steam the mercury. The technical specifications of the distillation tank are listed in the table below. In large mines, distillation furnaces are often used. There are many styles of distillation furnaces, one of which is shown in Figure 2. The furnace distillation cylinder has a cylindrical shape with a diameter of 225 to 300 mm and a length of 900 to 1200 mm. The bottom of the cylinder is provided with a sealing door, and the mercury paste is placed in an iron box covered with a porous iron sheet and placed in a distillation cylinder. At the other end of the cylinder is a lead-out iron pipe with a cooling water jacket for condensing and recovering sublimated mercury. Figure 3 shows an electric furnace for distilling amalgam.

Figure 1 Mercury paste distillation tank

1-tank; 2-sealed cover; 3-extracted iron pipe;

4-cooling water jacket; 5-cold water basin

Figure 2 Mercury cream distillation furnace

1-distillation cylinder; 2-furnace; 3-sealed door;

4-export iron pipe; 5-cooling water jacket; 6-iron box; 7-tubular bearing

Table Steam tank specification

Figure 3 Mercury paste distillation furnace

1-thermocouple; 2-insulation housing; 3-heating element; 4-distillation tank; 5-box;

6-box door; 7-fill tank; 8-spray tube; 9-coil tube; 10-relief valve;

11-settling tank; 12, 13, 16-valve; 14-injector; 15-line; 17-ball valve

When using a distillation tank, since the volume of the tank is small and the mercury paste is placed directly in the tank, the following points should be noted when steaming mercury:

A front loading amalgam, prior to the tank walls coated with graphite powder or paste chalk, talc, iron oxide powder, the distillation residue in order to prevent the adhesive tank wall.

Second, the thickness of the mercury paste layer in the distillation tank is generally 40 to 50 mm. If the thickness is too thick, the distillation of mercury is incomplete, and the gold is easily splashed into the outlet tube when the mercury paste is boiling.

Third, the mercury paste must be pure, and should not be mixed into the packaging paper, otherwise the recovered mercury will be easily pulverized when it is used again. When heavy minerals and a large amount of sulfur are mixed in the mercury paste, it is easy to cause the loss of gold caused by perforation at the bottom of the tank.

4. Since the decomposition temperature of AuHg ₂ (310 ° C) is close to the boiling point of mercury, the steamed boiler should be slowly heated. If the furnace temperature rises sharply, AuHg ₂ will enter the sublimation stage of mercury when it is decomposed, which will cause the intense boiling of mercury and splash.

5. The mercury content of mercury is continuously reduced by distillation, and the vapor pressure is continuously decreasing. At the end of the steaming, the vapor pressure in the tank is gradually balanced with the atmospheric pressure outside the tank. At this time, residual mercury and mercury vapor cannot be discharged outside the tank, resulting in incomplete distillation of mercury, and the mercury content in the slag often reaches 2% to 18%. . In order to reduce the mercury damage in the next process, Pingdu Gold Mine will install the inlet pipe with valve on the other side of the distillation tank. When the mercury vapor is not discharged in the later stage of steaming, open the valve and use a 0.25kW blower to the tank. The inside was blown for 1 min, and the distillation was continued for another 10 to 15 minutes. After the mercury is not discharged, the air is blown off, so that the residual mercury content in the steamed slag can be greatly reduced.

6. At the end of the steaming stage, when the pressure inside and outside the tank is gradually balanced, if the furnace temperature drops, the tank is negatively pressurized. The liquid mercury inserted into the cold water tray leads to the iron pipe, which will inject the cold water into the inner surface of the tank and cause an explosion. Therefore, the temperature of the late stage of steaming can only rise and not fall, at least should remain unchanged. If the temperature must be lowered during the steaming process, or before the steaming operation is terminated, the cold water pan should be removed first, or the outlet pipe can be lifted off the water surface for a certain distance before starting to cool down. At this time, a small amount of mercury vapor is discharged from the tube, and mercury should be prevented.

The mercury recovered by distillation is filtered to remove particulate impurities, washed with 5% to 10% of dilute nitric acid, and returned to the amalgam. Or dissolve it with hydrochloric acid to remove the base metal.

Distillation residue produced by distillation, also known as sponge gold. The gold content is about 60% to 80% (some mine products even have as high as 80% to 90%), and contains silver and a small amount of mercury, copper and other metals. It is common to use a graphite crucible to melt a composite gold ingot in a diesel or coke oven. And some gold, silver less, and distillation residue containing silica impurities, such as iron, sodium and a small amount of sodium carbonate may be added, borax and other oxidizing smelting slag, a large number of impurities to be removed after casting Crude Gold ingots. In large mines, smelting distillation slag is also used in converters or electric furnaces. In addition, in some mines, the distillation slag containing more impurities is pretreated by acid dissolution, alkali leaching, etc. to remove a large amount of impurities, and then the ingot is smelted.

The distillation slag containing more than 70% to 80% of the total amount of gold and silver may be first cast into an alloy plate and then subjected to electrolytic purification.

In view of the fact that gold and mercury can be solid-solid, the sponges produced by the above methods using the above method still contain mercury in the range of 1% to 18%. Even if the sponge gold is ingot after smelting, the amount of residual mercury in the composite gold ingot is still 0.01% to 0.1%, and the individual is as high as 2%. The presence of residual mercury is bound to cause secondary mercury damage to the environment and the refining of gold and silver. To this end, the Wuyi Third Plant handles the use of vacuum distillation from sponge gold and gold from various gold-selecting plants. It can be used to adapt to different amounts of residual mercury. The evaporation rate of mercury is nearly 100%. Figure 4 is a connection diagram of the distillation equipment of the plant. The method has the advantages of simple equipment, complete mercury removal, no pollution, and easy operation.

Figure 4 Schematic diagram of vacuum steaming

1-0.25 ~ 0.5kW blower; 2- coke oven; 3-坩埚;

4-condensed steel pipe; 5-mercury bottle; 6-2X-4 vacuum pump; 7-purification bottle

The coke oven used in the plant is a φ600mm×700mm steel shell furnace. The mercury is added to the 60 ^# stone shovel. The sluice is embedded in the seal ring sawn from the waste shovel and docked with the bell mouth on one end of the condensing steel pipe. The interface and seal ring are sealed with water glass and refractory mud. Start the steaming, first pass the condensed water, start the vacuum pump, and then start the blower to heat up the steam. As the temperature rises, the mercury vapor is pumped out by the vacuum pump and condensed as liquid mercury is dropped into a 10L water glass bottle. The exhaust gas is pumped into a 10L glass bottle containing 2% to 5% KI and K ₂ MnO ₄ through a vacuum pump, and is removed into the air after purification and mercury removal. The steaming operation was carried out until the 坩埚 was dark red (about 600 to 700 ° C) and maintained for 0.5 h. The temperature was raised to 1200-1300 ° C, the blower was turned off for 10 min to stop the temperature rise, and then the blower was turned on again. This was repeated until the turbidity gas in the mercury-containing bottle was terminated. After casting, the gold and silver are separated and purified.

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