We are a mold factory with over 20 years of experience. Our products are stable, corrosion-resistant, and very durable, and can be precision processed according to customized requirements. Special Graphite,Graphite Block,Graphite Tube,Graphite Seal Rings Yixing Xingfeng Carbide Mould Co.,LTD , https://www.xfcmould.com
Recovery of gold from silver-zinc shell by distillation and zinc removal
1. Distillation and zinc removal method
Pot eluted from lead bismuth or silver-zinc refining output pot shell, by pressing machine to squeeze liquid lead (bismuth) fire evacuation distilled zinc, lead alloy outputs noble metal-rich (called lead-rich). Lead is removed by ash, and gold and silver alloys are produced for separation and purification. This process is a conventional method for processing silver-zinc shells, and the process is mature, and is widely used at home and abroad. The process flow is shown in FIG. The silver-zinc shell is mainly an alloy of lead, zinc and silver (gold), and is mixed with a small amount of metals such as copper , arsenic and antimony and their oxides with lead and zinc. Zinc has a boiling point of 907 ° C at 101.325 kPa, which is lower than other metals. The silver-zinc shell is heated to 1000-1100 ° C under a reducing atmosphere to reduce oxides such as zinc and lead into metals, and then the metal zinc is volatilized at a temperature higher than its boiling point to separate zinc from lead, silver and the like. The volatilized zinc vapor is introduced into the condenser to be condensed into metal zinc for recovery. Usually with a tiltable furnace steamed distilled zinc, which is a refractory brick furnace body iron frame support, formwork sides on a pivot, by a gear and screw drive. The horizontal section of the furnace body is square, the top of the furnace is arched, the rear wall is provided with a flue, and the front wall is provided with a mouth of the distillation tank to install and take out the distillation tank. After the cans are packed, they are sealed with bricks and yellow mud. In the lower part of the furnace, there is an arch bridge supporting the distillation tank. Available powder coal, coke or heavy oil is heated. The furnace for burning oil and burning coal is equipped with a nozzle, the bottom of the coke oven has a furnace, and the top of the furnace is filled with a coke hopper, as shown in Fig. 2. When distilling, the distillation tank port is equipped with a condenser and is connected to the ingot car.
Figure 1 Distillation and ash blowing process of silver-zinc shell
Figure 2 Schematic diagram of coke distillation tank furnace
1-distillation tank; 2-arch bridge support; 3-condenser; 4-support frame and mold table;
5-coke material; 6-flue; 7-furnace
The distillation can is shaped like a flat-bottomed vial, made of a mixture of scaly graphite and refractory mud. Graphite: refractory mud = 0.55: 0.45, tank wall thickness of about 45mm, a tank height of 0.99m, belly diameter of 0.443m can be distilled 500-700kg of silver-zinc shell each time, each tank can be used about 60 times. Before using the new tank, it should be placed on the top of the stove for about 7 days to remove the moisture. Then slowly heat up for 24 hours. When the tank is dark red, the silver-zinc shell is added. When the empty tank is heated, a small amount of charcoal or coke is often added to the tank to prevent the inner wall of the tank from being excessively oxidized and shortening the service life. In the furnace, the distillation tank is placed at a 45° inclination on the support seat to increase the heating area of ​​the tank in the furnace. The distillation process is intermittent operation. After distilling a few tanks of raw materials, the tank is properly driven in a fixed direction and changed in the bracket. The contact area on the seat is damaged by the fact that the tank is not subjected to heavy pressure at some point.
After the distillation tank is installed, the furnace mouth must be sealed tightly, and then the condenser and the nozzle joint are sealed after feeding. The condenser can be replaced by an old distiller, but a cast iron tank or a steel shell is usually lined with a refractory mud or a refractory brick tank as a condenser. The shape of the condenser is generally frustoconical or cylindrical, with a vent at the top and a zinc vent at the bottom. The specifications of the condenser depend on factors such as the size of the distillation tank, the type of fuel, and the method of operation. The condenser continuously generates zinc vapor and gas by the distillation tank to maintain the temperature inside the tank, and is generally 450 to 500 ° C under normal conditions. If the temperature in the condenser is lower than the melting point of zinc (419.47 ° C), most of the zinc vapor entering the condenser condenses into blue powder; if the temperature is higher than 500 ° C, the zinc vapor is not completely condensed, and part of it is taken away by the gas stream to cause zinc. loss.
During the operation, the distillation tank is filled with a silver-zinc shell and a reducing agent (2% to 3% of broken charcoal or 3% to 4% of crushed coke), and then rapidly heated to a furnace temperature of 1100 to 1200 ° C (in-tank temperature of 1000 °) 1100 ° C). After the silver-zinc shell is softened, the raw material is added, until the tank is filled with the liquid alloy, and then the condenser is installed to start steaming. At the beginning of zinc smelting, the zinc vapor is thin, the temperature in the condenser is low, and the zinc vapor is initially condensed into blue powder. After zinc bromide for 1 h, the liquid zinc metal began to aggregate. During the zinc smelting process, the liquid zinc is released in several portions, about two-thirds of each time, leaving one third to maintain the temperature in the condenser, which is beneficial to the contact of zinc vapor with liquid zinc, which is beneficial to the formation of liquid phase and reduction of blue powder. The generation. When steaming zinc, if the exhaust port emits white smoke and a light blue flame appears, it means that the zinc vapor is burning, indicating that the zinc is running smoothly. White smoke gradually becomes shallower and finally turns into yellow smoke, indicating that lead is oxidizing and zinc smelting has reached the end. Each steaming operation takes about 6 to 8 hours.
After the zinc hydride is terminated, all the liquid zinc is released. Remove the condenser and tilt the furnace to pour the lead in the distillation tank into the steel drum. Remove the scum, make lead ingots, and send ash to extract gold and silver alloy. The slag adhering to the wall of the distillation tank was scraped off with an iron and returned to re-distillation. The furnace is then reset and ready for the next distillation.
The metal zinc produced by distillation still contains a small amount of silver and lead, which can be returned for the addition of zinc and silver. The blue powder is returned to the distillation or the gold is replaced from the cyanide solution and other solutions.
Each ton of silver-zinc shell consumes about 500-700kg of coke or 180-270L of heavy oil, and the recovery rate of zinc is 65%-80%. The zinc recovery rate is lower when the zinc is removed by distillation, and part of the zinc is lost by oxidation. The obtained lead-rich silver content is low, and the lead loss during ash blowing is large. And the productivity of the distillation furnace and the ash blowing furnace is low.
Second, low temperature vacuum distillation zinc removal method
The French Noelles lead plant uses vacuum distillation to remove zinc to treat silver-zinc shells. The composition of the silver-zinc shell which removes excess lead by pressing is: Ag10%, Zn30%, Pb60%. The silver-zinc shell is added to the pot with large depth and small diameter, and is melted under the cover of salt layer to produce 25% of silver. Zinc alloy 65%, lead 10% ternary alloy rich collective (T, A, C). This ternary alloy rich group is not oxidized even in a liquid state, and is easy to store. The ternary alloy is then enriched under low vacuum and low temperature, and the zinc vapor is condensed into liquid zinc. Due to the distillation under low vacuum conditions, there is almost no oxidized dross on the lead liquid surface after zinc bromide. The distillation furnace used in the plant is shown in Figure 3. The furnace body is a horizontal cylindrical body, and the outer casing is welded with steel plates, lined with refractory bricks, and heated by graphite electrodes. There is a feeding port in front of the furnace body, a lead opening in the lower part, and a condenser in the rear part. The condenser casing is a steel plate shell lined with refractory material. The condenser was heated in the condenser with a graphite electrode to start the distillation, and the temperature of the liquid zinc in the condenser was recorded by a thermocouple and an automatic recording measuring device. The temperature of the distillation furnace depends on the condensation rate of the zinc vapor in the condenser. The temperature in the furnace is controlled by the controller to make the evaporation speed of zinc in the furnace not more than the condensation speed of zinc vapor. The condenser is connected to the vacuum pump through a filter. All equipment interfaces are sealed with fluid, with a negative pressure of 1.333 kPa. Vacuum cryogenic distillation is an intermittent operation. Each furnace is charged with 1000kg of ternary alloy rich group, and 300kg of lead released by the melting country is added to reduce the melting point of the alloy and save electric energy. If the ternary alloy rich group contains lead, it can be added without lead. The temperature of the distillation furnace is 750 to 800 ° C, and the temperature of the condenser is about 450 ° C. In addition to feeding and metallizing, all automation operations have been achieved. In the later stage of distillation, the condenser temperature is lowered due to the decrease of zinc vapor. At this time, the controller will automatically raise the voltage to raise the temperature of the distillation furnace to accelerate the evaporation of zinc. After the distillation is completed, the vacuum pump is stopped, the liquid zinc and lead-rich are discharged, and then another batch of charge is charged and distilled. The distillation furnace automatically adjusts the evaporation rate of zinc according to the condensation speed of the zinc vapor in the condenser, so the zinc separation is relatively complete, the recovery rate is high, and the productivity of the furnace is also high. The annual average index of the plant is shown in Table 1. The recovery rate of zinc is greater than 95%, and the recovery rate of silver is 99%. The lead produced is blown to the ash. The plant processes 2000kg of ternary alloy rich collectives, and the power consumption per ton of alloy is 800-850kW·h.
Figure 3 Schematic diagram of vacuum distillation furnace
1-furnace; 2-graphite electrode; 3-feed port;
4-lead lead; 5-condenser; 6-zinc port
Table 1 Indicators of low temperature vacuum distillation
Raw materials and products
Weight / kg
content/%
Ag
Pb
Zn
Feed
Ternary alloy rich collective
Lead elution
1000
300
25
-
8
-
65
-
Discharge
Rich lead
Zinc
Blue powder
650
630
20
38
0.15
0.15
57
1.8
3
1.75
98
94
The low temperature vacuum distillation body itself is a distiller. Distilled zinc under low temperature vacuum conditions, low power consumption, low cost, complete separation of zinc and lead, high recovery of lead and zinc, small amount of zinc and lead oxide slag returned to the treatment, high recovery rate of silver, and improved operating conditions. If the large amount of lead in the silver-zinc shell is removed by other methods before vacuum zinc removal, and the zinc content in the alloy is increased, it will facilitate the vacuum distillation operation. It can increase the productivity of the distillation furnace and shorten the distillation time.