Recovery of gold from silver-zinc shell by lead-rich ash blowing

Silver zinc can by liquation, distillation rich primary output lead and lead silver, followed by a small amount of metal impurities and other zinc and gold as gold. The ash blowing method is commonly used to enrich the gold and silver in lead, and the reverberatory furnace that ash-rich lead is often referred to as ash blowing furnace.

Since the affinity of lead and oxygen greatly exceeds the affinity of silver and other metal impurities with oxygen, when lead is dissolved, lead is rapidly oxidized to lead oxide when a large amount of air is blown along the lead level. The ash blowing operation temperature is slightly higher than the melting point of lead (888 ° C), and the generated lead oxide has a small density and good fluidity, and the slag is continuously discharged from the slag mouth, and the precious metal is enriched in the molten pool. When ash is blown, the lead is mainly oxidized by air, but the decomposition of high-valent oxide of lead also plays a role. For example, PbO ₂ and Pb ₃ O ₄ decompose to form lead oxide and release active oxygen at a furnace temperature of up to 900 ° C, which accelerates the oxidation process of lead. When ash is blown, some arsenic and antimony volatilize as trioxide , and the other part is transferred into slag in the form of arsenite, tellurite or arsenate or citrate, and is discharged with lead oxide. About 25% of zinc is formed by zinc oxide volatilization, and 75% of zinc is oxidized and slag. When ash is blown, the affinity of copper and oxygen is smaller than that of lead, and its oxidation rate is very slow, and it is not oxidized into the slag until the late stage of ash blowing operation. Copper mainly reacts reversibly with lead oxide to form cuprous oxide into the slag:

PbO+2Cu≒Pb+Cu ₂ O

Cuprous oxide and lead oxide can form a low-melting eutectic (689 ° C) having a lead oxide content of 68%. Therefore, copper-rich lead is often ash blown at low temperatures, and the ash blowing rate is often faster than lead-free copper-free, which is related to the formation of cuprous oxide in the molten pool. Niobium can form a low-melting eutectic (262 ° C) with a silver content of 97.5%, and can also form a solid solution with a silver content of 5%. Therefore, bismuth and silver are copolymerized in the lead liquid during ash blowing, and are not oxidized into ruthenium trioxide into the slag until the end of the ash blowing. Therefore, lead-rich lead with high ash blowing content often requires a long working time. The affinity of strontium and silver is very large, and it is not easy to oxidize when ash is blown. In order to remove the cockroaches, the bismuth-free lead is added to the molten pool after the mashing to reduce the concentration of strontium, and then ash is blown. After the secondary addition of the lead ash, approximately one-third of the ruthenium is oxidized into the slag, and the remaining trace ruthenium remains in the silver.

During the ash blowing process, silver is first concentrated in the lead liquid. However, lead particles containing silver are often mixed into the slag, and lead oxide can dissolve a small amount of silver and silver oxide, which will reduce the recovery rate of silver. When ash is blown, gold is not oxidized and is enriched in silver. The ash slag contains trace amounts of gold and is mechanically mixed.

Ash blowers can be divided into French and British. The former is suitable for lead-rich production by ash blowing crystallization, but the crystallization method has been abandoned in most lead plants. In addition to some lead plants in France, modern ash-blowing furnaces are generally British-style ash-blowing furnaces, which are suitable for ash-blown, zinc-rich and silver-free lead-rich.

The structure of the British ash blowing furnace is shown in Figure 1. It is a small reverberatory furnace that burns heavy oil. The furnace wall, furnace top, base and flue are fixed. The hearth (ash ash tray) can be moved. The ash tray is rectangular. magnesium brick flat plate on the movable frame. The depth of the molten pool is 100-200mm, and the area of ​​the molten pool is determined by the weight of each batch of ash-rich lead. The side wall of the hearth and the slag opening are provided with a cooling water jacket (the small furnace is generally not provided). After loading the ash tray, all the ports are closed with mud, leaving only the oil nozzle holes on the side of the side wall, and the small holes on the side walls (only one hole for the small furnace). The air duct is supplied with high-pressure air. In addition to the oxidation of lead and the like, the slag produced by the oxidation is blown to the front end of the ash-blowing disc, so that the slag flows out from the slag opening on the water jacket. In order to reduce the damage of the slag mouth, several large slag ports are used alternately in the large furnace. The flue gas enters the dust collector through the flue and cooling system.

Figure 1 Schematic diagram of ash blowing furnace

1-furnace wall; 2-furnace top; 3-furnace (furnace blown disk); 4-air inlet; 5-underground chimney

After replacing the new ash tray, first use a small fire for 4-6 hours and then heat up until the furnace wall turns red. Then add the lead-rich ingot from the furnace mouth until the lead-rich liquid is filled with the ash-blow tray, and the scum is removed and the temperature is raised to 900 °C. Or higher, insert the air duct, for 1.47 ~ 1.96kPa of pressurized air oblique blowing of lead-rich liquid surface, lead is oxidized to form lead oxide floating in the liquid surface, was blown by the wind to the front end of the ash tray. When the molten pool level is covered by lead oxide by more than half, the small ditch of the slag blocked by the yellow mud is cut, and the lead oxide is successively discharged into the slag car in front of the furnace. With the oxidation of lead and the discharge of lead oxide, the molten pool level gradually decreases. The lead should be added to the slope of the ash tray in a timely manner so that it is slowly melted and replenished into the ash tray to maintain a proper level, and half of the surface of the bath is covered by lead oxide slag. Large furnaces do not have perfect dust collection equipment. The ash blowing temperature should be maintained at 1100 to 1200 ° C, and the small furnace should be maintained at 900 to 1000 ° C to accelerate the oxidation of lead. When the last batch of lead-rich is added, the feeding is stopped and the oxidation is continued until the molten pool is almost entirely gold-silver alloy. A small amount of saltpeter can be sprinkled to accelerate the oxidation of impurities such as copper, and finally evenly spread into a thin layer. Bone meal (or dry cement), the residual slag is absorbed and removed. After the completion of the slag, there is still a layer of lead oxide film covering the surface of the gold-silver alloy melt, which exhibits a color similar to that of the rainbow (due to strong oxidation). With the volatilization of lead oxide, the "rainbow" disappeared quickly and a "silver flash" appeared on the surface of the alloy. At this time, a layer of charcoal was added to cover the liquid surface, and it was allowed to stand for 0.5 h under a reducing atmosphere and a furnace temperature of about 1000 ° C to remove a large amount of oxygen absorbed by the silver liquid, and then cast in a preheated ingot mold. The gold-silver alloy ingot or the gold-silver alloy anode plate with a content of 96% to 98% is sent for electrolytic purification. The ash blowing operation time per ton depends on factors such as the volume of the hearth, the amount of silver rich in lead, and the speed of ash blowing. The production capacity of the hearth is related to the lead-rich composition and operation. Under normal conditions, 1m ² ash tray 24h can oxidize about 1t of lead. The loss rate of silver during the ash blowing process is about 0.5%, and the loss rate of silver can reach 1% when the ash is low-silver-rich. About 3% to 5% of the lead enters the flue gas, and the flue gas should be collected to reduce the loss of silver and lead and eliminate pollution.

When ash blowing low silver rich lead or sorghum rich lead, it is often carried out in two stages. The first section of ash is blown to a silver content of 50% to 70%, and then ingot is added to another small furnace for the second stage of ash blowing until the total amount of gold and silver is more than 99.5% of the alloy ingot or cast into gold and silver alloy. The anode plate is sent to electrolytic purification. The content of silver and antimony in the second stage ash slag is relatively high, and should be separated from the first stage ash slag to recover silver and strontium therefrom. Some plants use a two-stage ash blowing method for all lead-rich ones. The purpose is to reduce the loss of silver and volatilization. It is not necessary to control the deep slag ditch and damage the ash blown disk due to the continuous decrease of the molten pool level, so that some metals can be enriched in the later slag. In order to facilitate recycling.