Removal of lead and layered melt enrichment and electrolysis of carbanite by silver-zinc shell

1. Carnallite precipitation and lead removal method

The carnallite has a molecular formula of MgCl ₂ ·KCl·6H ₂ O, usually containing about 25% of water and having a melting point of about 400 ° C. When the silver-zinc melt in case carnallite liquid layer, the metal is not oxidized, and silver-zinc carnallite metal shell unreactive, even carnallite melt is lead oxide of 50% to 60% Nor does it lose fluidity when saturated with zinc oxide. Therefore, carnallite is a good covering agent when the silver-zinc shell is melted and layered. The former Soviet Union used a 150t melting pot to conduct industrial tests for the removal of lead by silver-zinc shell carnallite. In operation, the silver-containing returning ingot 30-40t and 2~3t carnallite are first put into the pot, and after heating and melting, 50-100t silver-zinc shell is added at 500-550 °C, and then heated to accelerate melting. The molten alloy was stirred to make the temperature uniform, and then heated to 580 ° C. After sedimentation, the slag, the silver-zinc alloy and the lead were well layered. The composition (%) of the three products obtained is shown in Table 1. It can be seen from the data in the table that 92% to 95% of the lead in the silver-zinc shell can be precipitated during the precipitation of the carnallite, and the silver-zinc alloy has a high silver content and a low lead content. Therefore, when the carbanite is melted and removed, the silver-zinc alloy produced by removing lead removes zinc, and the recovery rate of zinc is high. The yield of the distillation slag can be lowered, and the productivity of the distillation furnace and the ash blowing furnace can be improved.

Table 1 Composition of silver-zinc shell carnallite product (%)

Carnallite melting is not suitable for the treatment of oxide-depleted silver-rich zinc shells. Therefore, there is a large amount of zinc entering the slag layer, which affects the precious metal enrichment and increases the consumption of carnallite.

Second, the layered melting and enrichment method to enrich precious metals

The Cullilo lead plant in Bulgaria uses a layered melt-concentration method to produce silver-rich zinc shells containing 2% to 3% of high-lead silver-zinc shells (fertilizer shells). The layered segregation is carried out in a converter. Horizontal furnace has a cylindrical shell with a welded steel, magnesium brick lining, the processing capacity of 600kg. Use a small fan to supply air. Preheating to 750-800 °C before feeding, adding 500kg of fertilizer shell, 5-10kg of charcoal, covering the furnace cover, and heating the fuel at 850 °C for reduction and precipitation. The furnace body was rotated every 10 minutes during melting to stir the alloy. The heating is stopped after the alloy is completely melted. The furnace cover is removed, and the oxidized slag is taken out to allow the molten alloy to naturally cool and settle in the furnace. When the furnace temperature drops to 800 ° C, the silver-rich zinc shell begins to condense. When the temperature of the molten pool drops to 600 ° C, the silver-rich zinc shell is taken out. When the furnace temperature drops to 500-550 ° C, the upper zinc shell is taken out and returned to the next remelting. The lower part of the molten pool is lead liquid, containing about 500g/t of silver, and the returning zinc is added to the silver pot to produce a silver-zinc shell. The silver-rich zinc shell is distilled in a fuel distillation furnace to obtain a lead rich in silver content of about 42%, which is sent to the ash blowing furnace for ash blowing. The silver-rich zinc shell produced can be re-concentrated and concentrated if necessary, that is, it is placed in a vertical melting pot with a diameter of 0.3-0.5 m and a height of 1.3-1.5 m, and is heated to a low level under the cover of charcoal or other flux. Melting at 750 ° C can reduce the lead content of the alloy enriched in the upper layer to 5% to 8%, and the lower lead liquid is discharged through the siphon. The upper alloy was vacuum distilled at 1000 ° C and 266.64 Pa under negative pressure to remove 99% to 99.5% of zinc and 85% to 90% of lead. The recovery of silver was 98% to 99%. The composition of the silver, copper and lead alloys produced is shown in Table 2. It can be seen that the alloy has a high silver content and can be directly sent to the electrolytic purification without ash blowing.

Table 2 Composition of silver, copper and lead alloys (%)

The enrichment and enrichment operation time is about 3h, which is characterized by a reducing atmosphere in the furnace, which can prevent zinc oxidation. After segmentation and temperature stratification, it can produce products such as silver-rich zinc shell, which is rotated at high temperature. The method is stirred, and the lead is sedimented to the lower layer after layering.

Third, the melting-electrolysis method

Since the production of silver-zinc shells by the melt-electrolysis method in the San Gavino lead plant in Italy in the 1950s, the production cost of silver has been greatly reduced. The method produces a silver-zinc shell having a silver content of 3%, and produces a rich shell having a silver content of 6%. After the shell is crushed, zinc is added at 360-370 ° C to remove zinc (the amount of sodium hydroxide is 10% of the shell), and silver-rich lead and scum are produced. Lead-anode plates are cast together with silver-rich lead and silver-lead alloy, and lead is electrolyzed in a sulfamic acid electrolyte. The output of the anode mud composition is: Ag93%, Cu4%, Pb3%. The anode mud is desulfurized with sulfamic acid, and then nitrated in a graphite crucible to produce crude silver (containing 0.6% copper and 0.06% lead) for refining. Raking scum pulverized coal mixed with the first reduction in the converter, in addition to zinc oxide and hair, zinc oxide and silver output lead alloy.