Effects of Concentrate Impurities on Lead and Zinc Smelting

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1． Impurities in lead concentrate: copper: It exists as copper-containing sulfide in the concentrate. At the sintering and roasting temperature, the reaction is copper oxide, which is reduced to metallic copper during smelting and enters crude lead. If the crude lead contains high copper content (>2%), copper matte needs to be made and the copper recycled. Otherwise, it is difficult to separate lead and slag during smelting, and it is easy to block the siphon channel, causing processing difficulties, affecting the health of workers and causing large volatilization losses of lead. When the copper content in lead products is high, the lead will easily harden. Therefore, it is required that the copper content in the lead concentrate is <3%, and the copper content in the mixed concentrate is <1%. zinc: It exists in the state of zinc sulfide in lead concentrate and turns into ZnO when roasted. It does not undergo chemical changes during the smelting process, and most of it enters the slag, increasing the viscosity of the slag and narrowing the difference in specific gravity between the lead liquid and the slag, making it difficult to separate the two and affecting the recovery rate of lead. Part of the ZnO may condense on the furnace wall to form furnace knots, making operation difficult. When the zinc content in the raw materials is high, it will cause high-speed iron slag and increase the loss of lead in the slag. Zinc tends to harden lead metal and prevent it from being pressed into thin sheets, and promotes the corrosiveness of sulfuric acid to lead. Therefore, it is required that the zinc content of lead concentrate should not exceed 10%. arsenic: It exists in the form of arsenopyrite (FeAsS) and realgar (As2S3) in the concentrate. During smelting, it is partially reduced to As2O3 and volatilizes into the flue gas, forming extremely harmful atmospheric environmental pollution. Part of As enters crude lead and slag ； When the crude lead contains high As, alkaline refining method must be used to remove As. The Na3AsO4 contained in the produced scum is easily soluble in water and pollutes water sources, causing poisoning of humans and animals. Arsenic easily forms an alloy with lead to harden the lead, so it is required that the arsenic content in lead concentrate should not exceed 0.6%. Magnesium oxide (MgO): The melting point is 2800°C, which increases the melting point of the slag and tends to reduce the solubility of iron oxides in the slag, making the slag sticky. Generally, the MgO content reaches 3.5%, which results in frequent failures. Therefore, it is hoped that the lead concentrate contains no more than 2% MgO. Alumina (Al2O3): The melting point is 2050°C, which increases the melting point and viscosity of the slag, especially when combined with ZnO to form zinc spinel (ZnO·Al2O3). It is an infusible substance in the blast furnace, which significantly increases the melting point and viscosity of the slag, so the Al2O3 in the concentrate is required to be no more than 4%. 2. Impurities in zinc concentrate: copper: It often exists in the copper sulfide state in the concentrate. During roasting, different forms of cuprous oxide are mainly formed. The remaining copper sulfide easily forms matte, which lowers the melting point of the charge. During hydrometallurgical zinc smelting, Cu++ in the solution corrodes pipes and valves. During vertical tank distillation, a small amount often enters crude zinc, affecting the quality of commercial zinc. Therefore, zinc concentrate is required to contain no more than 2% Cu. lead: When the zinc concentrate contains high lead sulfide, fusible lead and sulfur are formed. The lead and sulfur first promote agglomeration and even melt the roasting material, preventing the removal of sulfur. Lead oxide easily forms a low melting point eutectic with many metal oxides and begins to melt at 800°C, causing the charge to agglomerate in the boiling furnace and flue. In lead hydrometallurgy, when the calcined sand is leached, it is converted into lead sulfate and sulfuric acid is consumed. In the fire method of lead smelting, the lead obtained by reducing lead oxides in the distillation tank is partially vaporized and condensed to become impurities in zinc ingots, which affects the quality of commercial zinc. The lead sulfate in the roasted ore is reduced to lead sulfide in the distillation tank, and can form matte with other metal sulfides, causing corrosion of the tank wall. Therefore, it is required that the lead content in zinc concentrate should not exceed 3%. iron: When iron exists as iron sphalerite in zinc concentrate, zinc ferrite is formed during roasting. In the process of hydrometallurgical zinc smelting, zinc ferrite is not dissolved when leached with dilute acid, which affects the leaching rate of zinc and increases the disposal cost of leaching residue. The free FeS in the concentrate is converted into Fe2O3 during roasting, and enters the solution as FeSO4 during sulfuric acid leaching. During oxidation and neutralization, flocculent Fe(OH)3 is generated, which affects the clarification speed of the thickener. During the fire-method vertical pot distillation, Fe2O3 in the roasted ore is reduced to FeO and metallic iron. The metallic iron forms iron deposits in the vertical pot, which affects the temperature rise of the vertical pot and causes insufficient evaporation of zinc, resulting in a high zinc content in the slag. ； When SiO2 exists in the ore, it is easy to form silicate with FeO to corrode the tank wall. ； When crude zinc enters the distillation tower, the iron content of the crude zinc directly affects the life of the tower. Therefore, it is hoped that the iron content of zinc concentrate is generally no more than 16%, and that of hydrometallurgical zinc is no more than 10%. arsenic: The concentrate contains arsenic. During boiling and roasting, arsenic enters the flue gas, causing V2O5 poisoning when it comes into contact with coal during sulfuric acid production. Most of the arsenic in the roasted ore is removed during leaching, but if the solution contains high As, a large amount of FeSO4 will be consumed (the amount of iron is 20 times the amount of arsenic). If there is more iron, there will be more slag, and more zinc will be taken away. As can discharge and precipitate on the cathode, causing plate burning (cathode reverse melting). Therefore, it is required that As in the concentrate mixture should not exceed 0.5%. Silica: Concentrates often contain free SiO2 and various combined silicates, which form zinc silicate with zinc oxide at high temperatures. During wet leaching, silicic acid enters the solution in a colloid form, concentrating the product and making the filtration process extremely difficult. Under the high temperature conditions of the distillation process, SiO2 forms silicates with CaO, FeO, etc., which corrodes the tank wall and hinders distillation. It is required that SiO2 in the concentrate should not exceed 7%. fluorine: The fluorine in the boiling roasting flue gas can easily corrode the tiles in the acid making system and damage the equipment. When the electrolyte contains high fluorine, the cathode zinc is not easy to peel off. It is required that F in zinc concentrate should not be greater than 0.2%.