Progress in metallurgical technology of low grade zinc oxide ore

Southwest of China is extremely rich mineral resources of zinc oxide, zinc oxide, high-grade ore grade is generally 25% to 35%, grade of less than 25% belong to low grade zinc oxide ores. Zinc oxide is a secondary mineral of zinc. Zinc oxide ore mainly to smithsonite (ZnCO _3), willemite (Zn-SiO _4), etc. are existed ore contains large amounts of metal impurities, such as lead, iron, cadmium, copper and the like. The oxidized minerals of zinc are complex and difficult to select, and the selection of flotation agents and the modification of mineral surfaces are difficult.

With the increasing demand for metals in the world and the shortage of mineral resources, the extraction of valuable metals from low-grade ores has become a research hotspot in recent years. For low-grade zinc oxide ore, the use of conventional acid leaching process is not satisfactory, mainly because of high acid consumption, high impurities, low zinc concentration, and difficult to handle leachate, so low-grade zinc oxide ore has not been very good. use.

Currently, there are zinc oxide ore treatment in two ways: The first is a zinc oxide ore beneficiation procedures to obtain enriched into the smelting of metallic zinc; second person is zinc oxide ore direct smelting process. The direct smelting method is divided into two types: fire method and wet method.

I. Pyrometallurgy

(1) Metal bath smelting reduction method

The metal bath smelting reduction method is a new metallurgical technology that has flourished in the past 10 years. The carbon-containing pellet iron bath smelting reduction method is one of the main research processes in non-blast furnace iron making. The metal bath smelting reduction method, in addition to the characteristics of rapid reduction of oxides (or carbon oxide-containing pellets) and good energy utilization, the metal bath itself has a large amount of sensible heat and its good heat transfer capability, which can be quickly replenished. One of the characteristics is that the heat consumed by the reduction of the oxide promotes the reduction reaction of the oxide. In the late 1980s, research on the use of iron or iron bath to reduce volatile zinc oxide was carried out. Japan's Ito Cong, Australia's Rankin and Canada's Donald have done research in this area, which provide a basis and reference for the treatment of zinc oxide ore by metal bath smelting reduction. Since zinc oxide ore is accompanied by a certain amount of lead, when it is treated with an iron bath or a temperature above the melting point of iron, all of the lead is substantially volatilized, so that the quality of the collected product is greatly reduced, so that zinc in the zinc oxide ore is obtained. Lead separation is the key to obtaining high-grade zinc oxide powder. Iron bath smelting reduction process using the high lead content of the zinc-lead dust treatment confirmed that the ZnO content was collected only 70% did not achieve the effect of treatment, the use of aluminum scrap iron bath to form an aluminum bath Alternatively, to achieve zinc oxide By smelting reduction, the ZnO content in the collected material reached 92.523%, and the zinc-lead separation and dust treatment were initially realized. The characteristics of the aluminum bath smelting reduction method: the melting temperature of aluminum is very low (660 ° C), its heat transfer capacity is very strong, and the reduction temperature can be selected at a lower temperature, which not only achieves the rapid reduction of zinc oxide, but also the purpose of non-volatile Pb. .

1. Iron bath smelting reduction method (hereinafter referred to as "iron bath method")

The principle of iron bath treatment of zinc oxide ore is similar to the mechanism of iron oxide iron bath reduction. In the iron bath method, the reduction method of zinc oxide is more than other fire process, and the reduction of zinc oxide is a heat-consuming process. The temperature of the iron bath method is high, the heat transfer mode is mainly conduction heat transfer, the heat transfer speed is fast, and the heat transfer is fully utilized. The sensible heat of molten iron, so the zinc oxide reduction reaction in the iron bath method is very fast. The following reactions occur in zinc oxide in the iron bath process:

(ZnO)+Cs=Zn(g)+CO (1)

(ZnO) ten CO=Zn(g)+CO ₂ (2)

C+CO ₂ = 2CO (3)

(ZnO)+[C]=Zn(g)+CO (4)

C=[C] (5)

(ZnO)+Fe=Zn(g)+(FeO) (6)

Wherein Zn- represents zinc oxide in the slag;

Cs- represents solid carbon;

[C]- represents saturated carbon in an iron bath.

2, aluminum bath smelting reduction method (referred to as "aluminum bath method")

In the aluminum bath method, zinc oxide is mainly reduced by means of carbon-containing pellets. The results of the study on the reduction of pure zinc oxide pellets and carbon-containing zinc oxide pellets in the aluminum bath show that the aluminum bath does not participate in the reduction of zinc oxide in the temperature range of 1000-1250 °C, and contains carbon pellets in the presence of aluminum bath. The reduction rate is faster than the absence of an aluminum bath. Due to the strong heat absorption during the reduction of zinc oxide, increasing the temperature is conducive to reduction, but increasing the temperature will also increase the vapor pressure of lead, increase the volatilization of the ship, and lead and its oxides enter the collection. The aluminum bath method is aimed at this. The characteristics of using aluminum with low melting point and strong heat transfer capacity are that zinc oxide can obtain sufficient heat at low temperature, thereby achieving both rapid reduction and reduction of lead volatilization.

(2) Electric furnace method

The electric furnace method uses an electric furnace to smelt and reduces zinc oxide in an electric furnace. The electric furnace method requires that the zinc grade of zinc oxide ore is very high, the lead content and other impurities are very low, the charge into the charge is metal powder, the metallurgical coke is required for reduction, and the raw material conditions of the electric furnace treatment method are strict. At present, most of the zinc oxide ore in China is of low grade. Difficult to choose oxidized ore, this raw material condition is difficult to achieve in many manufacturers.

(3) Weishi furnace method and rotary kiln method

The Wechsler furnace method and the rotary kiln method mainly deal with low-grade refractory minerals, and the raw material conditions are not limited, but due to economic benefits, the Zn content in the raw materials is also required to be greater than 15%. The furnace materials of the Wechsler furnace method and the rotary kiln method basically maintain the original shape during the reduction process, and cannot be softened or melted; the smelting temperature is low, reflecting mostly gas-solid phase reaction, and the smelting cycle is long; the heat of smelting is from coal. Combustion, heat transfer mode is mainly convective heat transfer, low capacity utilization. The electric furnace treatment method has high smelting temperature, the raw material is powder and melts rapidly; it reflects gas-solid phase reaction and liquid-gas phase reaction; the heat of smelting comes from electric energy conversion, and the heat transfer mode is mainly radiation heat transfer, so the reaction is required to be closed. In vivo, high capacity utilization.

Second, hydrometallurgy

(1) Alkaline leaching

Alkali leaching uses alkali to leach zinc oxide ore, equipment is not easy to corrode, solid-liquid separation is convenient, and leachate is easy to purify. It is one of the most promising treatment methods for zinc oxide ore. This work carried out an exploratory experiment on alkaline leaching of Lanping zinc oxide ore in Yunnan. The leaching agent used was sodium hydroxide and ammonia-ammonium carbonate. The aim was to find an effective method for direct zinc recovery from zinc oxide ore. .

Alkaline leaching is one of the effective methods for treating zinc oxide ore. It has the advantages of high leaching rate and environmental friendliness. Alkaline leaching tests on Lanping zinc oxide ore in Yunnan showed that both sodium hydroxide and ammonia-ammonium carbonate solutions are effective leaching agents for treating zinc oxide ore, and the zinc leaching rate can reach over 90%. This method can be used to treat similar ores.

(2) Sulfuric acid leaching

The waste electrolyte is used to leach the zinc oxide ore. The silicon in the ore is dissolved in a large amount to form colloidal silicon, which affects the liquid-solid separation of the slurry. The leaching of other impurities such as iron, calcium, magnesium and aluminum also increases the difficulty of purification. Therefore, a lot of research has been conducted at home and abroad on the wet treatment of zinc oxide ore, especially low-grade zinc oxide ore. In order to prevent the damage of silicic acid, zinc oxide ore should be avoided to produce colloidal SiO ₂ or to control the polymerization of silicic acid in the leachate, so that silicic acid can be removed before gelation to improve the solid separation performance of the slurry. At present, the zinc oxide ore leaching process commonly used in production has the Vieille-Montagne process, the neutralization coacervation method and the Radina method, all of which use different methods to gel the SiO ₂ in the slurry before gelation. Removed in different forms.

Leaching low-grade zinc oxide ore with sulfuric acid not only achieves the purpose of leaching metal zinc, but also reduces impurities and acid consumption, laying a foundation for the development and utilization of low-grade zinc oxide ore.

(3) Bacterial leaching

Some bacteria are used in leaching operations due to redox. For example, Thiobacillus ferrooxidans (Tf) is the most commonly used autotrophic bacteria in bacteria, and has been widely used in leaching of sulfide ore. on. Duncan et al. used Tf to leach zinc sulfide ore. Under certain conditions, the infiltration rate of zinc can reach 150mg/(L·h). The use of bacteria for the leaching of zinc oxide ore can not only promote a small amount of sulfide ore in a short time. Internal leaching, increasing the leaching rate of zinc, is also conducive to the precipitation of iron in the leachate, reducing the iron content in the filtrate. Tf can be directly or indirectly leached from zinc sulfide by the following formula:

ZnS+2O ₂ =ZnSO ₄ (7)

ZnS+Fe ₂ (S0 ₁ ) ₃ =ZnSO ₄ +2FeSO ₄ +S ⁰ (8)

Fe2+=Fe3+ (9)

(4) Percolation-extraction leaching

Percolation leaching is widely used in heap leaching of low-grade sulphide ore. Due to the reduction of the grinding process and the reduction of the cost of the leaching process, the production has the advantages of simple operation and easy management. Oxidized ore is difficult to achieve large-scale heap leaching due to poor permeability, but thin layer leaching can also be carried out.

(5) Low concentration ammonia leaching - direct electrowinning

In view of the development and utilization of these medium-low grade zinc oxide ore with 5% to 15% zinc in these highly alkaline gangues, the relevant units have carried out many years of research and proposed several treatment plans, all of which are due to economic or technical reasons. It failed to achieve industrial applications. In recent years, domestic experts and scholars have used the NaOH leaching-leaching solution used to extract zinc from steel-making zinc ash in France and Belgium in the 1980s. Low grade zinc oxide in alkaline gangue. Although the process has many advantages such as low production cost, NaOH regeneration and recycling, etc., since the lead in the solution cannot be effectively removed (lead solution contains 1 to 2 g/L of lead), it has no industrial application prospect.

Considering the solution removal, inhibiting the leaching of lead and increasing the leaching speed of zinc, Central South University used a stirred leaching-purification-electrowinning process in ammonia-ammonium chloride medium to study the treatment of zinc oxide ore. Due to the presence of Cl ^- , the anode reaction in the electrowinning process is mainly based on nitrogen evolution. Not only is the ammonia consumption large, but also the anti-corrosion requirements for the equipment material are high, in order to ensure the quality of the produced electrolytic zinc and maintain the high zinc concentration in the solution. When leaching, high free ammonia concentration has to be used, resulting in serious evaporation of ammonium in electrowinning zinc and poor operating environment, so the production cost of zinc is high.

For the treatment of low-grade zinc oxide ore with high silicate, low Ca and Mg content, domestic scholars have proposed a process of enhanced sulfuric acid leaching-P204 extraction-zinc stripping liquid electrowinning, and completed a small experimental study of the system. , achieved good test results. On the basis of the low-grade copper oxide heap leaching process, the relevant units carried out a heap leaching test study on zinc oxide ore, and relevant work is underway.

Third, the conclusion

In order to develop low-grade zinc oxide ore, many studies have been conducted at home and abroad. However, the metallurgical characteristics of high energy consumption and high pollution in pyrometallurgy are difficult to change at one time. The high consumption of hydrometallurgy and the low leaching rate also need to be overcome. These difficulties are the focus of low-grade zinc oxide ore metallurgy in the future. Only by solving these problems can we better develop low-grade zinc oxide ore.

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