Selection of Pipe Materials in Zinc Oxygen Pressure Leaching Process

In recent years, domestically developed and imported zinc -oxygen leaching processes have been industrialized. The main feature of this process is that zinc concentrate is heated and pressurized in the autoclave and oxygen is introduced into the dilute sulfuric acid medium. Zinc in zinc becomes soluble zinc sulfate, sulfur is oxidized to elemental sulfur, and other impurities are also leached accordingly. The slurry of the process has the characteristics of high temperature, strong corrosiveness and strong mechanical wear of the pulp. There are not many domestic discussions on the application of pipeline materials in this process. How to combine the process to select suitable pipeline materials for different working conditions is the primary task of the pipeline design.

1. Pulp parameters, corrosion characteristics and anti-corrosion materials of the main process pipe sections

(1) Pulp parameters of main process pipe sections

The slurry parameters of the main process sections are listed in Table 1.

Table 1 Main process pipe section slurry parameter table

Note: 1. According to different zinc concentrate components, the solution contains Cl ^- 400mg / L, F ^- 50 ~ 100mg / L; 2. In addition to the above components, it also contains more Zn ^{2 +} .

(2) Corrosion characteristics

It can be seen from Table 1 that the various types of pulp in the table belong to the dilute sulfuric acid system. Sulfuric acid corrosion is very different depending on the oxidizing and reducing metals. The main reactions of electrochemical corrosion are as follows:

Anode: M→M ^{2 +} +2e (1)

Cathode: 2H ⁺ + 2e → H ₂ (2)

4H ⁺ +4e+O ₂ →2H ₂ O (3)

M ^{n +} +e→M ^{(n -1 )+} (4)

According to the law of conservation of charge, the rate of anode reaction and cathodic reaction should be equal. If one of the pole reactions is restricted, the overall corrosion will slow down or stop. In general, sulfuric acid having a concentration greater than 70% is oxidizing, which promotes passivation of the metal surface, thereby slowing down the reaction (1). The low concentration of sulfuric acid is a non-oxidizing acid and does not have the above-mentioned accelerating effect. Therefore, anodic polarization is not the main controlling factor. Because the concentration of hydrogen ions in sulfuric acid is very high, the potential of the metal is lower than that of hydrogen, so the reaction (2) cathodic reaction Rate is the main control link. The lower the electrode potential of the metal is than the hydrogen potential, the more severe the corrosion. Since the corrosion phenomenon is an electrochemical reaction, of course, the influence of concentration and temperature, especially stainless steel, is particularly significant when changing from a passive state to an activated state. According to research, the corrosion rate of most stainless steels increases with the concentration of sulfuric acid. When the concentration of sulfuric acid is 60% to 70%, the corrosion of stainless steel is the most serious. The more concentrated sulfuric acid is oxidized, which accelerates the surface passivation of the stainless steel and causes corrosion to decrease.

In dilute sulfuric acid systems, the effect of temperature on the corrosion of metals is very obvious. As the temperature increases, the overpotential of hydrogen decreases. In general, for every 1 °C increase in temperature, the overpotential is reduced by 2 mV. Therefore, the temperature rises and dehydrogenation corrosion is intensified. In general, the relationship between metal corrosion rate and temperature can be expressed by the following formula:

C _t1 =C _t0 (t1-t0) ^xi

Where C _t0 and C _t1 are the corrosion rates (mm/a) at temperatures t0 and t1; Xi corrosion is the corrosion index, and when 70 ≤ t ≤ 85 ° C, Xi = 0.25. According to the study of classical dilute sulfuric acid stainless steel 904L, it has good corrosion performance under any concentration of dilute sulfuric acid at room temperature, but when the temperature is increased to 100 °C, the corrosion rate is greater than 5 mm/a when the concentration of sulfuric acid is 10%.

In addition to the above factors, impurities have an effect of accelerating or passivating the corrosion of dilute sulfuric acid. When the dilute sulfuric acid contains Cu ^{2 +} or Fe ^{2 +} plasma, the corrosion of the metal by the dilute sulfuric acid can be suppressed. Conversely, when a reducing ion such as F ^- or Cl ^{- is} contained, the overall corrosion of the metal is enhanced.

(3) Anti-corrosion materials

1, metal materials

(1) Titanium and titanium alloy

Titanium is not resistant to corrosion in sulfuric acid with a mass fraction of 5% to 98%. It can only be used in room temperature with a mass fraction of 5% dissolved sulfuric acid, but when heavy metal ions (such as Fe ^{3 +} , Ti ^{4 +} ) are present. Can significantly improve the corrosion resistance, corrosion rate of titanium in sulfuric acid: 10% concentration of sulfuric acid (chlorine saturation), at room temperature, annual corrosion thickness 0.0015mm, concentration of 10% sulfuric acid (chlorine saturation), at 190 At °C, the annual corrosion thickness is 0.05mm. Corrosion resistant titanium alloys have superior corrosion resistance than industrial pure titanium. If the alloy titanium containing A12.5% ​​can be used as the impeller and pump casing of the pump to transport the slurry containing certain sulfuric acid and fixed materials, the effect is better. In the heat exchangers with high requirements for wet smelting, the stirring slurry and the inner liner of the autoclave have applications of titanium alloy. However, the high price of titanium and alloys limits their use in these areas.

(2) Stainless steel

Most of the steel used in domestic wet smelting is 304, 316L steel. In general, ordinary type 304 stainless steel is not suitable for use in reducing dilute sulfuric acid. Common austenitic stainless steels usable in dilute sulfuric acid are at least type 316. The biggest difference between 316L and 304 is that the former has added molybdenum, an alloying element with strong passivation ability. At the same time, more nickel is added to maintain the austenitic structure. In 20% pure sulfuric acid, 316L stainless steel can only be used at room temperature. Classical dilute sulfuric acid is 904L stainless steel, which is widely used at home and abroad. 904L contains high chromium , nickel and molybdenum, and at the same time It is made of copper- assisted alloying and has good corrosion resistance. Pure sulfuric acid with a concentration of 0% to 98% at a temperature of less than 40 ° C can be used well. When the concentration of sulfuric acid is less than 5%, the use temperature can reach 100 °C. The corrosion of stainless steel is also affected by the impurities contained in the medium. In practical applications, it is also necessary to combine the specific working conditions. 316L and 904L stainless steel can use higher temperature than the above in the presence of heavy metal ions in the sulfuric acid system.

In addition to these three kinds of stainless steel, there are super austenitic stainless steel 926, 20Cb~3 of American Carpenter Company, Sanicro28 and other stainless steels of Sweden's Sandvik Company, and 2304, 2205, 2507 and other duplex steels developed by Sweden. Resistant to the corrosion of dilute sulfuric acid, but due to the high price of such products, the number of products made of pipes or fittings is not applicable to specific engineering practices on a large scale.

2, non-metallic materials

PP pipes (polypropylene), PVC pipes (polyvinyl chloride), glass pipes, and fluorine-lined composite pipes are often used for such materials. PP pipe, PVC pipe and glass steel pipe are very stable in chemical properties, corrosion resistant, and non-electrochemical corrosion. However, the temperature is not high, and it is not wear-resistant. It is not suitable for the transportation of pulp. It is often used for the transportation of acid and alkali at normal temperature. Although the maximum use temperature of the composite pipe can reach 150 ° C, it is also not wear-resistant, and it is also often used for high-temperature acid-base transportation.

Second, the choice of pipeline materials

The process of the zinc-oxygen pressure leaching process includes main processes such as oxygen pressure leaching, solution purification, and zinc electrowinning, and the pipeline includes a slurry pipe, a solution pipe, a steam pipe, an oxygen pipe, an exhaust pipe, and the like. The solution purification and zinc electrowinning process are similar to the conventional wet zinc smelting process. A selection of materials for the most representative process piping in the process is now presented.

(1) Autoclave slurry discharge pipe

The autoclave discharge pipe is the pipe section of the autoclave discharging to the flash tank. The working condition of the medium is the most complicated, the temperature and pressure are high, the temperature is 110-150 ° C, the pressure is between 450-1600 kPa, and the sulfuric acid is 1%. 7%, and ± 5% solids. Non-metallic materials can not adapt to such working conditions, and some manufacturers also choose titanium materials accordingly. According to the information provided by the foreign party and the corrosion characteristics of this medium, 904L material pipes and fittings were selected. Corrosion study of 904L material in dilute sulfuric acid, after the temperature is increased to 100 °C, the corrosion rate is more than 5mm/a when the concentration of sulfuric acid is 10%, indicating that 904L stainless steel is also difficult to handle such conditions, but according to the data Dexter Baye Mining Smelting Co., Ltd. uses 904L material to make the autoclave discharge tube with a service life of up to 5 years. It is analyzed that it is possible that the high-concentration heavy metal ions in the slurry increase the corrosion resistance of the 904L stainless steel, so it is required to minimize the amount of F ^- and Cl ^- entering the autoclave slurry to prolong the service life of the discharge pipe.

(2) Flash tank slurry discharge pipe

The high temperature and high pressure slurry is cooled to 100-120 ° C in the flash tank, and the pressure drop is 101-200 kPa. There is no change in the acidity, heavy metal ion content and solid content of the slurry. The temperature of the medium is high, and it is selected according to the corrosion characteristics. 904L stainless steel tube with special treatment on the outside.

(3) Thickener pulp outlet pipe

The slurry passes through the flash tank and the regulating tank to flow into the thickener, and the temperature and pressure are further reduced. The sulfuric acid at the outlet of one thickener and the second thickener is 1% and 7% respectively. According to the acid-containing situation, reference is made to similar production experience. According to the corrosion characteristics, the corrosion rate of 316L with temperature is calculated. The outlet pipe of a thickener is made of 316L stainless steel pipe, and the second section of thickener outlet pipe is still made of 904L stainless steel.

(4) Leaching supernatant tube

The leaching supernatant tube mainly refers to the thickener overflow pipe and the discharge pipe of each storage tank, the conveying pipe of each solution pump, etc., such a solution contains sulfuric acid 1% to 7%, the temperature has been reduced to below 85 ° C, according to corrosion Features and corrosion curves were selected from 316L tubes.

(5) Waste acid heating tube

The waste acid is the electrolytic waste liquid from the zinc electrowinning workshop. It is also necessary to add concentrated sulfuric acid in the waste acid storage tank. The waste acid contains 15%-20% sulfuric acid, 50g/L zinc, and the waste acid needs to be heated to 85°C. the corrosion control 316L and 904L, 904L is selected, 904L to strengthen the corrosion resistance, the spent acid storage tank in the ferrous sulfate solution was added.

Third, the conclusion

(1) The zinc-oxygen pressure leaching process is still a brand-new wet zinc smelting process in China. The choice of pipelines that can adapt to its high temperature, high corrosivity and high wearability pulp is still in the exploration stage. The pipeline also needs to be tested through production practices to sum up experience.

(2) The paper selects the media of several representative process sections in the zinc-oxygen pressure leaching process, and describes the selection of pipeline materials. Combined with foreign practical experience, 904L stainless steel is selected under the most extreme working conditions of the process. For the pipeline, the remaining sections are made of 316L stainless steel. This choice ensures that the zinc oxide pressure leaching process is smooth and smooth.

(3) Steel embryos used for the production of 904L pipelines are not yet capable of large-scale production in China. They need to be imported from abroad, the cost of raw materials is relatively high, and the delivery period is long, which affects the use of 904L stainless steel in such processes. Domestic steel mills have stepped up research and development of 904L steel or can find a non-metallic material that can replace 904L to accelerate the large-scale popularization and application of new zinc-oxygen leaching technology in China.

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