General operation and control of technical conditions for conventional leaching of wet zinc smelting

First, the conventional method of leaching

Conventional wet mixing zinc leaching process means the use of some neutral leaching, acid leaching period, leaching residue through a pyro metallurgical process reduction of zinc volatilized into zinc oxide and then subjected to a wet treatment, the principles of the process, see figure 1.

The wet process process is characterized in that a chute is arranged below the discharge opening of the roaster, and the hot material discharged from the furnace directly falls into the chute and is continuously passed through the dilute sulfuric acid solution of the chute (including 30-50 g). /LH ₂ SO ₄ , commonly known as oxidizing solution), is brought to the slurry classifier, graded and overflowed into neutral leaching, and the classified bottom liquid is subjected to acid leaching after ball milling. The main advantages are as follows: (1) the cooling equipment for hot baking is omitted; 2 the sensible heat leaching solution of the hot baking sand can be used to reduce the heating load of the leaching tank.

The newly built wet zinc smelting system of Zhuzhou Smelter Plant does not adopt the blasting process. The calcine is cooled and ball milled and sent to the leaching system. The excess calcine is sent to the calcine storage bin for storage. The advantages are as follows: (1) After dry ball milling, the fine particle size is fine, and a better leaching effect can be obtained. The zinc content of the leaching slag is 1.0% to 1.5% lower than that of the leaching leaching process; 2 due to the setting of the baking silo, The leaching portion is not affected by the shutdown of the roasting system.

Second, the technical conditions control of the leaching process

In order to ensure the quality of leaching pulp and increase the leaching rate of zinc, in general, there are three main aspects of technical control of leaching process: neutral leaching end point control, leaching process balance control and leaching technology condition control.

The pH of the neutral leaching control end point is 4.8-5.4, so that the ferric iron is hydrolyzed by Fe(OH) ₃ and aggregates and settles together with impurities such as arsenic , antimony and bismuth , so as to achieve the purpose of rapid sedimentation speed and high degree of solution purification. . In the past, the pH control of the leaching end point was determined by the operator using a test paper or a pH meter, and then adjusting the acid amount of the leaching process to achieve the purpose of controlling the pH value of the end point. As the level of automation increases, the pH control of the leaching process endpoint can be achieved by an automatic pH control system. After the pH value of each leaching tank outlet of the leaching process is set, the automatic control system can automatically adjust the amount of acid added according to the set pH value signal, so that the leaching end point reaches the set pH value.

The wet zinc smelting solution is a closed circuit, so the volume of the solution in the system, the amount of metal to be injected, and the volume of the concentrated sludge after clarification and concentration of the slurry are constant, that is, the liquid volume balance, the metal content balance, and the slag balance are generally referred to as leaching. The basic content of the process. The total volume of the wet zinc smelting solution, on the one hand, will continue to decrease with the process due to evaporation of water, slag running water and run, run, drip, and leakage losses, and on the other hand, due to cadmium- depleted liquid, washing slag, washing The low-acid and low-zinc wastewater collected by the filter cloth and washing equipment will bring a lot of new water to the system. The two must be balanced, that is, keep the volume of the solution in the system constant, otherwise there may be too much water brought in by the system. The amount of solution increases, causing the solution to fail to turn around, disrupting the production process, resulting in out of control of production technology conditions. If the water brought in is insufficient, the volume of the system solution is reduced, which also affects the normal solution turnover, which affects the control of normal production technical conditions. At the same time, the reduction of the solution volume is equivalent to the concentration of the system solution, which will lead to an increase in the zinc content of the solution. If it deviates from the allowable range, it will directly affect the leaching and subsequent purification and electrolysis processes. In practice, the temperature in the summer is high, the volume of the solution is easily reduced; the amount of evaporation in the winter is small, and the condensed water heated directly by the steam is increased, and the volume of the solution is easily expanded. Therefore, in order to maintain the volume balance of the solution, it is necessary to strictly control the amount of water washing, and maintain the water balance in time and in place. The metal balance in the leaching process means that the amount of metal that enters the solution after leaching of the calcine introduced in the leaching process is balanced with the amount of metal precipitated during the zinc electrolysis process. If the amount of metal input and the amount of precipitated zinc are not balanced, the amount of waste liquid produced by electrolysis will be unbalanced, which will affect normal production.

The slag balance refers to the balance between the amount of slag produced by the blasting of the calcine after two stages and the amount of slag discharged from the system through the filtration equipment . If the slag produced by leaching cannot be removed from the system in time, the volume of the thick mud in the concentration tank increases, which not only affects the quality of the supernatant, but also directly affects the production of the next process, and cannot continuously and stably carry out the leaching process. The change of the volume of the thick mud is often caused by the above-mentioned vicious cycle. For example, the volume of the acidic sludge is large, the clarification is difficult, and the solid content of the acidic supernatant is increased. When returning to a neutral leaching, the leaching of the slurry is further increased. The amount of solids, thereby reducing the liquid-solid ratio of the leaching slurry, makes it difficult to clarify the leaching slurry once. As a result, the suspended solids in the neutral supernatant are greatly increased, and the pressure-carrying burden of the cleaning process is aggravated, and even the cleaning operation cannot be completed.

The quality of the leaching process is closely related to the selected technical conditions. Practice has shown that only the correct selection of operating technical conditions, strict operation, and careful control can achieve good leaching results. The general leaching process conditions for conventional leaching are as follows:

(1) Technical conditions for neutral leaching

Leaching temperature 60 ~ 75 ° C

Leachate solid ratio (mass ratio of leachate to feed) 10 to 151

Leaching initial acid concentration 30~40g/L

Leaching end point pH 4.8 ~ 5.4

Leaching time 1.5~2.5h

(2) Technical conditions for acid leaching

Leaching temperature 70 ~ 80 ° C

Leachate solid ratio (mass ratio of leaching amount to material amount) 7 to 91

Leaching initial acid concentration 25~45g/L

Leaching end point pH 2.5~3.5

Leaching time 2~3h

Since the raw material and the acid are simultaneously added, the initial acid is controlled by the pH of the final acid from the outlet of the leaching tank. The product of the leaching process is a slurry, which is a suspension of zinc sulphate solution and insoluble residue. In order to meet the requirements of the next process, the slurry must be subjected to liquid-solid separation. The wet separation of the wet leaching zinc slurry is usually carried out by gravity sedimentation and filtration. After a neutral leaching slurry is solid-separated by neutral concentration liquid, the neutral supernatant is sent for purification, and the neutral concentrated bottom stream is sent for acid leaching.

After the second-stage acid leaching slurry is solid-separated by the acidic concentrated liquid, the acidic supernatant is returned to the neutral leaching, and the acidic underflow is filtered, dried, and sent to the rotary kiln for treatment, so that the zinc is reduced and volatilized into zinc oxide.

Third, the main technical and economic indicators of leaching

(1) Zinc leaching rate

The leaching rate is the ratio of the amount of zinc entering the solution to the total amount of zinc in the calcined ore after two stages of leaching of the calcined ore.

When the calcined ore contains 50% to 55% of zinc and the soluble zinc ratio is 90% to 92%, the zinc leaching rate is 80% to 87%. In design, continuous leaching may take 80% to 82%, and intermittent leaching may take 86% to 87%.

(2) Leaching slag rate

The leaching slag rate is the percentage of dry slag and calcined ore after leaching, filtration and drying of the calcined ore.

When the calcined ore contains 50% to 55% zinc, the corresponding leaching slag ratio is 50% to 55%. In recent years, the slag rate of each factory is generally about 52%.

(3) slag containing zinc

The zinc content of each slag fluctuates in the following ranges: 18% to 22% of total zinc; 2.5% to 7% of acid-soluble zinc; and 0.5% to 5.5% of water-soluble zinc.

4. Liquid-solid separation of leached pulp

The slurry obtained by leaching must be separated by liquid solids before being sent to the next process. The leaching slurry is concentrated once, the obtained supernatant is sent for purification, and the bottom stream is sent to the second leaching; the second leaching slurry is first concentrated, and the obtained supernatant is returned to the leaching once, and the bottom liquid is sent for filtration.

The concentration of the slurry is carried out in a concentration tank (also known as a thickener). The concentration efficiency of the leaching pulp depends on the sedimentation rate of the solid particles:

In the actual production process, the situation is more complicated, so there are many factors affecting the concentration, mainly:

(1) The pH of the pulp

The pH value of the pulp is generally controlled between 4.8 and 5.4, because this condition is most conducive to the aggregation and growth of fine colloidal iron hydroxide and silica gel particles, so the clarification speed is faster and the concentration effect is better.

(2) Grain size of roasting ore

The sedimentation rate of solid particles is proportional to its particle size. The larger the particle size, the faster the sedimentation, so the better the concentration effect. However, the particle size is too large, which easily blocks the concentration tank and damages the shaking device; conversely, the smaller the particle size, the slower the sedimentation and the worse the concentration efficiency.

(3) The difference in density between solid and liquid

The greater the difference in density between the solid and the liquid, the faster the sedimentation rate of the solid particles and the higher the concentration efficiency.

(4) Temperature of the pulp

Generally, the concentration of the neutral leaching pulp of the zinc roasting ore is preferably 55-60 ° C; the concentration of the acidic leaching pulp is preferably 60-70 ° C. As the temperature increases, the viscosity of the slurry decreases and the sedimentation rate of the solid particles increases.

(5) Liquid-solid ratio of pulp

The larger the liquid-solid ratio of the slurry, the smaller the viscosity of the slurry, and the more favorable the sedimentation of the solid, the better the concentration effect.

(6) Content of colloidal iron hydroxide and silica in solution

When the amount of iron hydroxide and silicic acid in the solution increases, the viscosity of the slurry rises, which makes the sedimentation of the solid material difficult and deteriorates the concentration process, so it should be strictly controlled. In this case, the slurry temperature can be increased and the liquid-solid ratio can be increased to lower the viscosity and facilitate the sedimentation of solid particles.

(7) Leaching time

When the leaching time is short, the residual solid particles are larger, and the sedimentation speed is faster; on the contrary, the solid particles are finer, and even the condensed large particles are crushed, which makes the concentration difficult.

(8) 3 ^# Addition amount of coagulant

3 ^# agglomerating agents are generally only added to the neutral concentration tank. It is used in an amount of 20 to 30 ppm. The addition of the coagulant can cause the tiny suspended particles to aggregate into larger particles, thereby accelerating the sedimentation speed, and the capacity of the concentrator is increased by 1.5 to 2 times.

The underflow obtained by concentrating the secondary leaching slurry is still a thick sludge containing a lot of zinc sulphate solution. In order to recover the zinc as much as possible, the underflow must be filtered to further perform liquid-solid separation. Filtration is to install the concentrated bottom stream in a filter with filter medium, and let the solution pass through the filter medium under a certain pressure difference, while the solid (leaching slag) is trapped on the filter medium to achieve the purpose of liquid-solid separation. .

The production capacity of the filter depends on the filtration speed, and the factors that affect the filtration speed are:

1. The nature of the filter residue

When the thick mud contains colloidal substances such as iron hydroxide, excessive silicic acid and excessive slag particles, the viscosity of the thick mud will be increased, and the fine colloidal particles will block the capillary channels of the filter medium during filtration; mud containing zinc, calcium sulfate, magnesium sulfate or the like is too large, the same concentration will increase the viscosity of the mud, and the generated fine crystals are easy to clog the filter pores of the filter media path, the filter which are difficult to reduce Filter speed.

2, the thickness of the filter cake

As the filtration time increases, the thickness of the filter cake increases as the filtration time increases, and the resistance of the solution in the filtered concentrated mud passes through the filter medium increases, thereby lowering the filtration rate. At the same time, it is also possible to damage the filter cloth because the filter cake is too thick. To this end, the filter cake thickness should be controlled. According to the practice of a factory, the reasonable thickness of the filter cake is 25~35mm.

3, filtration temperature

The temperature has a great influence on the filtration speed. It can be summarized as follows: high filtration temperature, good fluidity of the mud, and fast filtration speed; increasing the filtration temperature can eliminate small bubbles formed in the capillary channels of the filter cake and the filter cloth. In order to speed up the filtration rate; increase the filtration temperature, which is conducive to the aggregation and growth of suspended solid particles in the filtrate, so that the filtration speed is accelerated.

Therefore, higher filtration temperatures should be controlled in production practice, typically 70 to 80 ° C, and sometimes even up to 90 ° C.

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