Chemical principle Analysis of ammonia Zinc oxide process [Zinc oxide Forum]

SMM: at the ninth Green Zinc Salt and Zinc oxide Industry Summit Forum 2021 held by SMM, Shen Hongzhi, chairman and engineer of Baiyin Yuanyuan Technology Co., Ltd., introduced the topic of "Chemical principle Analysis of ammonia Zinc oxide process" for the guests. He respectively introduced the chemical principle of zinc oxide production process and ammonia zinc oxide process. The comparison between ammonia method and acid method and the possible technical problems are explained in four aspects.

I. Overview of Zinc oxide production

The production of zinc oxide is mainly divided into two types of process routes: fire process and wet process.

The fire method (indirect method) is a zinc oxide product prepared by air oxidation using zinc ingots as raw materials:

2Zn + O2 = 2ZnO

The method has the advantages of simple process and high purity of the product. Using zinc ingot as raw material, the cost is high. The particles are thicker and the specific surface area is small.

The wet production process uses zinc-containing materials such as secondary zinc oxide as raw materials to obtain zinc oxide products through leaching-purification-zinc precipitation-roasting.

The common wet process is to use sulfuric acid as the leaching agent, and then purify the aqueous zinc sulfate solution with zinc powder, remove the impurities, and then precipitate zinc, which is the complex of ZnCO and Zn (OH)-- Zny (OH) x (CO3) ZMui x, y, z represent different coefficients respectively. The precipitate Filter is washed, dehydrated, then roasted, and finally zinc oxide is obtained.

The wet process has the advantages of high purity, fine and uniform particles, large specific surface area and high activity. However, the process is slightly complicated, and a large amount of sodium sulfate wastewater is produced in the zinc precipitation process, which is difficult to treat.

Chemical principle of ammonia zinc oxide process

Leaching by zinc-ammonia complexation: ZnO + 4NH3 ·H2O = Zn (NH3) 4 (OH) 2 + 3H2O

In addition to NH ammonium (aq), some ammonium salts are often added as auxiliary leaching agents-the most commonly used of which is ammonium bicarbonate ZnO + NH4HCO3 + 3NH3 ·H2O = Zn (NH3) 4CO3 + 4H2O

The leaching solution still needs to be purified to remove impurities to ensure the purity of zinc oxide.

The more feasible purification process is divided into two stages:

1. Na sulfur (or ammonium sulfide) is used to precipitate the elements that are more insoluble in sulfides than zinc in the form of sulfides.

2. Zinc powder is used to replace the elements that can not effectively form sulphide precipitation.

III. Comparison between ammonia method and acid method

Comparison between ammonia method and acid method

Leaching difference:

The main results are as follows: 1. The price of sulfuric acid is much lower than that of ammonia and ammonium salt, so the solute taken away with the leaching residue is more expensive by ammonia method than by acid method.

2. The acid method needs heating to promote the reaction process, while the ammonia method does not need heating (in order to reduce the volatilization of ammonia, it should be kept at a low temperature as far as possible)

3. During acid leaching, impurities such as iron, silicon and aluminum react and dissolve with sulfuric acid and should be hydrolyzed under neutral conditions.

4. Ammonia leaching is a complex reaction process with high selectivity, and impurities that can not be complexed with ammonia will not participate in the reaction, which reduces the complex process such as removing impurities by water.

Effect of lead on leaching

Lead is a troublesome impurity in the production of electrolytic zinc by ammonia process. It can be partially dissolved during leaching and needs to be removed with zinc powder during purification.

In the production of ammonia zinc oxide, lead is difficult to react with ammonia because it does not have the structure of forming coordination bond with nitrogen atom, so it will not be leached.

Purification of arsenic

Arsenic adsorbent is a kind of deep arsenic removal reagent, which is only suitable for arsenic removal at low concentration. When the dosage is between 1~2g/L and stirring for about 90 minutes, the arsenic removal efficiency of Filter is between 65% and 90%.

Washing

In acid production, the precipitated zinc carbonate is washed many times to remove the attached salts and water-soluble impurity ions (such as sodium sulfate).

In ammonia production, the liquid after zinc precipitation is only water in theory, and there are few impurities, so the washing step is simple (even can be omitted without washing).

The cycle of water

The biggest problem in the zinc oxide industry is the discharge of waste water.

In acid production, after zinc is precipitated with sodium carbonate, the original zinc sulfate solution becomes sodium sulfate solution. This kind of wastewater has the advantages of high salinity, low value, high discharge pollution and high recovery cost. Taking into account all the washing wastewater, each ton of zinc oxide needs to be treated with waste water 7~10m3.

The zinc precipitation process produced by ammonia process is completed by ammonia decomposition. In theory, the liquid after zinc precipitation is only water, which can be cooled and used to absorb evaporated ammonia to form dilute NH (aq), and return to the leaching process. As a result, the closed-circuit cycle can be realized without discharge at all.

To sum up, compared with the acid method, the advantages of the ammonia method are as follows.

IV. Possible technical problems in ammonia process.

Although the ammonia process has many advantages, there are still some difficulties in the mature process.

1. Ammonia volatilization and workshop environmental problems

In ammonia production, the dissolution of zinc depends on the participation of ammonia. Therefore, increasing the concentration of ammonia can increase the leaching rate and the content of zinc in the leaching solution. Therefore, it is necessary to maintain a certain concentration of free ammonia in the solution system-the control of PH value is generally about 9.

But at the same time, ammonia is a kind of gas which is soluble in water and has a special odor, and its aqueous solution is highly volatile. When the concentration of ammonia is as high as a certain degree, the volatilization of ammonia will be more intense, emitting the odor of ammonia.

The stirring tank and the transfer tank can be set up as a semi-closed system, and then the ammonia can be collected and absorbed through the ventilation pipe. However, in the operation of Filter, how to avoid the unorganized spillover of ammonia is a difficult problem.

2. The efficiency of ammonia evaporation during zinc precipitation operation.

In ammonia production, the zinc precipitation process is completed by ammonia evaporation operation.

Ammonia is easy to volatilize, but it is not easy to volatilize completely-when the concentration of ammonia in the solution is low to a certain range, the rate of ammonia volatilization decreases. At this time, zinc has not been completely precipitated, how to improve the efficiency of this operation process is also a problem to be considered in production.

3. Enrichment of impurities

In theory, ammonia production can achieve closed cycle, but the actual use of zinc oxide raw materials (secondary zinc oxide or other zinc-containing materials) often contain a variety of impurities will lead to problems.

Among the ingredients listed in the above table, chlorine (Cl), sodium (Na), potassium (K) and sulfur (S) are all troublesome impurities. The trouble lies in:

1. There is no open circuit for these impurities in the above process, so it will remain in the solution all the time.

2. When the solution system is closed-circuit, the concentration of these impurities increases continuously, which will eventually cause systematic failure.

The existence of sulfur will change the main composition of the solution, resulting in crystallization and other failures; potassium (K), sodium (Na) at low concentration, are harmless impurities, will not have a negative impact on the production. However, in the closed cycle, its concentration continues to increase, which will eventually lead to the following consequences:

1. Attach to the precipitated surface of zinc carbonate in the form of salt, increase washing operation, increase water consumption and waste water discharge.

2. Affect the dissolution of zinc and reduce the leaching rate during leaching.

The presence of chlorine has no effect on the acid process, but the ammonia process is characterized by a closed cycle, reducing wastewater discharge-or even zero discharge. Therefore, chlorine does serious harm to the ammonia process. The details are as follows:

1. Adhering to the precipitated surface of zinc carbonate in the form of salt, increasing washing operation, increasing water consumption and waste water discharge;

2. It is easy to produce crystal-Zn (NH3) 2Cl2, which causes pipeline blockage and other faults.

3. Ammonia evaporation operation failure, ammonia volatilization is not complete, zinc is difficult to precipitate.

The use of auxiliaries

In order to ensure the activity of zinc oxide products, we need to pay attention to the dispersion and specific surface area of zinc oxide; larger specific surface area requires smaller and uniform particles, and try our best to avoid agglomeration.

The key to avoid agglomeration is to increase the dispersibility through the appropriate use of auxiliaries.

Common auxiliaries include surfactant dispersants, fatty acid salts modifiers and so on.