Manganese and silver resources are relatively rich in the manganese mining area of Yunnan Province, my country. If these resources are fully developed and utilized, it can benefit the country, local governments and enterprises. However, the symbiotic relationship between silver and manganese minerals in this type of ore is close, and it is difficult to obtain concentrates through mineral processing. The effect of direct cyanide leaching is not obvious, and the leaching rate of silver is only about 20%. Experimental research has found that after the concentrate is chlorinated and roasted in a rotary kiln, the leaching rate of silver can reach 88.54%, which greatly improves the economic benefits of the enterprise.

There are a series of problems in the use of rotary kilns, which affect the efficiency of rotary kilns, especially the lining of rotary kilns. It is not only the basis for ensuring the normal production of rotary kilns, but also a bulletproof vest to ensure the service life of rotary kilns. The main function of the lining of rotary kilns is to protect the kiln cylinder from damage by high-temperature gases and high-temperature materials. In actual production, since the gas temperature in the kiln is higher than the material temperature, the surface of the kiln lining will be subjected to periodic thermal shock every time the rotary kiln rotates one circle, and the temperature changes by about 150℃. Thermal stress is generated in the 10~20mm surface layer of the kiln lining. At the same time, it is subjected to radial and axial mechanical stresses generated by rotation, as well as erosion and wear of the roasting materials. Especially when local high temperature is formed in the kiln, the layered peeling of the hot surface in the high temperature zone causes the lining to fall off, resulting in kiln collapse and unplanned kiln shutdown for maintenance, which is a key factor affecting the annual operation rate of the rotary kiln. Cl- is generated during the chlorination roasting process, and chloric acid is generated in a high temperature and high humidity environment, which further aggravates the erosion and powdering of the rotary kiln lining and shortens the service life of the rotary kiln lining. Therefore, it is imperative to optimize the lining of the chlorination roasting rotary kiln.

1. Problems in the chlorination roasting process

The mixture sent from the concentrate batching process is sent to the rotary kiln through the hopper for chlorination roasting. The residence time of the mixture in the reducing atmosphere in the rotary kiln is 30~60min, and the roasting temperature is 780±50℃. The main reactions are as follows:

In the above production process, AgCl-containing gas is generated, which is easy to generate chloric acid in a high temperature and high humidity environment. The traditional rotary kiln adopts annular refractory brick lining, which has the main advantages of easy laying and disassembly and good torque resistance, but the annular seam is a straight seam, which has weak resistance to Cl⁻ gas and is easily corroded by chloride salts, causing local refractory damage, resulting in the collapse of the rotary kiln masonry, forcing the kiln to stop, and the operation rate is low. In addition, if simply setting anchors made of steel in the lining, the thermal expansion and contraction coefficients of steel and castable lining are inconsistent, which is easy to cause cracks. Chloric acid can easily penetrate into the interior of the kiln lining along the gaps in the lining, aggravating the powdering of the rotary kiln lining. The lining of the rotary kiln is in this working environment for a long time, which will cause the kiln lining to gradually become thinner, or even fall off completely, and the "red kiln" phenomenon will appear in some parts of the kiln body. Moreover, chloric acid will further corrode the cylinder of the rotary kiln at a high temperature of 600℃, reduce the strength of the rotary kiln cylinder, and may cause cylinder fracture accidents, causing greater losses. Therefore, the chlorination roasting rotary kiln lining is not only the basis for ensuring the normal production of the rotary kiln, but also a bulletproof vest to ensure the service life of the rotary kiln.

2. Selection of refractory materials for chlorination roasting rotary kiln

(I) Characteristics of refractory materials

The main factors that cause damage to the kiln lining during chlorination roasting process are mechanical stress, thermal stress and chemical erosion. The various stresses borne by the lining are different depending on the kiln type, operation, different positions of the kiln lining in the kiln, and the deformation state of the flame, kiln material and kiln shell during operation. In order to reduce the erosion of the kiln lining and shell and improve the operation rate, the refractory materials of the rotary kiln lining should have the following characteristics:

1. High temperature resistance. It can operate in a high temperature environment of about 800℃ for a long time.

2. High strength and excellent wear resistance. The refractory material needs to have a certain mechanical strength and be able to withstand the expansion stress at high temperature and the stress caused by the deformation of the rotary kiln shell.

3. Good chemical stability and thermal stability. It can prevent the corrosion of chemical substances in the flue gas and withstand alternating thermal stress.

4. Good thermal expansion stability. Although the thermal expansion coefficient of the rotary kiln shell is greater than that of the rotary kiln refractory material, the shell temperature is generally 150~300℃, while the temperature of the refractory material is generally around 800℃. This may cause the refractory material to have a larger thermal expansion than the rotary kiln shell, so it is easy to fall off.

5. Low porosity. High porosity will cause flue gas to easily penetrate into the refractory material, thereby corroding the refractory material.

(II) Physical and chemical properties of high-strength castables

According to the chlorination roasting process requirements of a certain project, the chlorination roasting rotary kiln uses acid-resistant high-strength castables as the basic material for the rotary kiln lining. Its physical performance indicators are shown in Table 1. As can be seen from Table 1, the high-strength castable has the following characteristics:

(1) The main chemical component is Al2O3, accounting for ≥78%, with high strength, high temperature resistance, and acid corrosion resistance.

(2) According to the relationship between bulk density and apparent porosity, if the bulk density is ≥2.85g/cm3, the apparent porosity is <20%, and the refractory temperature is >1550℃; low melting point materials can be prevented from adhering to the kiln lining.

(3) Good high temperature stability against heat load and hot wear

In summary, high-strength castables meet the requirements for lining, and the quality of their physical properties is directly related to the service life of the kiln lining.

Table 1 Optimization design of chlorination roasting rotary lining of high-strength castables

The above castables are cast on the inner surface of the rotary kiln cylinder with a thickness of not less than 250mm. The lining includes a plurality of acidic or neutral anchor bricks uniformly distributed axially and fixed to the inner surface of the cylinder. A plurality of uniformly distributed metal anchors are fixed on the inner surface of the cylinder on both sides of the anchor bricks. Then, acid-alkali-resistant high-temperature castables are used. The acid-resistant high-strength castable is connected to the cylinder as a whole through anchor bricks and metal anchors. The local lining structure is shown in Figure 1.

The structural design of Figure 1 is conducive to the metal anchors on the cylinder to drive the acid-alkali-resistant high-temperature cast body to roll and rotate with the cylinder. At the same time, since the thermal expansion coefficient of the anchor brick is similar to that of the acid-alkali-resistant high-temperature cast body lining, the thermal matching is good, which can avoid cracks caused by the inconsistency of the thermal expansion and contraction coefficients of the steel and the castable lining. In addition, when the rotary kiln is repeatedly started and stopped or in an abnormal operating state, the refractory material in the rotary kiln always maintains the best expansion balance, reduces the damage between the acid-alkali-resistant high-temperature cast body and the cylinder due to relative movement, and prevents the entire rotary kiln refractory material from deformation and displacement, ensuring the stability of the entire lining structure and improving the service life of the rotary kiln lining. Different from the conventional staggered masonry rules of refractory bricks, the anchor bricks are masonry with the same seam, each circle is self-contained, and does not overlap with the front and rear circles, so as to ensure that the refractory bricks are sheared due to asynchronous displacement when the rotary kiln is started and stopped.

Through the above technical measures, the cylinder is protected from Cl- corrosion, the service life of the rotary kiln lining is greatly improved, and the problem of refractory material shedding of the rotary kiln is effectively solved.

III. Conclusion

Combined with the process requirements of chlorination roasting, the influence of refractory materials on the life of rotary kiln is analyzed, and the physical and chemical performance indicators of more suitable refractory materials are given. By optimizing the design of the rotary kiln lining structure, its heat resistance and high temperature strength are improved, and it can resist the erosion of acid, alkali and salt, effectively increasing the service life of the rotary kiln refractory lining.