Common production problems of alumina ceramic balls
1、Spots:
Because the body of high alumina ceramic ball is white, if there are spots, it will be particularly obvious and easy to find. According to the number and size of spots, it can be roughly divided into the following three situations.
1. There are few spots and the diameter is less than 1mm.The spot defect is mainly caused by the iron impurity in alumina powder a and the iron content in various auxiliary materials exceeding u.3%;
2. There are many spots and the diameter is less than 2mm.This may be caused by the brick falling from the ball mill, or the slurry not removing iron in the process of ball milling and spraying.
3. Spots appear on the top layer, and the spots are large. This is generally caused by rust or firebrick falling into the product surface by the pre tropical air duct of the kiln.
To sum up, in order to reduce or eliminate the spot defects, pure a alumina powder and other auxiliary materials should be used; in the process of slurry discharge, powder spraying and discharge, it is necessary to remove iron for many times, and timely clean the iron remover; it is necessary to regularly clean the air ducts of the kiln.
2、 Blistering
Blistering defects usually appear in high alumina ceramic balls made by roll forming process. Because the finished product balls grow up and densify through the collision between many semi-finished products during roll forming, it needs to be polished for 10-120min before the final rolling to adjust the sphericity and surface finish of the semi-finished products. Due to the high temperature of the ball surface during sintering, the dense surface layer is formed by sintering first, which hinders the evaporation and discharge of the internal moisture, thus leading to the blistering of the surface of the finished ceramic ball. Based on the above principles, in order to eliminate blistering defects:
1. In the process of rolling high alumina ceramic ball body, the density and finish of semi-finished products should be considered, and the polishing time before leaving the pot should be minimized.
2. Reduce the content of low temperature solvent and high temperature volatile matter in the formula.
3. Dry the moisture content of the semi-finished product before firing, so that it cannot exceed 0.3%.
3、 Viscidity loss
The sticking defects of ceramic balls are common. High alumina ceramic balls are fired at high temperature. In order to ensure the performance of the fired products, it is often necessary to conduct 2-10h heat preservation at high temperature. Once there are more liquid phases at high temperature, there will be adhesive damage between the ceramic ball and the ceramic ball, as well as between the ceramic ball and the silicon carbide boron plate.
How to deal with this problem:
1. Between the ceramic ball and the ceramic ball, between the ceramic ball and the silicon carbide boron plate, between the ceramic ball and the column, the isolation is carried out by spreading corundum sand, placing corundum base plate, etc;
2. At the same time, the loading height of each layer of ceramic ball is reduced;
3. The formula of solid phase sintering was designed to reduce the glass phase inside the ceramic ball, and the sintering was carried out at the lower limit of the firing temperature range, and the holding time was extended properly.The above measures can solve the sticking defects and improve the product qualification rate
4、 Aeolian crystal
The air crystal defect refers to the explosion of air crystal in the cooling zone of kiln.This is mainly because the product has reached densification under high temperature firing, the sintered high alumina ceramic ball has a small porosity, and when it is cooled to 570 ℃ and 790 ℃, the quartz phase transformation produces a large volume change, which leads to the product cracking.Therefore, in case of air crystal explosion, if it is roller kiln or tunnel kiln, it is necessary to timely reduce the speed of the kiln car, reduce the air intake of the cooling belt, adjust the height of the product loading, and avoid the hot products directly blown by the cold air; if it is shuttle kiln, it is only necessary to reduce the air intake of the cooling air.The results show that the blast fracture occurs more in the large-scale ceramic balls, but less in the semi isostatic pressure or isostatic pressure formed high alumina ceramic balls.This is because it is difficult to release the stress produced by quartz transformation.Therefore, the possibility of free SiO2 should be reduced as far as possible in the formulation design of roll forming method to prepare high alumina ceramic balls. It is better to consider to promote the formation of a certain amount of mullite or spinel in the formulation design to prevent the occurrence of specific wind crystal defects
5、 Lumbar fissure
Waist crack is a kind of defect which is difficult to be found and detected directly in the kiln products. It is a ring crack on the surface of high alumina ceramic ball after pre grinding.The crack is more about 5mm below the spherical crown, hence its name.When the defect is serious, there will be a 0.5mm wide crack around the ceramic ball.In the process of using the ceramic ball containing the defect, there will be serious quality accidents such as broken ball and cracked ball.According to our experience, this kind of crack has existed in semi-finished products, mainly in the fully automatic semi isostatic pressing ceramic ball.The main reason is the large hollowness and poor fluidity of the powder, which results in a large number of high-pressure airtight gas produced in the molding process, which can not be eliminated smoothly and timely. Therefore, in the process of firing and exhausting, there are pieces of distributed cracks to remove the domain. After the surface layer of high alumina ceramic ball is pre ground, the waist cracks will appear.In case of waist crack, the preparation process of raw materials in the early stage shall be adjusted in time to increase the fluidity of powder and extend the pressure relief and holding time of the press; the semi-finished products shall be soaked and screened with kerosene (the semi-finished products can be seen only when the semi-finished products are smashed); if the semi-finished products are deformed, the rubber mold shall be replaced; the heating time of the products in the temperature range of 1000-1300 ℃ shall be prolonged when they are fired.The above measures can completely solve the problem of waist crack of high alumina ceramic ball.
6、 Raw fever
Burning defect refers to the situation that the physical and chemical properties of high alumina ceramic balls are not up to standard due to insufficient sintering and densification.Generally, it can be judged by testing the water absorption and specific gravity of ceramic balls.If the high alumina ceramic ball is burnt, the direct causes of technical analysis may be: ingredient error, slurry particle size is too coarse, ball milling time is not enough, forming pressure and furnace temperature fluctuation, etc. besides the production factors such as simple power failure and gas shutdown.When all the above process control points are confirmed to be error free, it is necessary to consider replacing a alumina powder and formula.
7、 Great abrasion
Large abrasion defect refers to that under the normal production of high alumina ceramic ball, the abrasion of some products suddenly becomes larger and exceeds the normal fluctuation range, so it cannot be judged as qualified products.Wear, as an important index of grinding media, directly affects the quality of high alumina ceramic balls.The main reasons for the large abrasion are as follows;
1. The performance index of raw materials has declined sharply and there are many impurities;
2. The high alumina ceramic ball is burnt;
3. The grain inside the high alumina ceramic ball grows abnormally, and a large number of pores are produced by overburning;
4. The formula system is unstable and the firing temperature is narrow.In view of the sudden increase of wear, a stable raw material supplier shall be selected to ensure that each batch must be inspected. In case of any unqualified raw material, it is necessary to use as little as possible. In case of raw firing, high alumina ceramic balls shall be backfired. In addition, the maximum firing temperature of the kiln shall be reduced, the holding time shall be prolonged, and the abnormal growth of grains due to overburning shall be avoided. At the same time, the internal firing uniformity of high alumina ceramic balls shall be ensured. The firing shall be re selectedWhen MgO is added into the formulation system with a wide temperature, MgAl spinel is formed at the grain boundary to inhibit the abnormal grain growth.
