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The common quality defects of high alumina ceramic balls are divided into:

(a) appearance quality defects such as spots, blistering, adhesion and wind crystal;

(b) performance defects, including waist cracks, raw burning, large wear and nail lines.

Through the inspection of high alumina ceramic balls at home and abroad, it is found that nail pattern defects exist in the large-size products of high alumina ceramic balls to varying degrees, and it is difficult to completely eliminate them. Therefore, this article focuses on the causes and countermeasures of nail pattern defects. analysis.

Cause analysis and treatment methods of common quality defects

 1. spots

Because the body of the high alumina ceramic ball is white, spots will be particularly obvious and easy to find. According to the number and size of spots, it is roughly divided into the following three situations.

(1) There are fewer spots, and the diameter is less than 1mm. The spot defect in this situation is mainly caused by the iron impurities in the α alumina powder and the iron content in various auxiliary materials exceeding 0.3%;

(2) There are many spots, and the diameter is less than 2mm. This may be caused by the ball mill dropping bricks, or the slurry did not remove iron during the ball milling and spraying process.

(3) Spots appear on the top layer, and the spots are larger. This is usually caused by rust or refractory bricks falling into the surface of the product by the kiln preheating duct.

In summary, in order to reduce or eliminate spot defects, pure alpha alumina powder and other auxiliary materials should be used; in the process of discharging, spraying and discharging, it is necessary to remove iron several times, and clean the iron remover in time; clean regularly Each duct of the kiln.

2. blistering

Blistering defects generally appear in high-alumina ceramic balls made by the roll forming process. Since the finished ball billet grows through the collision between many semi-finished balls during roll forming and completes the densification process, it needs to be polished for 10 to 120 minutes before the final rolling is completed to adjust the sphericity of the semi-finished ball billet. And surface finish. Due to the high surface temperature of the ball blank during firing, it is first sintered to form a dense surface layer, which hinders the evaporation and discharge of internal moisture, resulting in blistering on the surface of the finished ceramic ball. Based on the above principles, in order to eliminate blistering defects:

(1) In the process of rolling the high-alumina ceramic ball body, the density and smoothness of the semi-finished product should be considered, and the polishing time before the pot should be minimized.

(2) Reduce the content of low-temperature flux and high-temperature volatile substances in the formula.

(3) Dry the moisture of the semi-finished product before firing so that it cannot exceed 0.3%.

3. Stickiness

Adhesion defects of porcelain balls are more common. The high-alumina ceramic ball uses a high-temperature firing process (the temperature can be as high as 1520°C). In order to ensure the performance of the fired product, it is often necessary to carry out heat preservation at a high temperature for 2-10 hours. Once there are more liquid phases at high temperature, there will be adhesion damage between the ceramic ball and the ceramic ball, and between the ceramic ball and the silicon carbide boron board. To deal with this problem: (1) Between the ceramic ball and the ceramic ball, the ceramic ball and the silicon-boron carbide plate, the ceramic ball and the column, use corundum sand, put corundum backing plate, etc. to isolate;

(2) Reduce the kiln height of each layer of ceramic balls while taking into account the output;

(3) Design a solid phase sintering formula to reduce the glass phase appearing in the ceramic ball, and fire at the lower limit of the firing temperature range to appropriately extend the holding time. The above measures can well solve the sticking defect and improve the product qualification rate.

4. Wind Crystal

Aeolian crystal defect refers to the explosion of aeolian crystal in the high alumina ceramic ball in the cooling zone of the furnace. This is mainly because the product has reached densification under high temperature firing. The sintered ceramic high alumina ceramic ball has a small porosity, and when cooled to 570°C and 790°C, the phase transformation of quartz produces a larger volume. The change caused the product to burst [3]. Therefore, in the event of wind crystal explosion, if it is a roller kiln or tunnel kiln, it is necessary to reduce the speed of the kiln car in time, reduce the air intake of the cooling zone, adjust the height of the product installed in the kiln, and avoid direct cold air blowing to the hot product; if it is Shuttle kiln only needs to reduce the cooling air intake

That’s it. Aeolian crystal burst occurs more frequently in roll-molded large-size ceramic balls (generally above φ20mm), and rarely occurs in semi-isostatic or isostatically-pressed high-alumina ceramic balls. This is because it is difficult for roll-molded ceramic balls to release the stress caused by the phase transformation of quartz. Therefore, in the formulation design of the high alumina ceramic ball prepared by the roll forming method, the possibility of free SiO2 should be minimized, and it is best to consider the promotion of the formation of a certain amount of mullite or spinel in the formulation design. , To prevent the occurrence of wind crystal defects.

5. waist crack

Waist crack is a defect that is difficult to be found and detected directly in the kiln product. It is an annular crack on the surface of the high alumina ceramic ball after the pre-grinding. The cracks are mostly located about 5mm below the spherical crown, and hence the name. When the defect is serious, there will be a crack with a width of 0.5mm around the ceramic ball. Serious quality accidents such as broken balls and cracked balls will occur in the ceramic balls with this defect. According to our experience, this kind of crack already exists in the semi-finished ball billet, and mainly appears in the fully automated semi-isostatically pressed ceramic ball.

The main reason is that the powder has large hollowness and poor fluidity, resulting in a large amount of high-pressure airtight gas generated during the molding process, which cannot be smoothly removed in time, so that there are fragments of cracked areas during the firing and exhaust process, and pre-grinding After removing the outer surface of the high-alumina ceramic ball, the waist crack was revealed.

When waist cracks occur, the preliminary raw material preparation process should be adjusted in time to increase the fluidity of the powder and prolong the pressure relief and holding time of the press; use kerosene for soaking and screening of semi-finished products (smash the semi-finished products to see); in case of semi-finished ball stones If the shape is abnormal, the rubber mold needs to be replaced; extend the heating time of the product in the temperature range of 1000 to 1300 ℃ during firing. Taking the above measures can completely solve the problem of waist cracks in the high alumina ceramic ball.

6. Burning

Burning defect refers to the situation that the physical and chemical properties of the high alumina ceramic ball are not up to standard due to insufficient sintering and densification. Generally, it can be judged by testing the water absorption rate, specific gravity and other indicators of the ceramic ball.

If high-alumina ceramic balls are burnt, in addition to simple power outages, gas outages and other production factors, the direct causes of technical analysis may include: incorrect ingredients, coarse slurry particle size, insufficient ball milling time, molding pressure and furnace temperature Fluctuations and so on. When all the above process control points are determined to be error-free, it is necessary to consider replacing the α alumina powder and formula

7.Large wear loss

Large abrasion defect refers to the abrasion of some products suddenly becomes larger under the normal production of high alumina ceramic balls, which exceeds the normal fluctuation range and cannot be judged as qualified products. As an important indicator of grinding media, abrasion directly affects the quality of high alumina ceramic balls. The main reasons for large abrasion are:

(1) The raw material performance index drops sharply and there are many impurities;

(2) The high-alumina ceramic balls are burnt;

(3) The internal grains of the high alumina ceramic ball grow abnormally, and a lot of pores are generated during over-burning;

(4) The formula system is unstable and the firing temperature is narrow. In view of the sudden increase in wear, a stable raw material supplier should be selected, and each batch must be inspected. If unqualified raw materials are encountered, they should be used as little as possible; high alumina ceramic balls that appear raw should be refired; Reduce the maximum firing temperature of the kiln, extend the holding time, and avoid abnormal growth of crystal grains due to over-firing; at the same time, ensure the uniformity of the internal firing of the high alumina ceramic ball; re-select the formula system with a wider firing temperature, and add a small amount of MgO , The formation of magnesium-aluminum spinel at the grain boundary inhibits the abnormal growth of crystal grains.

8. Nail pattern

Nail line defects are common in high alumina ceramic ball products at home and abroad, and the larger the product size, the more nail lines. Nail patterns cannot be detected in semi-finished products. They can only be found after the high alumina ceramic ball is pre-polished. The curved and intermittent cracks distributed on the surface of the product will become deeper and deeper as the grinding time increases Widen. High-alumina ceramic balls with nail patterns will have problems such as slag drop and large abrasion when used as grinding media. The appearance of nail patterns is related to the entire technical process system for the production of high alumina ceramic balls.

In summary, there are mainly:

01. Raw material usage problem

The high alumina ceramic ball uses α-phase alumina powder as the main raw material,

The conversion rate of α-phase alumina powder directly determines the number of nail lines in the product. If the α-phase conversion rate is less than 92%, it contains a large amount of γ-phase alumina powder which will produce shrinkage phase change during the firing process of the product [4], which will lead to nail lines.

02. Problems with the milling process

The powder of the high alumina ceramic ball is prepared by pressure spray drying with slurry. The viscosity and moisture of the slurry determine the particle gradation, sphericity, bulk density and powder stickiness of the obtained powder. If most of the powder has insufficient sphericity, poor fluidity, and hollow powder, closed pores will appear in the pressed semi-finished product. In the sintering process of the high alumina ceramic ball, the pores will be transferred along with the grain boundaries, and finally a connected closed pore will be formed. The closed pore will be in the process of ball milling because the strength is lower than the surrounding environment, resulting in curved nail lines.

03. The binder problem

High alumina ceramic balls are formed by dry powder. In order to improve the semi-finished product rate and the strength of the semi-finished product in the semi-isostatic pressing and isostatic pressing process, a large amount of binder is often added to the formula. The binder will generate a large amount of ash and high-pressure gas during the sintering process of the product, which cannot be discharged smoothly inside, resulting in nail lines, so the binder should be reduced

The amount of use or the use of additives without residue after burning.

04. Molding pressure problem

Nail line defects seldom occur in rolled products, because the impact of the rolling process destroys the original powder structure. When high alumina ceramic balls are formed by semi-isostatic pressing and isostatic pressing, the current design pressure cannot achieve the internal structure of completely crushing the raw powder, making the semi-finished product not dense enough, resulting in nail lines inside the fired product. By comparing the high alumina ceramic ball products at home and abroad, it is found that the greater the molding pressure, the fewer nail lines, and the isostatic pressure is better than the semi-isostatic pressure.

There are few nail lines in the pressed product.、

05. Burning problem

Firing is also very important when high-alumina ceramic balls eliminate nail line defects. Eliminate kiln design problems. The firing temperature, holding time, and heating curve directly determine the number of nail lines. In the firing process of high alumina ceramic balls, there are complicated processes such as volatilization of free water and crystal water, shrinkage, phase change, and recrystallization [5]. For this reason, in the process of rapid firing shrinkage in the range of 1000 to 1350°C, the holding time should be extended, and the temperature difference of the firing zone should not exceed 5°C to avoid inconsistencies in firing as much as possible.

Conclusion

Through the above analysis and summary of the common quality defects of high alumina ceramic balls at home and abroad, it is not difficult to find that the control and elimination of these defects mainly need to start from the following aspects. Use the optimal raw material and formula system to increase its stability and practicability; strictly control and adjust the milling process to prepare a granulated powder with good fluidity, high sphericity, and solid; according to the requirements of the product, use the appropriate corresponding high-alumina ceramic ball blanks are prepared by the forming process. The types of firing defects in the ball blanks caused by different forming processes are different, and the measures to eliminate the defects are also different; a reasonable firing process is especially important, which is directly Determines the final performance of the product. If the above-mentioned countermeasures are effective, it will greatly improve the product qualification rate of high alumina ceramic balls and the overall performance of the product.

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