Abrasive Grains 101 | Manufacture of Abrasives

All abrasives contain particles with a range of sizes. In general, the more uniform in size the abrasive, the more expensive and difficult it is to manufacture. Sizing or grading refers to making the particle sizes within an abrasive more uniform so that the majority of the particles fall within a given range of sizes.

Furnacing Aluminum Oxide: The Electric Furnace Process

Fused Aluminum Oxide is produced in an electric furnace. For brown aluminum oxide, the furnace is mainly charged with bauxite. Additives are included to increase the amount of alumina and toughen the abrasive qualities of the end product. At approximately 2000C, the furnace charge melts and fuses together.

Typically, a brown aluminum oxide furnace produces two products. Some of the additives included in the furnacing combine with the impurities in the bauxite to produce ferrosilicon. This is much heavier than aluminum oxide and settles to the bottom of the furnace.

When the process is complete, the furnace is tilted slightly, pouring the molten alumina into a pot. The pot is allowed to cool. After cooling, the crude ingot is removed from the pot, crushed and sized. The ferrosilicon is removed from the furnace by tilting the furnace fully to remove the heavier material from the bottom. After cooling, it is also crushed and sized.

For the production of white fused alumina, white alumina ore is used as the raw material. It has been refined from bauxite by the Bayer process, which removes almost all impurities. No additives need to be included in the furnace charge, and the operation is the simple electric furnace fusion of the white alumina ore. Pouring, cooling, crushing, and sizing are the same as for brown fused alumina.

Silicon Carbide is formed through an electro-chemical vapor phase reaction which takes place above 15000C. Silicon oxide (sand) and coke are mixed together and an electric current applied to the mix to bring it to the required temperature for the formation of silicon carbide. After the reaction is complete, any remaining unreacted graphite is removed, and the silicon carbide pile (fused into a solid mass) is crushed for removal in layers. Generally, the innermost portion of the pile has the purest silicon carbide. After initial crushing, further size reduction and classification are as for fused aluminum oxide.

Fused abrasives, as discussed above, must undergo size reduction typically by crushing before being classified into sizes. The equipment used includes jaw crushers, gyratory crushers, hammermill crushers, and roll crushers. The goal is to crush the large ingots of fused material into smaller particles.

Abrasives such as calcined alumnium oxide are typically agglomerated after furnacing. These agglomerates undergo size reduction of a different sort, meant to separate the agglomerates into individual crystals. This is typically accomplished by milling, for example in a ball mill or jet mill. In a jet mill, streams of material are shot into each other at high pressure. In a ball mill, the abrasive is added to the grinding media in a closed container which is then agitated (by vibration, rolling, etc.).

Milling may also be used to reduce all the particles in a material to a given size or smaller. Because grinding is selective (larger particles receive more force than smaller particles), size reduction tends to proceed to a given size and then slow down, making it possible to obtain a reasonably tight size distribution.

After size reduction, the material is separated into discrete size ranges. This is accomplished by a variety of means, including most prominently screening, air classifying, and water classifying.

In screening, the material to be separated is passed over a series of screens with decreasing opening sizes. At the first, coarsest, screen, most of the material passes through, with only the largest particles retained on the screen and eventually collected. At the second screen, the next coarsest fraction is removed, and so on.

In air classifying, the material is blown across a series of openings. The coarsest particles fall first; the finer particles fall later. Thus, size separation is achieved.

There are two main forms of water classification, fractional sedimentation and elutriation. In fractional sedimentation, the material is mixed with water and a dispersing agent to allow for discrete settling. The agitation ceases, and the material begins to settle. After a given period of time, all particles over a given diameter will have settled at least to a given depth. The material and liquid above that depth are then removed, and the material removed from the liquid, while the remaining material is then remixed for further gradual material removal.

In elutriation, the same principle applies, but the column of water in which the material is settling is itself moving at a fixed rate (as more water is added near the bottom of the column). Particles small enough to settle more slowly than the column of water is moving upward are floated off and collected.

In some cases, it is desired to agglomerate smaller abrasive particles together into larger chunks. This is frequently done by sintering. In sintering, the abrasive is mixed with a quantity of another material which has a lower melting point than the abrasive. The mixture is then heated beyond the lower melting point. This causes the mass to adhere together. The sintered mass can then be formed into granules of the desired size and shape, and allowed to cool.