Powder Handling: Size Up Size Reduction

Understand the operation and appropriate application of mills and pulverizers

By Amin Almasi, rotating equipment consultant

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Chemical plants often must reduce the size of materials to create dramatically greater surface area of particles to improve processing. Indeed, many operations require pulverized or fine feed materials. Providing suitable materials may call for multiple crushing, milling and pulverizing stages. The necessary size reduction equipment frequently is massive, expensive and energy intensive. Moreover, such units suffer unavoidable wear and tear that incurs substantial operational and maintenance costs. Therefore, it’s crucial to select the most appropriate, efficient and reliable units.

Optimized mill or pulverizer performance is a prerequisite for ensuring the best efficiency and performance in subsequent processing units. At many plants, mills and pulverizers form the heart of the processing unit. Often, the root causes of out-of-spec products, poor performance, inefficiencies or frequent shutdowns lie with the mills or pulverizers. Unfortunately, such issues usually result because the teams responsible for design, operations, etc., underestimated the importance of size reduction equipment and didn’t pay adequate attention to such units.

There are three basic types of size reduction:

1. Crushing, in which material usually is forced between two machine elements;
2. Impaction, in which equipment components or an outside force hit the material; and
3. Attrition or grinding, in which rubbing or friction reduces material size.

Fineness is an indicator of the quality of the mill or pulverizer action. It denotes the percentage of a milled or ground material that passes through a set of test sieves usually designated by specific sizes — e.g., 50, 100, 200 and 400 mesh (equivalent to 300, 150, 75 and 40 microns, respectively). For example, a specification for pulverizer performance may require more than 70% of material passing 200 mesh (75 microns) and more than 98% passing 50 mesh (300 microns).

Many types of mills and pulverizers are available. Proper selection, design and operation requires considering numerous characteristics and factors such as capacity, effects of moisture content, material sizes, air/gas inlet details, fineness of produced material, outlet specifications, environmental emissions, etc. So, it’s common to find that poorly performing equipment was selected with minimum attention to details and installed carelessly. However, you can take steps to quantify, monitor and improve the operation of size reduction units such as crushers, mills or pulverizers.

Fine Mills and Pulverizers

These often are located close to processing units because moving pulverized materials over long distances usually is difficult. Pneumatic conveyers, pneumatically operated piping systems and others methods can transfer fine or pulverized materials but they are expensive and hard to operate. So, it’s best to keep the transfer distance to the absolute minimum. Often, materials to be milled are transferred by conventional conveyers to bunkers (or hoppers) close to processing units and then passed by feeders to mills (pulverizers) located somewhere below the bunkers. Milled materials then go a very short distance to bins near the processing unit; sometimes, the fine materials directly feed into a processing unit.

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You usually should arrange mills/pulverizers to enable doing maintenance and replacement of internal parts and other components subject to high wear without removing the units from their foundations. This demands great care not only because they are near the processing unit but also because mills/pulverizers include auxiliaries and accessories such as acoustic enclosures, lubrication system, etc. and usually require local platforms, walkways, stairs or ladders.

Fine mills or pulverizers often come as standard models, with multiple units used to provide the necessary capacity. For instance, one manufacturer offers eight models covering a 150–800-kW-rated power range, with each model around 20–30% larger than the preceding one. Plants commonly opt for “n+1” or “n+2” arrangements. The one or two spare/standby machines also can be used to cope with harder or more difficult feed materials that reduce the milling capacity of machines in operation. For instance, a large processing unit with five operating and two standby mills sometimes puts all seven machines to work for harder-than-rated feed materials.

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