Powder and Bulk Engineering, February 2001 21 0 0 73 2.

Michael Schuelke Koppern Equipment

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Waste by-products from manufacturing plants, coal-burning power plants, and other operations can create bulky, costly, or even hazardous disposal problems. Agglomeration equipment can treat these by-products so they can be recycled - either by sending them back to your process or using them to make new products. This two-part article concen- trates on using one type of high-pressure agglomer- ation equipment, the roller press, to help you recycle low-moisture waste by-products by compacting them into various high-density shapes. Part I1 will appear in the March issue.

In many cases, the by-products are very fine or are otherwise hard to return to the process. A high-pressure roller press can compact these by-products into briquets, sheets, or granules that are easy to handle and recycle to the process.

Roller press basics for recycling by-products The roller press provides high-pressure agglomeration for low-moisture by-products in a wide particle size range. The resulting compacts have high strength to withstand rough handling during recycling to your process or trans- porting to another plant for use in another process. De- pending on your needs, you can install a roller press to compact the by-products in your plant or contract a toll processor to provide this service.

oller presses have long been used to increase the bulk density of powders, granules, and shredded R materials so they’re less dusty and easier to handle

and transport without degrading. But increasingly, these high-pressure machines are helping processors agglomer- ate low-moisture waste by-products for recycling. Recy- cling can include sending the by-products back to the original process, creating new products from them, or sell- ing the by-products for use as secondary raw materials in other products.’

Several kinds of by-product waste streams are found in processing plants. They include undersize raw materials, such as fines from salts or fertilizer processes, dried sludges from waste-water treatment plants, chips and turn- ings from metal manufacturing processes, and dust from primary and secondary dedusting systems in processing plants. If these by-products aren’t recycled to the process or sold, their value is lost, and they often must be discarded or landfilled at the processor’s added cost.

What the equipment includes. For most recycling appli- cations, the roller press is installed in a separate circuit lo- cated near the manufacturing process. In addition to the roller press, this circuit can include various types of con- veyors, cleaning and drying equipment, a mixer, a size re- ducer, a screener, and other equipment.

Which equipment you need depends on the by-product’s characteristics. For instance, a circuit for a by-product with fine particles can require a pneumatic conveyor or screw conveyor to move the material from your process to the roller press, while one for fibrous by-products or chunks can require a belt conveyor. A circuit for metal swarf requires cleaning equipment to remove contami- nants, such as machine oil, that can prevent good com- paction. Such a material then must be dried to ensure that the swarf contains 5 percent or less moisture, which is the moisture limit the roller press can handle.

Many circuits include a mixer to combine by-products with a binder or other materials. Large compacts can require

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breaking or granulating in size reduction equipment located after the roller press. Screening equipment is typically lo- cated further downstream to remove fines from the finished compacts and send them back to the roller press. Finally, other conveyors are required to transport the compacts.

Powder and Bulk Engineering, February 2001

The roller press is typically part of a package that includes a feeder, which is available in various types to suit the by- product’s characteristics. An enclosed housing as well as flange and pipe connections allow the roller press to be connected to the plant’s dust collection system.

The roller press consists of two rolls that counter-rotate. As material is fed between the rolls, the rolls apply pres- sure to it, compressing the material into individual briquets or a densified sheet. The briquets are discharged from the rolls for recycling. The sheet is discharged to a crushing and classifying operation for reduction to granule-sized agglomerates before recycling.

How compaction works. The pressure (called pressing force) applied to the material during roller press operation brings individual particles close together. This high pres- sure facilitates molecular attraction forces, called Van der Wuulsforces, that cause the particles to compact into bri- quets or sheets.

This is shown in Figure 1, in which a material is fed into the gap between two equally large rolls that counter-rotate at the same speed. The pressure P builds up from the mate- rial intake point a toward the point where it reaches maxi- mum pressure, Pmax, just before the narrowest gap between the rolls. From this point, the pressure decreases sharply. In the figure, s is the gap width, D is the roll diam- eter, a is half the nip angle (also called the intake angle), P, is the mean pressure, F is the pressing force, and x is the processing time.

How much pressing force is required to form the compact depends on the feed. For recycling applications, the typi- cal pressing force is between 1 1.2 and 33.6 tons per linear inch. Using a greater amount of binder can reduce this re- quired pressing force.

The roller press can have surfaces with oval, droplet-, pil- low-, or rod-shaped pockets (also called molds) to produce briquets, or it can have smooth or corrugated surfaces to produce a sheet. The briquets typically are the final prod- uct, while the sheet is typically an intermediate product that is then crushed and classified into the final product.

The compacts must have high strength to withstand rough handling during subsequent recycling, whether they’re fed

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back to your process or transported to another user. In some cases, the roller press can compress the by-product at a high pressing force so no binder is required; in others, the by- product can only be pressed into a high-strength compact by mixing the material with a binder or by compressing it while hot so it deforms and compacts more easily (in a process called hot briquetting, to be discussed later).

Powder and Bulk Engineering, February 2001

When the feed material is corrosive, abrasive, or hot, all parts in the roller press that contact the feed material must be designed to withstand these stresses. The roll surfaces consist of rings (with pockets for producing briquets, as shown in Figure 2a, or smooth or corrugated profiles for producing sheets) or segments with briquet pockets, as shown in Figure 2b. Carefully choosing the construction materials for these components helps them to withstand corrosion, abrasion, and heat from the feed material. Heat treatment can also improve the surfaces’ ability to with- stand these stresses. A hot feed material also transfers heat to the roller press, which must be removed by a suitable cooling system. To help remove heat, the roll diameter typi- cally must be 19.685 inches or larger. [Editor’s note: Con- tact the author for further detail on briquet geometry, roll width, roll speed, pressing force, and roller press sizing.]

The following sections explain how a roller press com- pacts low-moisture by-products for recycling. The by- products include aluminum swarf, steel mill residues (discussed here), other metallic fines, burnt lime fines, flue-gas-desulphurization gypsum powder, dried sewage sludge, and metal residues and dusts (discussed in Part 11 in next month’s issue).

Compacting aluminum swarf Aluminum foil and aluminum bar manufacturing plants, as well as manufacturing plants that turn the bars into alu- minum products, use cutting tools that produce fine alu- minum cuttings and chips of various sizes and shapes. These by-products are called aluminum swm$ In an alu- minum foil or bar plant, remelting the swarf in the plant’s hearth furnace for recycling into new aluminum can re- duce manufacturing costs. However, the swarf’s bulk den- sity is only about 15.61 lb/ft3, which makes it hard to feed the swarf into the furnace. Rather than sink into the molten aluminum and melt, the lightweight swarfis more likely to rest on the furnace’s dross layer (that is, the layer of waste or impurity, such as an oxide) on the molten aluminum’s surface and then simply burn off in the form of a gas. Using a roller press to compact the swarf into a sheet can increase the by-product’s density to more than 143.58 1 w f t 3 .

How theprocess works. The pressure applied by the roller press easily deforms and compacts the soft aluminum swarf, so no binder is required to form a strong compact. After exiting the roller press, the aluminum sheet can pass to a cutter or other size reduction machine that breaks the

sheet into narrow, approximately 1-foot-long pieces that are easy to feed to the furnace.

When the aluminum pieces enter the furnace, their greater bulk density allows them to penetrate the dross layer on the molten aluminum. The pieces then sink into the alu- minum, where they melt completely with only negligible burnoff losses.

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Specialrequirements. The aluminum swarfmust be clean and dry to compact properly in the roller press. This typi- cally requires preliminary washing and drying steps be- fore the swarf is conveyed to the roller press feeder.

The feeder that delivers the aluminum swarffrom the con- veyor to the roller press must be chosen based on the swarf’s particle size and shape. When the swarf includes very fine particles, it must be force-fed to the roller press by a screw feeder. When the swarf includes chips or long turnings, which can form bird’s nests in a screw feeder, the swarf must be gravity-fed through a chute. To achieve good intake of the chips and long turnings, the roller press must have a roll diameter of at least 39.37 inches.

Briquetting steel mill residues Metal-bearing dusts -including furnace and flue dust and dust from secondary metallurgy, such as oxide by-products from steel mills -are common by-products in steel manu- facturing. Like aluminum swarf, these dusts are hard to

handle. Compacting the dust into briquets allows it to be re- cycled back to the process, which cost-effectively reuses the dust and can eliminate aneed to landfill this waste.

How the process works. The dust can be sent from the process to a mixer in the roller press circuit. In the mixer, the dust is blended with a binder, such as sodium silicate or molasses combined with hydrated lime; shredded waste paper; polymer; or another natural or synthetic material. A cement-based binder can be used to provide high strength in some applications. The choice of binder strongly de- pends on binder availability and price, as well as on which binder will minimize contamination of the briquets.

This mixture is then fed to a roller press for compacting into briquets that contain from 50 to 65 percent iron. Depending on the dust’s composition and the binder type, the briquets can be recycled to the manufacturer’s steel mill or to a blast furnace, where they can be melted along with iron ore to form pig iron. This process is shown in Figure 3.

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28 Powder and Bulk Engineering, February 2001

Before it’s fed to the roller press, the dust can be mixed with other steel manufacturing by-products for recycling to the process. These by-products include dried sludge from wet dedusting equipment, sludge from various furnaces, pig iron residues, and mill scale (coarse residue from the outer surface of rolled steel). The resulting briquets can be of var- ious sizes and shapes, depending on the steel residue’s characteristics and the end user’s requirements.

The process is somewhat different in a stainless steel man- ufacturing plant. Here, dust collects in filters near the arc furnace in the melting shop and in other dust collection systems throughout the plant. The dust’s valuable alloy content makes it cost-effective to recycle the dust to the arc furnace.

During the melting process, zinc begins to accumulate in the dust collected in the filters. After the dust reaches a cer- tain zinc content, the dust is discharged and melted to ob- tain waelz oxide. The hot (662°F to 752°F) waelz oxide is then fed to a roller press and formed into briquets. The bri- quets are fed to a shaft furnace for producing zinc. Figure 4 shows a roller press for briquetting waelz oxide; the hinged frame has been opened to allow workers to service the rolls.

extensive cooling systems to prevent high temperatures from damaging the equipment.

The choice of binder depends on the performance charac- teristics the briquets must have and the binder’s relative availability and cost. The binder quantity depends on the application’s binding force and mechanical requirements and should minimize contamination of the briquets.

If the steel dust is coarse enough to flow by gravity, the dust can flow through a gravity chute to the roller press. If the dust is fine, a screw feeder must force-feed the dust to the unit. PBE

Next month: Part I1 describes additional roller press recy- cling applications, including briquetting other metallic fines, burnt lime fines, and flue-gas-desulphurization gyp- sum powder; compacting dried sewage sludge; and hot- briquetting metal residues and dusts.

Endnote 1. The roller press can also be used in landfilling applications. The unit

can form a by-product into tough, dust-free compacts that will withstand rough handling and transportation to the landfill.

For further reading Find more information on high-pressure compaction equipment and agglomerating waste by-products in arti- cles listed under “Agglomeration” in Powder and Bulk En- gineering’s comprehensive “Index to articles” (in the December 2000 issue and at www.powderbullc.com).

Michael Schuelke is vice president of technology at Kop- pern Equipment, 2201 Water Ridge Parkway, Suite 500, Charlotte, NC 2821 7; 704/357-3322, fax 704/357-3350 (mas95@ bellsouth.net). He completed Technical College, Bochum Universig, in Bochum, Germany, and has been with Koppem since 1980.

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Special requirements. Because metal dust is abrasive, the roll surfaces that form metal dust compacts wear more rapidly and must be replaced more frequently. For this rea- son, the roller press must be designed and installed to pro- vide easy access to the rolls. The roller press for compacting hot waelz oxide must also have heat-resistant surfaces and