Press rolls

A novel comminution device, the roll press, has achieved commercial success [Schoenert, International J. Mineral Processing, 22, 401-412 (1988)]. It is used for fine crushing, replacing the function of a coarse-baU mill or of tertiary crushers. Unlike ordinary roll crushers which crush individual particles, the roll press is choke fed and acts on a thick stream or ribbon of feed. Particles are crushed mostly against other particles, so wear is veiy low. Energy efficiency is also greater than in ball mills. 

Roll presses are now manufactured by most cement-equipment manufacturers, for example Krupp-Polysius AG. For drives up to 1200 kW they are equipped with V-belt drives, allowing speed to be changed. Rolls in one particular crusher are 48 inches (1200 mm) in diameter and 30 inches (700-mm) wide and each weighs 90 tons. Machines with up to 2500 kW installed power and 1000 ton/h (900... 

The advent of the roll press has greatly improved the economy of cement milling. More information is given later under cement. 

FIG. 20-25 Size distribution of roll-press products. Feed is quartz, 300-2500 im, dry. [Schonert, International J. Mineral Processing, 22, 401 12 (1988).]... 

FIG. 20-26 Hybrid flow sheet to comhine a roll press and hall mill with a classifier. Conroy, 31st IEEE Cement Industry Technical Conference, 509-542, 2988. Copytight IEEE.)... 

Only once in 15 years does a truly novel method of size reduction become successful. The roll press is a truly successful example of a novel mill, and stirred-bead mills are older mills that have reached a new state of development. Many more methods are proposed and studied some of these are described below The information may be useful to judge other novel methods that may be proposed. 

If the rock is abrasive, the first stage of crushing may use gyratory or jaw crushers, otherwise a rotor-impact mill. Their reduction ratio is only 1 12 to 1 18, so they often must be followed by a hammer mill, or they can feed a roll press. Rotor crushers have become the dominant primaiy crusher for cement plants because of the characteristics. All of these types of crushers may be installed in moveable crusher plants. 

Finisli-Grinding of Cement Clinker Typically the hot clinker is first cooled and then ground in a compartment mill in closed circuit with an air classifier. To crush the chukers, balls as large as 5 in may be needed in the first compartment. A roll press added before the ball mill can reduce clinkers to a fine size, ana thus reduce the load on the ball mills. The main reasons for adding a roll press has been to increase capacity of the plant, and lower cost. 

Roll press >1 Up to 50 ton/hr sensitive to powder organic chemicals. 

Transmission of Forces As pressure is applied to a powder in a die or roll press, various zones in the compact are subjected to differing intensities of pressure and shear. Compaction stress decreases with axial distance from the applied pressure [Strijbos et al.. Powder Tech., IS, 187 209 (1977)] due to frictional properties of the powder and die wall. For example, the axialpressure experienced within a cyhndrical die with an applied axial loaa Oq may be estimated to a first approximation by... 

Roll presses can produce large quantities of materials at low cost, but the product is fess uniform than that from molding or tableting presses. The introduction of the proper quantity of material into each of the rapidly rotating pockets in the rolls is the most difficult problem in the briquetting operation. Various types of feeders have helped to overcome much of this difficulty. 

Roll presses provide a mechanical advantage in amphfying the feed pressure Pq to some maximum value P, . This maximum pressure... 

FIG. 20-91 Roll-force factor as a fimction of compressibility K and dimensionless gap distance d + h)/D. [Vietsch (ed.). Roll Pressing, Fowder Advisoiy Centre, London (1987).]... 

TABLE 20-52 Some Typical Capacities (tons/h) for a Range of Roll Presses ... 

ASTM A123 Zinc (Hot Galvanized) Coatings on Products Fabricated from Rolled, Pressed and Forged Steel Shapes, Plates, Bars and Strips. ... 

It is added to steel and other metals as an alloy to make the metals easier to roll, press, pull into wires, and turn on a lathe. It is also used in the semiconductor industry and to make permanent magnets. 

TABLE 12.15. Alphabetical List of Some of the Materials that Have Been Successfully Compacted by Roll Presses... 

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