The Briquetting Process
It is more surprising that typical ionic solids, with their strong well-ordered crystal lattices, are so easily formed into satisfactory briquettes. Sodium chloride, for example, is extensively briquetted for recharging Zeolite-type water softeners. Potassium chloride is briquetted and compacted for the production of granular fertilizer. Calcium oxide is also extensively briquetted, but at much higher pressures.
Hot briquetting is generally understood to mean the briquetting of materials that have been heated to temperatures above ambient where plasticity has developed or become imminent. It is not surprising that materials heated to such temperatures should form better briquettes. Temperatures well below these, however, may also be beneficial. This seems to be particularly true of the ionic compounds. Sodium and potassium chlorides and cyanides, for example, form better briquettes when they are heated to 100-200° C. Temperature affects a number of properties of dispersed solids and is an important variable in the briquetting process.
Continuous bars and strips can also be rolled from particulate materials with roll type briquette machines. If the rolls are arranged one above the other, as in a rolling mill, the strips will issue from the machine horizontally, where they can be fed into a roller hearth, or other furnace, for subsequent processing. Portland cement, sponge iron, coke, metals and alloys, sintered ores, dusts from steel making and other metallurgical processes, and many other products can be made or processed in this way. Dust losses can be reduced and thermal efficiencies can possibly be increased. No application for this technique outside the metals industry appears to be practiced commercially at this time, but with our growing concern about ecology and the conservation of energy and materials, a fresh look at established industrial processes does not seem to be amiss.
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The Briquetting Machines
Roll type briquette machines apply pressures to particles by squeezing them between two rolls rotating in opposite directions. Cavities or indentations cut into the surfaces of the rolls form the briquettes.
Characteristics of Roll Type machines
In early briquette machines, the axes of the rolls were always horizontal and their centers were fixed in the frame. A simple feed box or hopper mounted above the rolls held the material to be briquetted and it flowed from there into the rolls by gravity.
Modern briquette machines usually have only one roll in a fixed position in the frame. The other roll is moveable, but is restrained by hydraulic cylinders. The rolls may be arranged horizontally or side by side in the frame, or they may be arranged vertically or one above the other as in rolling mills. The rolls additionally may be located symmetrically between the bearings or they may be mounted outside the bearings at the end of cantilevered shafts. Each of these four arrangements has certain unique properties. Other features of the machines as well can be varied to satisfy special process conditions. Six characteristics in all determine the behavior of roll type briquette machines.
Briquette machine rolls are classified according to their construction as integral, solid or segmented. Integral rolls, as the name implies, are made integral with the shafts. These rolls usually have a band of stainless steel or some corrosion or abrasion resistant material welded to their circumference or working face. Since they have no joints or mating surfaces, integral rolls are frequently used for briquetting food or pharmaceutical products where cleanliness is of primary concern. Integral rolls can easily be steam heated or water cooled. They are not generally suitable for abrasive materials.
Solid rolls or tires are the most commonly used briquetting rolls and consist of replaceable rings keyed or shrink fitted to the shafts. The rolls are made from a variety of abrasion and corrosion resistant materials. Unlike integral rolls, which require some compromise in materials of construction, solid rolls and shafts can each be made from the most suitable material.
Segmented rolls are made in a series of sections or segments which are mechanically clamped to the shafts. The advantages of segmented rolls are obvious to anyone who has ever changed conventional rolls, so rolls of this type have been the subject of continuing investigation since the beginning of the briquetting industry. Segmented rolls are recommended for briquetting hot or abrasive materials and are made from materials suitable for such applications.
The mechanical construction of the rolls determines such important characteristics as reliability, ease of maintenance and cost of operation. The effect that the rolls will have upon materials passing through them, however, depends on their geometry.
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In most briquette machines, the moveable roll is pressed against a fixed roll by hydraulic cylinders. Stops located between the bearing blocks prevent the rolls from coming in contact with each other. Material passing between the rolls attempts to spread them apart. The hydraulic cylinders resist this effort until the force exerted by the material exceeds that exerted by the cylinders. The moveable roll is then displaced and in turn displaces the pistons in the hydraulic cylinders until both efforts become equal. The oil displaced by the pistons is stored under pressure in a gas filled accumulator. It returns from there as needed to push the moveable roll back against the stops.
The hydraulic system acts like a spring. The initial force holding the rolls together can be adjusted by the pressure of the oil in the cylinders. The incremental force necessary to displace the moveable roll is also adjustable by the volume of the gas in the accumulator.
The success of the modern roll type briquette machine is due in no small part to this ability of the hydraulic system to match the slope of the force-displacement curve of the moveable roll to the requirements of the briquetting process.
When roll type briquette machines were limited to compacting materials, which were mixed with binders, the simple gravity type feeder was usually adequate. Briquetting in this case is primarily a forming or molding process and little change in the density of the product occurs as it passes through the rolls. The pressure required for such applications is low and the virtue of simplicity frequently outweighs the advantages possible from more sophisticated control. Gravity type feeders consequently are still used for some purposes.
For dry or finely divided materials, screw or auger type feeders are commonly used. These feeders in addition to controlling the mass of material passing between the rolls, frequently have important secondary effects. They may precompress the material before it reaches the rolls. They may crush the particles to achieve a more favorable size consistency. There is speculation that the mobility of the particles in the feed screw allows the crystal axes to align themselves more favorably so as to produce better quality briquettes. Heating of the particles in the screw feeder may also have a significant effect. Whatever the mechanisms may be, briquettes of better quality frequently can be made by using a screw feeder.
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