Briquetting

the processing of a material into briquettes—pieces having a uniform, geometric shape and practically identical weight. Briquetting creates additional raw material resources from fine materials, primarily fuels and ores, the use of which would otherwise be inefficient or difficult; it also makes it possible to use waste products, such as dust, slags, and metal chips. In all cases, the usefulness of briquetting is judged by economic factors.

Depending on the material to be processed, briquetting may be done with binders at medium pressures (10–50 meganewtons per m²) or without binders at high pressures (100–200 meganewtons per m²). In order to produce briquettes of high quality, the material used must meet specific requirements for fractional composition, moisture content, temperature, and the like.

Briquetting was proposed in Russia in the 1830’s by the Russian inventor F. P. Veshniakov, who developed a method of producing hard briquettes from waste wood charcoal and hard coal; Veshniakov called the fuel produced karbolein. The first factory for the briquetting of brown coal was put into operation in Germany in 1858; a briquetting factory for hard coal that used roller presses was set up in 1860. Briquetting was widely used in the second half of the 19th century to nodulize ore fines.

The mechanism of the principal stage of briquetting—compressing of the raw material into the desired shape—may be described as follows. At low pressures there is an external compacting of the material caused by reduction of the spaces between particles. The particles themselves are then compressed and deformed, and a molecular bond is created between the particles. High pressures at the end of the process cause a transition from elastic deformation of the particles to plastic deformation, which strengthens the briquette and enables it to retain a given shape. The physical and chemical properties of the raw material are a major factor affecting the nature of the deformations.

The following fuels are briquetted in factories primarily for power engineering and home and municipal use: residues of hard coals and of nearly hard, old brown coals with a relatively strong mechanical structure; common weak-structured, young brown coals; and peat. The ash content of the briquettes produced may reach 20 percent. The briquettes stand up well to being transported, withstand long storage in the open, air, and do not fall apart until they have finished burning. Briquetting is used in various new coking processes designed to produce metallurgical coke from gas coals and weakly sintering coals. Lean coals, anthracite, old brown coals, and semicoke are briquetted with a binder—solid or liquid pitch from a hard coal, bitumen, or the like.

Hard coal is briquetted with a binder in the following stages: delivery of the raw material (charging of different types of coals); sorting of the coal and pulverizing to dimensions of 6 mm or smaller; drying of the coal to a residual moisture content of 3–4 percent; preparation of the binder (crushing and liquefaction); batching and mixing of the heated coal with the binder (6–10 percent) at a temperature of approximately 100°C to produce a homogeneous mass, or charge; cooling of the charge to 80°–90°C; compacting in roller presses at pressures of 15–30 meganewtons per m²; and cooling of the briquettes to 40°C. Egg-shaped briquettes weighing 70–75 g are the most commonly used type; they withstand shipment well.

Fundamental disadvantages of briquettes containing a pitch or bitumen binder are the production of soot upon combustion and a low thermal stability, which seriously limit their use. Methods are being introduced for the treatment of such briquettes with hot gases containing a specific quantity of oxygen or with a solid heat-transfer agent. This causes an oxidation polymerization of the binder, as a result of which the briquette becomes stronger and is made to burn with a smokeless flame. Hot briquetting is widely used to produce a high-quality smokeless fuel or coke without a binder. The process consists in compacting sintering coals that have been preheated to a plastic state or a mixture of such coals and nonsintering coals (anthracite and lean and brown coals) and semicoke.

Young brown coals with high moisture contents (45–60 percent) ordinarily fall apart during storage and transporting. They may be briquetted without a binder for fluidized-bed combustion in the following manner: delivery of the coal; batching; sorting in rotary or screen sieves and pulverizing in impactors to a grain size under 6 mm; drying in steam-heated or gas-fired tube dryers to an optimum moisture content of 14–19 percent; supplementary crushing of large coal particles; cooling of the coal, which emerges from the dryers at a temperature of 85°–90°C, to a temperature of 35°–45°C in cooling installations (omitted in some cases); compacting at pressures of 100–200 meganewtons per m² in auger extruders or, less often, rotary presses; cooling of the briquettes, which leave the presses at a temperature of 70°–80°C, to a temperature of 40°C in cooling troughs and on screen conveyors; and delivery of the briquettes to the consumer. The briquettes produced have the shape of a parallelepiped with rounded edges and weigh 500–600 g.

Auger extruders are used to fashion semibriquettes from milled peat with a moisture content up to 25 percent. When the moisture content is higher (up to 50 percent), the peat is first dried in dryers to a moisture content of 12 percent. The dryers used—pneumatic steam-heated, steam-heated tube-type, gas-fired steam-heated, or pneumatic gas-fired—constitute the principal component of the briquetting process.

When pneumatic gas-fired dryers are used, the briquetting process for peat consists in the following steps: sorting and (in some cases) pulverizing of the delivered peat to dimensions of no more than 6–10 mm; drying by means of smoke-producing gases in a pneumatic gas-fired dryer (tube-type or with a grinding fan or hammer mill) with recovery of the dried peat in cyclone separators; compacting at pressures of 7–20 meganewtons per m²; and cooling of the briquettes to a temperature of 40°C in cooling troughs.

The technology of briquetting has progressed through development and introduction of new processes and intermediate stages, new binders, and new equipment, both for the production of a high-quality smokeless fuel for household use and for the manufacture of a fuel for continuous coking processes; the latter is designed to broaden the raw material base and improve the economy of the coke industry.

The world production of coal briquettes amounts to approximately 110 million tons a year, of which briquettes made from brown coal account for 85 percent. In 1968 the USSR produced approximately 8 million tons of coal briquettes (70 percent from brown coal) and approximately 7 million tons of peat briquettes.

In the briquetting of metal ores, the raw materials used include iron ore fines (separate or mixed with a fuel), finely ground and powdered nonferrous ores, charge dust from blast furnaces, and other metallurgical wastes. The binders used include lime, various cements, and liquid glass. The briquetting is done in roller presses or auger extruders according to the following plan: batching and mixing of the ores with binders, compacting of the mixture, and consolidation of the briquettes to provide strength; the last step includes seasoning, roasting, steaming, and drying. Iron ore briquettes are used in open-hearth and blast furnaces; non-ferrous ore briquettes are used in water-jacketed and reverberatory furnaces.

Metal chips and ferrous and nonferrous metal wastes may also be efficiently briquetted. The direct use of a loose, high-volume mass of metal presents several difficulties: oxidation during storage, inconvenience of transportation, the production of carbon monoxide fumes during combustion, and others. The briquetting of metal chips eliminates such problems and makes it possible to manufacture a totally useful metal product from the chips. The briquetting consists in compacting chips that have been finely crushed and freed of such impurities as oil; both hydraulic and mechanical presses are used. In some instances the process is facilitated by heating the material directly during compacting.

The briquetting process is also used for various other materials (seeFOOD CONCENTRATES, FEED BRIQUETTES, and FEED BRIOUETTER).