Furnaces processing

A breakthrough came in 1957 with the introduction by TVA of superphosphoric acid and of base solutions made by ammoniation of that acid (90,91). This superphosphoric acid, which at first was made by the electric-furnace process, contained 76% P2 5 compared to 54% P2 s commercial... 

This reaction is self-sustaining without the need for additional heating once the reactants are heated to 430—450°C 10—12% HCl is produced by this process as compared to 30—60% HCl from the Mannheim furnace process. 

Opera.tlon, Because of the long residence time of the materials (8—10 h), the blast furnace process can exhibit considerable inertia, and control is usually appHed where the goal is maintaining smooth, stable input conditions. One of the most important aspects of blast furnace control is supply of consistent quaUty raw materials, which is why there is a strong emphasis on quaUty control at coke plants, peUeti2ing plants, and sinter plants (see Quality ASSURANCE/QUALITY control). 

The most common method of converting iron ore to metallic iron utilizes a blast furnace wherein the material is melted to form hot metal (pig iron). Approximately 96% of the world s iron is produced this way (see Iron). However, in the blast furnace process energy costs are relatively high, pollution problems of associated equipment are quite severe, and capital investment requirements are often prohibitively expensive. In comparison to the blast furnace method, direct reduction permits a wider choice of fuels, is environmentally clean, and requires a much lower capital investment. 

Manufacture. Phosphoric acid, H PO, is the second largest volume mineral acid produced sulfuric acid is the first. The greatest consumption of phosphoric acid is in the manufacture of phosphate salts, as opposed to direct use as acid. Markets are differentiated according to the purity of the acid. Phosphoric acid is produced commercially by either the wet process or the thermal (furnace) process. Thermal acid, manufactured from elemental... 

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

Phosphorus [7723-14-0] is a nonmetaUic element having widespread occurrence in nature as phosphate compounds (see Phosphoric acid and phosphates). Fluorapatite [1306-03-4], Ca F(P0 2> is the primary mineral in phosphate rock ores from which useful phosphoms compounds (qv) ate produced. The recovery from the ore into commercial chemicals is accompHshed by two routes the electric furnace process, which yields elemental phosphoms and the wet acid process, which generates phosphoric acid. The former is discussed herein (see Furnaces, electric). Less than 10% of the phosphate rock mined in the world is processed in electric furnaces. Over 90% is processed by the wet process, used primarily to make fertilisers (qv). 

The electric furnace process generates four streams that can be considered by-products slag, ferrophos, precipitator dust, and carbon monoxide off-gas. The approximate composition of the slag and precipitator dust are given in Table 3. These vary somewhat among different phosphoms manufacturers. 

Some phosphides, such as titanium phosphide [12037-65-9] TiP, can be prepared bypassing phosphine over the metal or its haUde. Reaction of phosphine with heavy metal salt solutions often yields phosphines that may contain unsubstituted hydrogens. Phosphides may also be prepared by reducing phosphoms-containing salts with hydrogen, carbon, etc, at high temperatures, the main example of which is the by-product formation of ferrophosphoms in the electric furnace process for elemental phosphoms. Phosphoms-rich phosphides such as vanadium diphosphide [12037-77-3] may be converted to lower phosphides, eg, vanadium phosphide [12066-53-4] by thermal treatment. 

Carbon Black. This is the principal reinforcing filler used in mbber. Carbon black is made by three processes the furnace process, the thermal process, and the channel process. Over 97% of black is made by the furnace process (see Carbon, carbon black). 

The furnace process involves injecting low end fraction of cmde oil, eg. Bunker Euel C, into a heated chamber. The temperature, shape of the injectors of the oil, rate of injection, and other factors are controlled to produce black fillers of different particle si2e and stmcture. The particle si2e and stmcture control the reinforcing character of the carbon black. There are 30 common grades of carbon black used in the mbber industry. There are numerous specialty grades produced, and several hundred are used in plastic, conductive appHcations, and other uses. 

Several systems are under development that will reduce or negate casing movement with the carbon (34—36). These will be more acceptable to furnace processes where product contamination by the casing material is undesirable. 

The oil-fiimace process, based on the partial combustion of Hquid aromatic residual hydrocarbons, was first introduced in the United States at the end of World War II. It rapidly displaced the then dominant channel (impingement) and gas-furnace processes because it gave improved yields and better product quahties. It was also independent of the geographical source of raw materials, a limitation on the channel process and other processes dependent on natural gas, making possible the worldwide location of manufacturing closer to the tire customers. Environmentally it favored elimination of particulate air pollution and was more versatile than all other competing processes. 

Table 10. Furnace Process Special Grades for Pigment Applications in Inks, Plastics, Paints, and Paper...

Table 11 Hsts the types and appHcations of special pigment-grade carbon blacks. Included in this Hst are thermal black and lampblack. Over 40 special black grades have been developed based on the furnace process having a broad range of surface areas, from 20 m /g to over 1500 m /g. The lower surface area products are used in printing inks and tinting. The high area, more expensive products find use in high color enamels and lacquers.

The earliest method for manufacturiag carbon disulfide involved synthesis from the elements by reaction of sulfur and carbon as hardwood charcoal in externally heated retorts. Safety concerns, short Hves of the retorts, and low production capacities led to the development of an electric furnace process, also based on reaction of sulfur and charcoal. The commercial use of hydrocarbons as the source of carbon was developed in the 1950s, and it was still the predominate process worldwide in 1991. That route, using methane and sulfur as the feedstock, provides high capacity in an economical, continuous unit. Retort and electric furnace processes are stiU used in locations where methane is unavailable or where small plants are economically viable, for example in certain parts of Africa, China, India, Russia, Eastern Europe, South America, and the Middle East. Other technologies for synthesis of carbon disulfide have been advocated, but none has reached commercial significance. 

Depending on energy and raw material costs, the minimum economic carbon disulfide plant size is generaHy in the range of about 2000—5000 tons per year for an electric furnace process and 15,000—20,000 tons per year for a hydrocarbon-based process. A typical charcoal—sulfur facHity produces approximately 5000 tons per year. Hydrocarbon—sulfur plants tend be on the scale of 50,000—200,000 tons per year. It is estimated that 53 carbon disulfide plants existed throughout the world in 1991 as shown in Table 2. The production capacities of known hydrocarbon—sulfur based plants are Hsted in Table 3. The United States carbon disulfide capacity dropped sharply during 1991 when Akzo Chemicals closed down a 159,000 ton per year plant at Delaware City, Delaware (126). The United States carbon disulfide industry stiH accounts for about 12% of the total worldwide instaHed capacity. 

Crambe was introduced to the United States in the 1970s. It is an oilseed, the oil of which is very high in emcic acid [112-86-7] (13-docosanoic acid), C22H42O2. This oil can be used to provide industrial lubricants, especially those needed for the basic oxygen furnace process for making steel (qv). Crambe is grown in relatively small volume in the midwestem United States. 

S 111 fi 1 r-reco ve ry p lants Carbon-black plants (furnace process) Primary lead smelters Fuel-conversion plants Sintering plants Secondary metal-production plants Chemical-process plants ... 

The electric arc furnace process accounted for about 25% of the 1982 U.S. steelmaking capacity (14). Most of the raw material used for the process is steel scrap. Pollutants generated by the electric furnace process are primarily particulate matter and CO. The furnaces are hooded, and the gas stream containing the particulate matter is collected, cooled, and passed to a bag-house for cleaning. Venturi scrubbers and ESPs are used as control devices at some mills. Charging and tapping emissions are also collected by hoods and ducted to the particulate matter control device. 

Some gas processes use direct fired furnaces. Process fluid flows inside tubes that are exposed to a direct fire. In this case radiant energy is important. Furnaces are not as common as other devices used in production facilities because of the potential fire hazard they represent. Therefore, they are not discussed in this volume. 

These formerly involved the use of banks of externally heated, horizontal retorts, operated on a batch basis. They were replaced by continuously operated vertical retorts, in some cases electrically heated. Unfortunately none of these processes has the thermal efficiency of a blast furnace process (p. 1072) in which the combustion of the fuel for heating takes place in the same chamber as the reduction of the oxide. The inescapable problem posed by zinc is that the reduction of ZnO by carbon is not spontaneous below the boiling point of Zn (a problem not encountered in the smelting of Fe, Cu or Pb, for instance), and the subsequent cooling to condense the vapour is liable, in the presence of the combustion products, to result in the reoxidation of the metal ... 

This is a more advanced partial combustion process. The feed is first preheated and then combusted in the reactor with a limited amount of air. The hot gases containing carbon particles from the reactor are quenched with a water spray and then further cooled by heat exchange with the air used for the partial combustion. The type of black produced depends on the feed type and the furnace temperature. The average particle diameter of the blacks from the oil furnace process ranges between 200-500 A, while it ranges between 400-700 A from the gas furnace process. Figure 4-4 shows the oil furnace black process. 

Figure 4-4. Carbon black (oil black) by furnace process of Ashland Chemical...

Selected properties of carbon black from an oil furnace process... 

Twin-fluid medium pressure 1-lOMW Yes All types (mainly class G) 1-4 20 <5 1 Wide range of shapes mainly used for process apphcations Kilns, furnaces, processes requiring special flame characteristics... 

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