J. L. Sundholm, H. S. Valia, F. J. Kiessling... Manufacture of Metallurgical Coke and Recovery of Coal Chemicals

Carbon as a Reducing Agent

Although the oxides of iron may be reduced to metallic iron by many agents, carbon (directly or indirectly) is the reducing agent found to be best suited for the economical production of iron. Car-bon of suitable reactivity and physical strength was at one time produced from wood by distillation, yielding wood charcoal; but for the operation of a modern large blast furnace the carbon required for the smelting of iron is obtained from the destructive distillation of selected coking coals at temperatures in the range from 900°C to 1095°C (1650°F to 2000°F).

Chemical Effects of Coking

Coal is made up principally of the remains of vegetable matter which has been partially decomposed in the presence of moisture and the absence of air and subjected to variations in temperature and pressure by geologic action, see Chapter 6. It is a complex mixture of organic compounds, the principal elements of which are carbon and hydrogen with smaller amounts of oxygen, nitrogen and sulfur. It also contains some noncombustible components called ash. The ash consists primarily of inorganic compounds which became imbedded in the coal matrix during the coalification process.
The chemical compounds making up coals, like most of those in animal and vegetable life, are unstable when subjected to a high degree of heat or thermal treatment. When heated to high tem peratures, in the absence of air, the complex organic molecules break down to yield gases, together with liquid and solid organic compounds of lower molecular weight and a relatively non-volatile carbonaceous residue (coke).

Coke, then, is the residue from the destructive distillation of coal. Structurally, it is a cellular, porous substance which is heterogeneous in both physical and chemical properties. The physical properties of metallurgical coke, as well as its composition, depend largely upon the coal used and the temperature at which it is carbonized. Not all coals will form coke, and not all-coking coals will give the same firm, cellular mass characteristic of coke suitable for metallurgical purposes.

Some coals will produce an acceptable coke without blending with other coals, while others are usable only as constituents of blends. The type and method of operation of coking facilities also exert a profound influence on the quality and yield of coke for the blast furnaces.

Kinds of Coke

There are three principal kinds of coke, classified according to the methods by which they are manufactured: low-, medium- and high temperature coke. Coke used for metallurgical purposes must be carbonized in the higher ranges of temperature (between 900°C and 1095°C) (1650°F and 2000°F) if the product is to have satisfactory physical properties. Even with good coking coal, the product obtained by low temperature carbonization between 480°C and 760°C (900°F and 1400°F) is unacceptable for good blast furnace operation.

Important Properties of Metallurgical Coke

Probably the most important physical property of metallurgical coke is its strength to withstand breakage and abrasion during handling and its use in the blast furnace. In the United States, the standard ASTM tests used to evaluate these properties are the stability index for breakage and the hardness index for abrasion. Both of these tests involve tumbling coke of selected size in a standard drum rotated for a specific time at a specific rate. The stability index and the hardness index are the percentages of coke remaining on 1 in. and 1/4 in. screens, respectively, when the coke is screened after tumbling.

In modern blast furnace practice, the trend is toward use of iron-bearing burden materials of controlled size such as sinter and pellets; thus, the size of the coke used in the burden assumes more importance then in the past when only crude ore was used. The size of coke produced in byproduct ovens is somewhat dependent upon the type of coal, heating rate, width of the ovens, and the bulk density of the coal charge, greater amounts of low-volatile coal, wider ovens and greater bulk density of the coal charge generally tend to produce larger coke while faster heating rates tend to produce smaller coke. Because relatively uniform size is desired, crushing and screening of the coke must be resorted to when controlled size is desired. Most blast furnace operators prefer coke sized between about 18.5 and 76 mm (3/4 in. and 3 in.) for optimum furnace performance. Other physical properties of the coke such as porosity, density and combustibility are controllable only to a small extent, and their importance in affecting blast furnace operation has not been definitely established.

Methods of Manufacturing Metallurgical Coke

There are two proven processes for manufacturing metallurgical coke, known as the beehive process and the byproduct process. In the beehive process, air is admitted to the coking chamber in controlled amounts for the purpose of burning therein the volatile products distilled from coal to generate heat for further distillation. In the byproduct method, air is excluded from the coking chambers, and the necessary heat for distillation is supplied from external combustion of some of the gas recovered from the coking process (or, in some instances, cleaned blast furnace gas or a mixture of coke oven and blast furnace gas). With modern byproduct ovens, properly operated, all the volatile products liberated during coking are recovered as gas and coal chemicals, and, when coke oven gas alone is used as fuel, about 40% of the gas produced is returned to the ovens for heating purposes. While the beehive process was the leading method for manufacture of coke up to 1918, largely the byproduct process as discussed later in this chapter now has replaced it. There is a difference of temperature of coking in the two processes, that of the byproduct being somewhat Manufacture of Metallurgical Coke and Recovery of Coal Chemicals lower than the beehive. Beehive coke is usually larger, though not as uniform in size. In general, properly carbonized beehive coke and byproduct coke both are silvery gray in appearance. A modification of the beehive technology, known as non-recovery ovens, is gaining prominence and is discussed in Section 7.8.

Other processes for producing metallurgical coke are known as continuous processes; many variations have been proposed but none has been adopted on a commercial scale. In one continuous process, finely pulverized coking or non-coking coal is dried and partially oxidized with steam or air in fluidized bed reactors to prevent agglomeration when coking coal is used. The reactor product is carbonized in two stages a successively higher temperatures to obtain a char. Using a binder produced from tar obtained in the carbonization stages, the char is briquetted in roll presses. The “green” briquettes are cured at low temperatures, carbonized at high temperatures, and finally cooled in an inert atmosphere to produce a metallurgical coke of low volatile content. This type of coke often is referred to as formcoke. Briquetting will be discussed again later in this chapter.

Products of Coal Carbonization

The reactions occurring during the carbonization of coal for the production of metallurgical coke are complex. The process can be considered as taking place in three steps: (a) primary breakdown of coal at temperatures below 700°C (1296°F) yields decomposition products some of which are water, oxides of carbon, hydrogen sulfide, hydroaromatic compounds, paraffins, olefins, phenolic, and nitrogen-containing compounds; (b) secondary thermal reactions among these liberated primary products as they pass through hot coke, along hot oven walls and through highly heated free space in the oven involve both synthesis and degradation. A large evolution of hydrogen and the formation of aromatic hydrocarbons and methane occur in the stage above 700°C (1296°F).Decomposition of the complex nitrogen-containing compounds produces ammonia, hydrogen cyanide, pyridine bases and nitrogen; (c) progressive removal of hydrogen from the residue in the oven produces hard coke.

During carbonization, from 20–35% by weight of the initial charge of coal is evolved as mixed gases and vapors which pass from the ovens into the collecting mains and are processed through the coal-chemical recovery section of the coke plant to produce coal chemicals. When the production of coke is accomplished in modern byproduct coke ovens with equipment for recovering the coal chemicals, one ton of coking coal in typical American practice yields about the following proportions of the coke and coal chemicals presented in Table 7.1, depending upon the type of coal carbonized, carbonization temperature and method of coal-chemical recovery.

The coke oven gas contains the fixed gases so classified because they are gases at 760 mm (29.92 in.) pressure and 15.5°C (60°F). They are: hydrogen, H ; methane, CH ; ethane, C H ; carbon 2 4 2 6 monoxide, CO; carbon dioxide, CO ; illuminants which are essentially unsaturated hydrocarbons, such as ethylene, C H ; and acetylene, C H . Other fixed gases present are: hydrogen sulfide, H S; 2 8 2 2 2 ammonia, NH ; oxygen, O ; and nitrogen, N .

Other substances in the raw gases and vapors leaving the ovens, which are liquids at ordinary temperatures and pressures, are discussed here.

Ammonia Liquor
Primarily this is the water condensing from the coke oven gas and is an aqueous solution of ammonium salts of which there are two kinds—free and fixed. The free salts are those which are decomposed on boiling to liberate ammonia. The fixed salts are those which require boiling with an alkalisuch as lime to liberate the ammonia.

Tar
Tar is the organic matter that separates by condensation from the gas in the collector mains. It is a black, viscous liquid, a little heavier than water. The following general classes of compounds may be recovered from tar: pyridine, tar acids, naphthalene, creosote oil and coal-tar pitch.

Light Oil
Light oil is a clear, yellow-brown oil somewhat lighter than water. It contains varying amounts of coal-gas products with boiling points from about 40°C to 200°C, and benzene, toluene, xylene and solvent naphthas are the principal products recovered from it.

Recovery of Coal Chemicals
In the recovery of coal chemicals, the first step is the recovery of the basic crude materials (coke oven gas, ammonia liquor, tar and light oil) as a primary operation in accordance with commercial practice. Secondary operations consist of the processing of these primary products to separate them into their components as discussed in detail in Section 7.7 of this chapter.

<< На каталог

В начало ^^