COKE MAKING

THIS INVENTION relates to coke making. In particular, the invention relates to a method of making coke and to a coke product.

Conventional coke making methods or processes of which the applicant is aware require selecting an appropriate mixture of hard and soft coking coals to obtain coke of desired properties. A drawback of these conventional processes is thus that hard coking coal is always needed as part of the coking coal mixture, which has a cost implication. However, for industries that rely on market coke or coke breeze, coke properties similar to metallurgical coke are not required. A coke making method which does not require hard coking coal will be advantageous. Coke obtained by such a method can be used as a reductant for instance as required by the ferro alloy industry.

According to one aspect of the invention, there is provided a method of making coke, the method including feeding an admixture of coking coal and non-coke fines susceptible to microwave volumetric heating and/or induction heating into a coke oven; and heating the admixture in the substantial absence of air to drive off volatile compounds from the coal thereby producing coke, the heating being at least partially by microwave irradiation and/or induction heating.

The coking coal may be soft coking coal. Instead, the coking coal may be an admixture of hard coking coal and soft coking coal. The admixture may also include non-coking coal. Although the non-coke fines may include coke fines, the non-coke fines will comprise predominantly non-coking materials.

The non-coke fines may be selected from the group consisting of a ferro alloy, a ferro alloy ore, a chromium compound, a chromium ore, a nickel compound, a nickel ore, a manganese compound, a manganese ore, a titanium-containing material, and mixtures of two or more of these.

The manganese compound or the manganese ore, when present, may include one or more of a manganese oxide, a manganese carbide, silica manganese, or silica ferromanganese.

The ferro alloy or ferro alloy ore, when present, may include one or more of

FeCr, FeMn, FeSi, FeMo, FeTi, FeNi, FeW or FeV. The ferro alloy or ferro alloy ore may be low in carbon content, i.e. less than 3 %, or high in carbon content, i.e. more than 3 %.

In one embodiment of the invention, the fines material is a ferrochromium alloy or ore fines. In another embodiment of the invention, the fines material is a ferromanganese alloy or ore fines. Typically, ferrochromium and ferromanganese alloy fines include significant amounts of carbon, e.g. up to 8%, which improve their microwave heating susceptibility and possibly also their induction heating susceptibility.

When titanium containing materials are used, the use of the coke in blast furnaces may advantageously lead to the protection of refractory linings of the blast furnace through formation of TiC not taken up by formed iron.

The admixture may also be heated, in addition to the microwave or induction heating which heats the admixture volumetrically at a molecular level, by means of hot gas, e.g. by combusting a coke-oven gas obtained from the coal being coked.

Advantageously, fines susceptible to microwave heating or induction heating, and being subjected to such microwave irradiation or induction heating, act as microwave receptors enhancing heat transfer and also as a seeding material during coking. Recycling of coke as seeding material may thus be eliminated or reduced.

The admixed non-coke fines may be pre-reduced during coking, which may be advantageous in a downstream metallurgical process in which the coke is used.

The fines may be obtained from a downstream metallurgical process in which the coke is used.

The method may include compacting the admixture of coking coal and non-coke fines, typically prior to feeding the admixture into the coke oven. In other words, the method may include feeding compacted or stamped artefacts of the admixture into the coke oven. These high density artefacts will enhance the properties of the coke.

The admixture may include at least one flux.

The at least one flux may be selected from the group consisting of silicates, dolomites, lime, limestone, clays, sodium, lithium, halide fluxes, electric arc furnace dust and two or more of these.

According to another aspect of the invention, there is provided a coke product comprising particulate material which includes coke in admixture with a non- coke material selected from the group consisting of a ferro alloy, a ferro alloy ore, a chromium compound, a chromium ore, a nickel compound, a nickel ore, a manganese compound, a manganese ore, a titanium containing material and mixtures of two or more of these.

The non-coke material may be present in a concentration of at least 1 % by mass, preferably at least 5 % by mass, more preferably at least 10% by mass. Typically, the non-coke material will be present in a concentration of less than 70% by mass, preferably less than 50% by mass, more preferably less than 30% by mass.

Care should be taken to restrict the concentration of the non-coke material to ensure coke formation, otherwise agglomeration of coke particles will be sacrificed. The concentration of non-coke material can be empirically determined, through routine experimentation and is also a function of the type of coking coal used as well as respective particle size distributions.

The non-coke material is preferably present predominantly as a metal or alloy in a reduced state. The non-coke material may even be present as one or more ores.

The coke product may include at least one flux. The at least one flux may be selected from the group consisting of silicates, dolomites, lime, limestone, clays, sodium, lithium, halide fluxes, electric arc furnace dust and two or more of these.

The invention will now be described, by way of example only, with reference to the following examples and the accompanying schematic drawings in which

Figure 1 shows a method of making coke in accordance with the invention; Figures 2 - 5 show typical dielectric constant and dielectric loss factor values for various materials, as a function of temperature; and

Figure 6 shows a typical microwave heating curve for a 10% admixture of high carbon ferrochrome with 90% Grootegeluk Mine semi-soft coking coal.

Referring to Figure 1 of the drawings, reference numeral 10 generally designates a method of making coal in accordance with the invention.

In the method 10, an admixture of coking coal and non-coking coal and metal-containing non-coke fines, generally indicated by reference numeral 12 is fed to a coke oven 14. The metal-containing non-coke fines material is typically selected from the group consisting of a ferro alloy, a ferro alloy ore, a chromium compound, a chromium ore, a nickel compound, a nickel ore, a manganese compound, a manganese ore, a titanium-containing material, and mixtures of two or more of these. In the coke oven 14, the admixture is heated using microwave irradiation or induction heating, generally indicated by reference numeral 16. Such microwave heating or induction heating can be effected in conventional fashion, e.g. using microwaves at 915 MHz or 2.45 GHz, in monomode or multimode microwave applicators. Coke oven gas 18 is driven off. The coal is allowed to coke for a relatively short period, e.g. from 1 to 4 hours, compared to 21 to 65 hours for conventional slot coke ovens or stamp non- recovery ovens, respectively, whereafter coke 20 is removed. The metal oxide of the metal-containing non-coke fines material is at least partially reduced during the coking process.

In the coke oven 14, the coking coal is converted to coke as a result of the heating of the coal in the absence of air, to drive off volatile compounds. The resultant

coke is a hard, porous carbon material that can be used for many purposes, including reducing iron oxides or ores in a blast furnace.

Coking coals are the coals which, when heated in the absence of air, first melt, go into a plastic state, swell or contract and then re-solidify to produce a solid coherent mass called coke. Coking coal is typically carbonised at a temperature around 900O - 1 100O to reach a desired degree of devolitisation to produce coke of desired mechanical and thermo-chemical properties. During carbonisation, coking coals undergo a transformation into a plastic state at around 350 ⁹ C - 450 ⁹ C, swell or contract and then re-solidify at around 450O - 550O to give semi-coke and then coke. In a typical conventional coke oven, after coal is charged into the oven, plastic layers are formed adjacent to hot walls of the oven, and with the progress of time, the plastic layers move towards the centre of the oven. The quality and quantity of plastic layers in conventional coke ovens are of extreme importance and determine the inherent strength of the coke produced. For producing coke of good quality, coals should have a certain degree of maturity, good rheological properties, a wide range of fluidity and low concentrations of inerts.

In order to assist with the heating of the admixture of coking coal and non- coking coal and fines in the oven 14, a portion of the coke oven gas 18 can be combusted. One conventional way of doing this is to combust the coke oven gas in heating flues in side walls of the coke oven 14, with waste gases from this combustion passing out through a stack or chimney. Typically, when this approach is used, at about 20 or 30 minute intervals, the flows of coke oven gas, air and waste gas are reversed to maintain uniform temperature distribution across the wall of the coke oven.

A feature of the present invention is that the fines material admixed with the coking coal and the non-coking coal (when present) is a material susceptible to microwave volumetric heating and/or induction heating. Thus, at least a portion of the heat 16 is provided by microwave irradiation or induction heating, allowing the relatively short coking time in the oven for the method of the invention. When this happens, the fines material acts as a microwave receptor enhancing heat transfer and also as a seeding material during coking. This allows the conventional recycling of coke as seeding material to be eliminated or reduced.

Dielectric constant and loss factor measurements can be determined in order to establish if a selected material can be heated through microwaves at a preselected frequency.

The dielectric constant, or relative permittivity, is the ratio of the permittivity of the material to the permittivity of free space. It is also the property of a material that determines the relative speed that an electrical signal will travel in that material. The lower the dielectric constant the easier the signal will pass through that material and vice versa. The dielectric constant of a material depends on the intrinsic property and moisture of the material, frequency and temperature of the material. The loss factor gives the microwave power that can be dissipated in a given material. The higher the loss factor the easier the material absorbs the incident microwave energy. Materials with loss factors between 0.01 and 5 are, in general, considered as good candidates for microwave heating. Materials possessing high loss factors of >5 may lead to thin surface heating of only a few mm.

Figures 2 to 5 give typical dielectric constant and loss factor values obtained, as an example, for various materials that may be employed in the method of this invention. These properties were determined with a Hewlett Packard 8753A Vector Automatic Network Analyzer operating at 915 MHz for materials up to 1 10OO. Figures 2 to 5 show the high microwave susceptibility as a function of temperature for selected materials which varies significantly from material to material.

Example 1

Materials used were -1 mm Grootegeluk Mine semi soft coking coal containing a high level of volatiles (35.7%). A microwave heating receptor, i.e. high carbon FeCr containing 7% carbon was milled down to -500 μm in order to optimise contact with the coal fines during thorough mixing at a weight ratio of 90% (dry basis) coal and 10% high carbon FeCr. 5 kg of pressed briquettes, of 40 mm length and 25 mm width, was transferred to a 12 litre higher order single mode microwave reactor and heated at 4 kW effective forward power for 3 hours from cold start at 915 MHz. The Applicant believes that heating periods may be reduced during a continuous microwave heating process. Figure 6 shows the microwave heating profile of the coal bed at three

different positions during a cold start process, indicating the commercial viability of the process.

When the temperature of the mixture reached 400O, the coal plasticized and the microwave heating rate of the mixture increased rapidly up to 67OAnJn, enhancing the overall heating rate of the mixture and driving the coking process to significantly earlier completion to 1000O than through conventional heating mechanisms, indicating the unique heating benefit obtainable through utilizing coking coal and microwave heating receptor admixtures.

Example 2

Theoretical calculations, using Factsage (commercially available software and database), indicated that chromite ore will be partially reduced at 1000O in a coking coal environment as set out above to form Fe metal, Cr ₂ O ₃ , and Fe and Cr carbides. This partially reduced material may form in a short period as illustrated above.

The theoretical calculations were tested by conducting an experiment using similar conditions as described in Example 1 but with chromite ore. The composite coke was reacted until a temperature of 1 100 degrees Celsius was reached and the product was allowed to cool down under nitrogen without the presence of microwaves.

An SEM micrograph of the coke matrix which formed showed a reaction zoning effect obtained around a Fe chromite particle embedded in the coke matrix. This zoning effect is due to Fe migration during partial reduction of the chromite where about 8% Fe was removed from the chromite particle, leading to a Cr enrichment of about 4%. The micrograph also showed the precipitation of the formed Fe metal in the middle of the Fe chromite particle with the following composition: 89 wt% Fe, 5.4 wt% Cr, 1 wt% C, 0.8 wt% Mg, 0.6 wt% O ₂ , and 0.5% Al. Fe metal also was precipitated around the edges of the particle as a Fe sulphide (most probably pyrrhotite). (The sulphur originated from the coal feed material as well as some sulphides associated with the chromite ore).

Example 3

Heating of a similar 5 kg sample as described in Example 1 in an induction heating coil fitted with a graphite sleeve and an alumina crucible for 4 hours at 27 kHz, again from a cold start, resulted in coking of the coal mixture at 1 100O with 5 kW power. Small scale tests (25g samples) showed that higher amounts of ferro alloy added improved coupling in the heating coal, indicating a lesser dependency on the thickness of the graphite sleeve and radiation heating therefrom.

It is envisaged that the method 10 may advantageously be used with a downstream metallurgical process in which the coke and admixtures are employed as a reductant and a source of thermal energy. Particularly, the downstream metallurgical process may be a process generating ferro alloys such as ferro chromium and ferro manganese fines. Both ferro chromium fines and ferro manganese fines contain significant amounts of carbon, even up to 8%, which will improve the microwave heating susceptibility and possibly also the induction heating susceptibility of the admixture. Thus, by using these fines as microwave receptors during coke formation, fines generated in the metallurgical processes can be recycled and be built into the reductants during coke formation and then used in the metallurgical processes, and recycling of coke as seeding material can be eliminated or reduced. Such reductants can advantageously include a flux already selected for the metallurgical process in which the coke as reductant is to be used.

By way of development, the admixture of coking coal and fines may be provided as compacted or stamped blocks of high density material. Coke formed from these compacted or stamped blocks will show enhanced properties.

An advantage of the process 10, as illustrated, is that it provides the opportunity to recycle fines generated by metallurgical processes and to include the fines into the structure of a reductant that can then be used in the metallurgical processes. The method of the invention, as illustrated, can also contribute towards alleviating problems being experienced regarding coke shortages and environmental pollution. The presence of pre-reduced fines forming part of the coke may also be

advantageous when the coke is used in a downstream metallurgical process, such as enhancing the reactivity of the coke reductant, as well as inducing partial reduction of admixed ores. Furthermore, as the fines material is used as a microwave receptor and a seeding precursor for coke formation, recycling of coke can be eliminated or reduced. Lastly, but not the least, the method of the invention requires significantly less time than conventional coking processes to produce coke.