US 3240566 A
Description (OCR text may contain errors)
March 15, 1966 v. L. BULLOUGH ETAL 3,
METHOD OF OBTAINING MAXIMUM SEPARABILITY OF ORGANIC MATTER FROM ASH IN COAL EXTRACTION PROCESSES Filed April 23, 1965 3 Sheets-Sheet 1 COAL RECEIVING WATER CRUSHING AND DRYING SOLVENT MIXING L'GHT DIGESTION I CENTRIFUG CENTRIFUGE (souo BOWL) (DISC) SLUDGE COKER SOLVENT STILL L. PITCH SLUDGE COKE COKER GAS COKE TAR 74 fie/(imam INVENTORS.
March 15, 1966 v. BULLOUGH ETAL 3,240,566
METHOD OF OBTAINING MAXIMUM'SEPARABILITY OF ORGANIC MATTER FROM ASH IN GOAL EXTRACTION PROCESSES Filed. April 23, 1963 5 Sheets-Sheet 5 ('WLV) BHFISSEIEId EIQI'IVE) M M INVENTORS.
United States Patent 3 240 566 METHOD OF OBTAININGMAXIMUM SEPARABIL- ITY OF ORGANIC MATTER FROM ASH IN COAL EXTRACTION PROCESSES Vaughn L. Bullough, Florence, Ala., and Wilburn C.
Schroeder, College Park, Md., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Apr. 23, 1963, Ser. No. 275,094 6 Claims. (Cl. 23-209.9)
This invention relates to the production of high purity carbon of the type employed in carbon and graphite electrodes, and it deals more particularly with a novel process for the removal of impurities from bituminous coal to produce a substantially ash-free carbon which can be used in the manufacture of carbon electrodes suitable for the electro-metallurgical industries, especially in the aluminum industry.
In the production of aluminum metal by the electrolysis of aluminum oxide, there is employed a carbon anode conventionally made from petroleum or coke-oven pitch coke. The carbon anode is consumed during the electrolysis, causing many of the impurities present in the carbon, such as silicon, vanadium and iron, to pass into the aluminum.
Generally, a purchase specification of 1% total ash is imposed on coke used in the manufacture of electrodes for the aluminum industry. An auxiliary specification of less than 2% sulfur in the calcined coke is also imposed. This severely limits the raw materials that can be used in the manufacture of electrodes, and suitable materials heretofore have been confined to coke produced from certain grades of petroleum residuum, coke produced by carbonization of coal tar or coke-oven pitch, and coke produced by the carbonization of gilsonite. There are reported instances where cokes of marginal purity have been produced from certain seams of coal by froth floatation and chemical cleaning, but there are no major seams of coal in the United States that can be cleaned adequately by froth flotation to produce coke that will meet the specification as to ash content of anodes for use in the aluminum industry. Chemical cleaning of coals, for example, with mineral acids, is an expensive procedure which limits its application to emergencies; and few seams of coal are available which are amenable to this process.
Because of its relatively high purity and availability, petroleum coke has generally been the material of choice for the preparation of anodes of the type heretofore discussed, but not all petroleum crudes yield a residuum of satisfactory quality for these purposes. In fact, the availability of crudes which yield a satisfactorily low sulfur and vanadium content for the production of carbon of desired purity is so limited that sources of pure carbon for future expansion of domestic aluminum production are uncertain.
An advantage of the present invention resides in the production of carbon having virtually no vanadium and sufficiently low sulfur for use in the aluminum industry.
It is an object of the present invention to provide a method for the economical production of anode quality coke from coal.
It is another object of the invention to provide a method for the solution of raw coal and the separation of ash from said solution, employing a self-sustaining regenerative solvent system.
It is a further object of the invention to provide a method for the solution of raw bituminous coal, employing a solvent derived from said coal, and coking the coal solution, whereby electrode quality coke can be obtained with the coal itself serving as substantially the sole raw material.
These and other objects will appear from the ensuing description.
The fundamental problem in the conversion of coal into virtually ash-free coke is that of effective separation of the ash and other mineral components of the coal. The ash-forming components comprise two groups (a) inherent ash, constituting the mineral materials which occurred in the living plant and which became incorporated in the coal upon coalification, and (b) extraneous ash, including material codeposited with the plant materials originally, or which has subsequently found its way into coal seams. Inherent ash contains iron, phosphorus, sulfur, calcium, potassium, and magnesium, and other plant nutrients. The extraneous ash may include common minerals such as kaolinite, pyrite, and calcite, clay and shale.
Among known methods for separation of the ash from coal are those based upon physical removal, such as floatsink, and froth flotation. Among chemical methods, there have been proposed treatments with mineral acids, and with alkalis. These methods have not resulted in providing a coke residue sufficiently low in ash to meet the specifications for carbon electrodes, or have been too expensive for commercial practice.
Coal solution processes have also been proposed for ash removal from coal. These are based upon the principle that coal, and particularly bituminous coal, is a complex mixture of polymeric molecules of unknown structure, which are present together with fusain or mineral charcoal. It is known that certain coal-based hydrocarbons act as solvents for some or all of the polymeric molecules, such solvents including, for example, creosote oil and some of its higher hydrocarbon constituents, such as anthracene oil and phenanthrene. A method of coal treatment of this type is disclosed in Rose et al., Patent 1,925,005 (1933). Lahari et al. (India Patent 49,729, December 14, 1954) disclose the separation of fine particles of ash from solutions of coal in solvents by settling, filtration or centrifuging. However, the solutions produced by the methods of Rose et al. and Lahari et al. are highly viscous, and contain large aggregates of colloidal coal particles with the fine ash so entrained that ordinary separation methods are unsuccessful.
In accordance with the invention it has been found that the soluble constituents of coal, particularly bituminous coal, can be dissolved in an aromatic hydrocarbon oil of relatively low viscosity and boiling range, under superatmospheric pressure, and at a temperature maintained at above the temperature of maximum solubility of the coal constituents.
The aromatic hydrocarbon oil which is advantageously employed for the solution treatment of coals, in accordance with the invention, is preferably a lower boiling fraction obtained by the fractional distillation of coal tar or coke oven tar. Preferably the fraction is a creosote type fraction from which light oils have been substantially removed, i.e. benzene, toluene, and mixed xylenes, so that the creosote type oil has an initial distillation temperature of about 180 C. at atmospheric pressure. The creosote type extraction oil which has been found suitable for the purposes of the invention may, however, depending upon the selection of operating conditions, include high boiling anthracene oil fractions, and in fact, it is within the contemplation of the invention that any make-up to the process necessitated by loss or removal from the system of the solvent-valuable lower boiling creosote type oil fractions, can be supplied from relatively inexpensive heavy residue creosote oil.
Thus, in accordance with a first aspect of the invention, bituminous coal can be extracted under critical and carefully controlled conditions, up to a solubility of more than 80%, in an aromatic liquid hydrocarbon creosote oil type solvent having a relatively low viscosity and a low initial boiling point of about 180 C. Moreover, as will be described more fully below, this aromatic liquid hydrocarbon extractant can be generated in the sustained operation of the process, so that bituminous coal becomes the only raw material needed in the process.
For a better understanding of the invention and its various objects, advantages, and details, reference is made to the accompanying drawings, which are exemplary of the present preferred embodiments thereof, and indicate typical operating conditions. In the drawings:
FIG. 1 is a flow sheet of the process of the invention, with the principal steps thereof illustrated in diagram form.
FIG. 2 is a plotted series of curves graphically illustrating the effect of temperature on the amount of coal dissolved in the solvent employed and the purity of the coke produced therefrom.
FIG. 3 is a typical heating curve for the digestion of coal in accordance with the invention.
In the practice of the first, or extraction aspect of the invention, there is selected a suitable bituminous coal, or other carbonaceous material of similar composition. Preferably freshly mined coal is used, and the raw coal is crushed and pulverized to optimum particle size. Examples of suitable bituminous coals include Alabama and Kentucky high volatile types. Coals of this type may analyze, for example, on the as-received basis, from 3545% volatile matter, 7284% carbon, and 2-10% ash, depending upon quality.
The coal is advantageously crushed and pulverized, then preheated, if desired, to reduce its moisture content. The crushing may be carried out in conventional equipment, for example, an air-swept ball mill with air classifier, and the moisture reduced by a flow of heated flue gas through the ball mill, to a level of about 1%.
The crushed and dried coal is then fed to a suitable mixing tank and mixed with a suitable amount of a creosote type extraction oil of the character described, or with an extraction oil fraction generated from within the process system. The coal extraction oil mixture is then transferred to a pressure digester. Alternatively, the coal and extraction oil can be fed directly into the pressure vessel, which serves both as mixer and digester. The pressure vessel comprises an autoclave equipped with heating elements, a cooling coil, a stirrer, and pressure and temperature indicating devices.
The finely divided coal is heated in the digester with the solvent at a temperature slightly in excess of the temperature required to achieve maximum solubility of the coal constituents in the extraction oil, preferably at a temperature from about 5 to about 50 C. in excess of the maximum solubility temperature. Where the coal and oil are premixed, the mixing may be carried out at a temperature of about 50 C. if the mixing tank is open, up to about 150 C. when the tank is closed.
The mixture of coal and extracting oil in the digester is brought to a temperature suitable for maximum solubility, as previously indicated. This temperature will generally lie in the range from about 380 C. to about 450 0., depending upon the type of coal. The pressure in the extraction vessel at these temperatures will rise to about 750 to 2000 lbs. per sq. in. gage. The mixture is stirred at a velocity sufiicient to maintain the solids in suspension. The mixture is held at the extraction temperature for a period of time from about 2 to 60 minutes but a retention time of 10 to 30 minutes is preferred. The temperature of the mixture is then reduced by cooling to between about and about 220 C. Any gaseous or liquid hydrocarbon material present having a boiling temperature below 160-220 C. is distilled off, and collected for use as fuel or as a valuable light oil byproduct of the process. The charge is then transferred to to a centrifuging system, described below, for separation of insolubles.
The weight ratio of extracting oil to coal may range from about 1:1 to about 6: 1, but is preferably maintained between about 2.511 to 4:1.
The temperature to which the coal and extracting oil mixture is heated has been found to have a marked influence in the purity of the carbon or coke ultimately obtained. As previously mentioned, this temperature must vlJe above that of maximum solubility of the coal constituents to produce a final carbon or coke which is substantially ash-free. It is thought that finely disseminated particles of ash in the hot extraction mixture at these elevated temperatures may act as nucleating sites for carbonization, and thus some material which would otherwise contribute to ash content in the final carbon product is trapped Within a partially carbonized matrix and removed by subsequent processing.
In accordance with a second aspect of the invention, there is provided a novel method of purification of the extraction mixture of digested coal solution, to separate therefrom a clarified coal solution, and a residue of undigested coal, carbon, and mineral ash. It has been found that the separation of the digested mixture of coal and extract is best carried out by a two-stage centrifuge treatment. The digested mixture from the pressure vessels, after removal of gas, volatile liquids, and cooling to 160- 220 C., as previously described, is fed as a steam, in a first stage, to a disc-type centrifuge. The overflow from this centrifuge, which is the clarified coal solution, is transferred to heated tanks to be held for distillation and coking The underflow, which contains about 20% solids comprising undigested coal, 'carbon or charcoal, and mineral ash, is fed, in a second stage to a solid bowl centrifuge where it is separated into an overflow which is cycled back to the disc centrifuge for further clarification The underflow comprises a sludge containing from 40-50% solids which is discarded from the system. As mentioned, both centrifuge stages operate at about 160-220" C.
The final step of the process of the invention comprises the carbonization of the purified coal solution discharged from the disc type centrifuge. This step converts the purified coal solution partly to a distillate and partly to a solid residue of carbon or coke. The amount of coke produced will generally range from about 13% to about 15% by weight of the amount of coal solution charged to the coking means. The carbonization of the coal solution can be carried out by any conventional type coking apparatus such as, for example, rotating drum, delayed or fluid cokers of the type used in the carbonization of coal, coal tar pitch, or petroleum residue, or in by-product coke ovens. If the latter are used, it (is desirable to subject the coal solution to a preliminary distillation or topping step to remove all possible solvent before charging the coke oven. Coking temperatures will generally lie in the range of 550700 C.
The distillate, which constitutes from about 7585% of the coal solution charged comprises a creosote type aromatic hydrocarbon containing liquid which meets the requirements previously disclosed for a suitable coal solvent. Accordingly when the process is in full operation this distillate serves as the solvent for the treatment of the raw coal and it is necessary only to add make-up heavy residue creosote oil, as indicated previously to offset process losses. Hence the novel process of the invention is virtually completely self-sustaining in that the required coal solvent is obtained from the coal itself in the operation of the process, so that raw coal is substantially the only raw material which need be employed.
The following table shows the properties of cokes obtained from Kentucky and Alabama c-oal after calcining the coke to 2450 F., in comparison with a typical petroleum coke:
TABLE I Properties of cakes from Kentucky coal and Alabama coal after calcining to 2450 F.
1 In ohms/inch/sq. inch.
The coke manufactured from bituminous coal in accordance with the invention Was found to be Well suited for use in anodes of alumina reduction cells of both Soderberg and pro-bake types, performing as well as or better than the petroleum coke used in regular anodes.
The following examples serve to illustrate the invention but are not to be regarded as limiting.
EXAMPLE 1 100 lbs. of high volatile Western Kentucky bituminous coal (volatile matter 40.5%, ash 9.9%, sulfur 3.1%) was pulverized to minus 48 mesh particle size and introduced into a heated stirred autoclave together with 300 lbs. extraction oil having an initial boiling point of 180 C. and the mixture was heated at 400 C. and at a pressure of 1300 p.s.i.g. for 30 minutes. The reaction mixture was cooled to 200 C. and centrifuged at that temperature in a disc-type centrifuge equipped for continuous solid discharge. The purified coal solution constituting the centrifuge overflow, upon coking, in a rotary steel drum at a temperature of about 900 F., was found to yield a coke which contained 0.42% ash.
EXAMPLE 2 The treatment described in Example 1 was repeated using an amount of extraction oil equal to twice the weight of the coal. The calcined coke obtained from the purified coal solution contained 0.26% ash.
EXAMPLE 3 The process of Example 1 was repeated employing three times the weight of the coal of an extraction oil distillate produced from the coking of the coal solution of Example 1. The product of the disc centrifuge, when coked, yielded a coke containing 0.39% ash.
EXAMPLE 4 100 lbs. of high volatile Alabama bituminous coal (35.4% volatile matter, 3% ash) was treated with three times its weight of extraction oil having an initial boiling point of 180 C. by heating in a stirred autoclave at 400 C. for 30 minutes at 1000 p.s.i.g. The coal solution mixture was centrifuged in a disc-type centrifuge at 200 C. and the purified solution was coked in a rotating steel drum at about 900 F. and the resultant coke was calcined at about 2400 F. in an induction furnace. The resulting coke analyzed 0.33% ash. When prepared in a suitable aggregate, the coke produced test electrode specimens of the following properties when compared to typical petroleum coke treated in comparable manner:
Petroleum Coal Coke Apparent Density, gJcc 1. 50 1. 54 Resistivity, ohms/m./mm. 59. 4 58.0 Compression strength, lbs/in. 7,088 7, 564
EXAMPLE 5 A purified solution of the same coal as used in Example 4 was prepared in a continuous digestion operation, then coked in a slot-type by-product coke oven and calcined to about 1100 C. Samples from carbon paste prepared from this coke for test in a commercial Soderberg cell were found to yield electrode specimens having the following properties:
Pctroleum Coal Coke Coke Apparent density, g./cc 1. 56 1. 60 Resistivity, ohms/m./mm. 61. 5 51. 5 Compression strength, lbs/in. 5, 750 8,
EXAMPLE 6 The same digested coal solution as in Example 5 was fed at 200 C. to a commercial size tar decanter centrifuge of the disc-type, equipped for continuous discharge of solids. The underflow stream from this centrifuge, containing essentially all of the mineral matter, undissolved coal, and mineral charcoal was fed to a solid bowl centrifuge with continuous solids discharge features. The cake discharged from the machine was approximately 50% solids. The overflow from the solid bowl machine was recharged to the disc-type centrifuge for further purification. This procedure was found to give the maximum purity and minimum loss of product from the system drag-out with the undesirable solids, and the highest centrifuge capacity per unit machine.
What is claimed is:
1. Method for the preparation of substantially ash-free carbon suitable for electrodes from bituminous coal, comprising the steps of:
(a) forming a mixture of a finely divided bituminous coal and from about 1 part to about 6 parts by weight of the coal of an aromatic hydrocarbon creosote type extraction oil having an initial boiling point of about 180 C.;
(b) heating said mixture for a period of from about 2 to about 60 minutes at a temperature from about 5 C. to about 50 C. in excess of the temperature required to achieve maximum solubility of the coal constituents in the oil and at a pressure from about 750 to about 2000 p.s.i.g. to attain maximum separation of soluble coal constituents from insoluble residue;
(c) reducing the temperature of the mixture to between about 160 C. and about 220 C.;
(d) separating the resulting solution at between about 160 C. and about 220 C. from the insoluble residue of the coal to form a clarified carbonizable substantially ash-free coal solution; and
(e) distilling and carbonizing said coal solution to recover extraction oil and a residue of substantially ash-free carbon therefrom.
2. Method of forming a substantially ash-free bituminous coal solution, comprising the steps of:
(a) forming a mixture of a finely divided bituminous coal and from about 1 part to about 6 parts by weight of the coal of an aromatic hydrocarbon creosote type extraction oil having an initial boiling point of about 180 C.;
(b) heating said mixture for a period of from about 2 to about 60 minutes at a temperature from about C. to about 50 C. in excess of the temperature required to achieve maximum solubility of the coal constituents in the oil and at a pressure from about 750 to about 2000 p.s.i.g. to attain maximum separation of soluble coal constituents from insoluble residue;
(c) reducing the temperature of the mixture to between about 160 C. and about 220 C. and
(d) separating the resulting solution at between about 160 C, and about 220 C. from the insoluble residue of the coal to form a clarified carbonizable substantially ash-free coal solution.
3. The method of claim 1 in which the coal solution is separated from the insoluble residue of the coal by centrifuging at between about 160 C. and about 220 C.
4. The method of claim 3 in which the centrifuging is performed in two stages, the sludge discharged from the first stage being fed to the second stage.
8 5. The method of claim 3 in which the centrifuging is performed in two stages, the effluent from the second stage being combined with the feed to the first stage.
6. The method of claim 1 in which the recovered extraction oil is recycled to the mixing step (a).
References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Campbell et al.: Coal as a Source of Electrode Carbon in Aluminum Production, Bur. Mines Rep. of Investigations 5191, US. Dept. Int., 1956, pp. 10-13 and 3251.
Warnes: Coal Tar Distillation, 3rd E., pp. 56-58, Van Nostrand Co., New York (1924).
MORRIS O. WOLK, Primary Examiner.