US2424746A - Cement manufacture - Google Patents

Description

July 29, 1947.@ F. A. DOWNES 2,424,746

CEMENYT MANUFATURE Filed Sept. 25, 1940 CENTR/F065 OVF/PS/Zf' FITUR/V' l INVENTOR. F/PH/V/f lq. DOW/Vfl?,

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Patente-cl July 29, 1947 UNITE STABS CEMENT MANUFACTURE Frank A. Downes, Vfest port, Conn., assigner, by

mesne assignments, to Bird Machine Company,

Vt alpole, Mass., a

corporation of Massachusetts 8 Claims.

The present invention relates to the manufacture of cement and more particularly to the sizing or fractionating of the ground raw materials. In the manufacture of cement, limestone, shale or other necessary materials are first crushed and then the material is fed into a ball or tube mill Where itis ground down to a suitable particle size.

It is important that there be few if any cement particles materially coarserthan will pass through a 100 mesh sieve because in the subsequent burning of the material in the kiln to produce finished cement clinker, coarse particles are discharged Without being burned to the core and these incompletely burned particles make an inferior quality of cement. Also by reducing all the materials entering the kiln to a very finely divided state, the chemical reaction that takes place in the kiln is greatly facilitated in that the accomplishment of the reaction is largely dependent upon the face-to-face surface contacts or areas presented by the particles themselves. In general, the smaller the particles, the more speedy is the chemical reaction for it is a contact phenomenon. Where particles that are relatively coarse are contained in the materials entering the kiln, these particles require too muchtime in fusing by the available heat whereas Without fusing they would pass through the kiln relativelyunreacted and passive. To a lesser extent the type of the kiln and other practical considerations have a bearing upon the nature of the reaction. It follows, therefore, that if the slurried materials fed to the kiln are of 'a uniformly small size, the materials will react quickly and thus may be passed through the kiln at a greater rate resulting in that the kiln may be operated at its full capacity. I

Until recent years practically all cement raw materials were ground dry in multi-compartment tube mills operating in open circuit. That is, the material passes through the mill .but once, which means that all of the material must be held in the mill long enough to be sure that substantially all of the particles are reduced to the desired mesh.

Later wet grinding tube mills replaced to a considerable extent the dry tube'mills but until very recently these Wet mills were also operated in open circuit with the same Waste in power due to useless over-grinding. While Wet grinding eliminates-the dust problems of dry grinding, the material that is unavoidably over-ground to extremely iine meshes, presents a further problem of being very diiiicult to deWater by filtration ahead 4of the kiln or, if not dewatered prior to going into the kiln, the presence of the extremely iine material With the high Water content of the slurry tends to form mud rings in the kiln, a most troublesome operating problem.

The only possible Way to avoid the over-grinding discussed above is by operating the mills in closed circuit, This means operating the mills so that only a part of the material being discharged is suiiiciently ground. This mixture of line and coarse material is delivered to a classier Where the particles that are ne enough are removed and sent on to the kilnswhile the coarse particles are sent back into the mill for further grinding. Thus no particle can get out of the closed circuit until it has been ground to the desired lneness but since the ne material is discharged from the circuit immediately it is suiciently fine, Wasting power by undesirable over-grinding is reduced to a minimum.

Closed circuit dry grinding has been practiced by using air separators to classify the mill discharge but these are very costly to install andY operate so are not used to any extent in the cement eld.

Closed circuit Wet grinding has become common practice in metallurgy for many years Where various types of classiers are used to separate the sufficiently ground material from the oversize but it is this separation step that xes the required dilution.

Newhouse in his U. S. Patent No. 1,184,656 discloses applying this procedure to the grinding of cement raw materials and it has been adopted in very recent years in newly constructed cement plants, but has found very little application in already existing cement plants due to the difliculty of nnding suiiicient space in which to install the necessary dewatering equipment required by the Newhouse process. Due to space limitations such a procedure is not adaptable to some existing plants. The principal objection to all processes of this character is that for most efficient grinding of the materials clean classication is requisite and such classification requires the presence of considerable Water Whereas the less water in the slurry, the more eiiicient is its burning in the kiln. So in current processes it is costly to remove mechanically the excess Water in order to render the ground material. suitable for introduction to the kilns.

The present invention is designed to overcome the above-mentioned operating inefciencies that are attendant upon the process employed by Nevv'house and other similar processes and toward this end it contemplates the production of a relatively thick slurry of cement forming materials suitable for directintroduction into the kilns, the solid components thereof being of finely divided uniform size and containing substantially no particles larger than a predetermined size, and the water content thereof being no greater than that required for eflicient grinding of the cement materials in the mills, while at the same time falling within the limits prescribed by the kilns for efcient evaporation thereof and for subsequent reaction producing burning of the dehydrated solids. In this manner, the efficiency of the kilns may be materially increased, while at the same time the expense that is attendant upon the use of a dewatering apparatus such as a thickener is eliminated altogether.

In a cement plant the consistency of the slurry in an open circuit grinding mill is usually the maximum or thickest at which it can Still be pumped and handled as a fluid. In most cases the maximum consistency happens to be about 65% solids by weight. However, there are many instances of cement grinding mills operating on maximum consistency slurry containing about 50% solids. The difference is due primarily to the granular or amorphous nature of the cementmaking materials. In the closed circuit process of Newhouse, the grinding mill functioning at (iO-65% solids would be diluted to about 15% solids for eicient classification. Similarly, with the grinding mill functioning at Li5-50% solids in the Newhouse process the slurry would have to be diluted to l0-l5% solids for eiiicient classication., In either case the -15% solids slurry is vtoo dilute to mix and hold in storage basins andV much too dilute for calcination and therefore must Abe dewatered by means of a thickener.

At any rate, up to the present time there has been no feasible method of classifying the materialbeing discharged from a wet grindingkmill without materially increasing the dilution of the slurry for classification. The proper dilution in the mill for efficient grinding is from 30% to 50% water whereas to make the required 100 mesh classication in a classifier, the slurry must be diluted to 80% to 90% water and this extra water must again be decanted or otherwise removed from the finished slurry before it is fed to the kiln as otherwise the fuel consumption to evapcrate it could be prohibitive.

It isanl object of this invention to provide an improved closed circuit grinding flow-sheet in cement manufacture comprising means for separating or segregating the suiciently ground material from the insufficiently ground particles discharged from a wet grinding mill without the addition of water to the mill discharge. Another way of saying this is that by this invention it is proposed to eliminate the classilier and the thickener from present day cement plants and to use in their stead a single machine that classifies continuously at mill dilution, namely, a continuous centrifuge that separates or fractionates solids of one group of sizes from solids of another group of sizes while all the solids are in suspension. Hence, it is also an object to provide an extremely compact closed circuit grinding systemthat can be installed in an existing cement lplant where only small space is available for this operation. The object is attained by the aforementioned use of a continuous centrifuge for the fractionation of sizes at mill dilution.

Thus by the use of this invention the nished mill product can be sent on to the slurry tanks and the kiln at substantially the same dilution as if the mill were operated in open circuit and the insufliciently ground product can be returned to the mill without changing the dilution in the mill.

In order to attain these objects there maybe used in the now-sheet a continuous centrifuge of the type disclosed in the patent to Laughlin No. 1,710,315, issued April 23, i929, or its equivalent in function.

By utilizing a centrifuge in closed circuit with the mill, not only is the degree of dilution in the circuit materially lessened without reducing the efficiency of the mill, but in addition the slurry issuing from the centrifuge and conducted to the kiln is in a condition that it may be passed directly to the kiln without further dewatering.

The present invention is applicable to the production of cement from natural cement forming material-s or cement rock in which the proper proportions of lime and silica for the finished product are inherently present. It is also applicable to the production of cement by processes wherein the basic limestone and clay products are separately ground and mixed in the proper proportions to give the necessary ration of lime and silica to meet the chemical requirements for the yielding of dicalcium silicate. Irrespective, however, of the particular ingredients from which the cement is produced, the essential features of the invention are always preserved.

Other objects and advantages of the invention, not at thi-s time enumerated, will become apparent as the nature of the invention is better understood. In the accompanying drawing, for the purpose of example, there has been illustrated the best embodiment of the invention as it is now known` Such embodiment, however, is not to be regarded as limiting but only typical of many possible embodiments.

The figure of the drawing is a flow-sheet diagrammatically illustrating a so-called closed circuit arrangement of apparatus suitable for practicing the process of the invention.

The How-Sheet as herein shown comprises a Crusher, a ball or tube mill, and a continuous centrifuge.

Raw material, such as raw cement rock, limestone, clay, marl, blast furnace slags, or other combinations of materials, enters the crusher where it is reduced to about 1A to 1/2" size. The crushed material leaving the crusher is pulped with water to a consistency of say 67 solids and fed to the mill. Also supplied to the mill is a quantity of oversize fractionated in and returned from the centrifuge.. The mixture is then ground. The ground material leaves the mill as a fluid to enter the centrifuge which is operated to remove from the slurry those particles of a size substantially larger than mesh since only smaller sizes are desired in the centrifuge end product slurry. Consequently, the centrifuge will discharge along one path as an end product, a slurry containing the fines at a consistency of about 65% solids by Weight (or 35% dilution), which is slightly lower than that of the feed to the centrifuge, and along another path a fraction of oversize material which is returned in the closed circuit to the feed end of the mill. l

The classified slurry delivered from the centrifuge will carry a slightly higher dilution than the feed to the centrifuge due to a slight concentration of water in the fine product and, of course, the coarse material returned from the centrifuge to the mill will carry slightly less water than the feed :to the centrifugeybutin mostfcases the mill can Ibe operated at a slightly lower dilution due tov the 1- predominance of coarse particlesin `the mill While operating in closed circuitfas-compared with open circuit operation so that theresulting finished product will carry very little, if any, more water than would be the case for open circuit grinding;`

The slurry that is the end-product of the-centrifuge is passed -to a kiln where yit is burned to yield clinker. In thispassage; theslurry may be collected in slurry tanks or treated'in stationslfor blendingor mixing it with other slurry or slurries as is usual in current cement making practice.` Clinker fromthe-kilnis--then ground into-finished cement.

Because of the fact thatusual cement making kiln maybe advantageously operated to receive, dehydrate and burn-to clinker, slurry that-contains as high as approximately 40% moisture, and because of the fact that the water passing in the closed circuit through the mill and centrifuge is confined to this circuit, the water content of the slurry issuing from the centrifuge is maintained within a range that the kiln can treat economically, and it is unnecessary to resort to rthe use of a dewatering apparatus in the process.

In carrying out the above-described process, it is not essential that the materials issuing from the centrifuge be returned to the mill for regrinding. These materials may be discarded from the process if desired and only the fine materials smaller than 100 mesh that are initially produced in the mill may be segregated by the centrifuge and conducted to the kiln, but in general this would be uneconomical.

It will be understood that the details of the process comprising the present invention will vary for different plants and different raw materials, although the same general principles will apply in all cases. The invention is not to be limited, therefore, to the exact process as described herein. The invention should, therefore, be interpreted Ibroadly within the scope of the appended claims.

The operation of a centrifuge suitable for the present purpose is substantially clear from the above-mentioned patent to Laughlin, No. 1,710,315, disclosing a conical outer shell surrounding a spiral shaped conveyor blade, both the shell and the conveyor blade rotating about the identical horizontal axis, although at a speed differential so measured as to cause the emergence and removal of the coarse or oversize from the narrow end of the shell while the linished slurry containing the lines leaves the shell by way of an annular Weir plate disposed at the wide end of the shell. The material is fed axially into the centrifuge, and under the centrifugal force an annular liquid body will form in the wider end portion of the shell and as dened by the inside diameter of the annular weir plate. The oversize particles collect in the shell due to the centrifugal force, and from there are gradually pushed by the spiral conveyor toward and out of the small end of the shell.

In practice I have found that a continuous centrifuge such as that manufactured by the Bird Machine Company is capable of making the required separation of approximately 100 mesh at the consistency or. dilution at which the mill should operate of about 50% to 70% solids by weight (or from 30% to 50% water), which separation under such conditions is not possible by any other known means. Such a type is shown (1):. feedingV the in V.the patentto'EipersNo'. 1,962,461, ,grantedf'Juner-i havea.-r primary millfeed to the centrifugeand the centrifugethen;l t0'. have the oversize from` passato secondary'mill for re-grinding and thence back tothe centrifuge.

I claim:

1.. The process ofv making cement which comprises grindingthe cement forming materials v at the dilution at.whichrthe grinding mill operates most efciently, classifying the mill dischargein a centrifuge capable of separating suspended solids and removing such solids substantially as fast as separated without substantially increasing the dilution of the mill discharge, sending the sufficiently ground material on to process and returning the insuiciently ground material from the centrifuge back to the mill in closed circuit for further grinding.

2. The process of making cement which comprises wet grinding cement forming materials at a consistency of dilution substantially directly suitable for calcination operation, classifying the ground materials substantially at said dilution in a continuous centrifuging step whereby coarse particles are emerged and separated from slurry containing the fines suitable for calcination, returning the separated coarse particles for regrinding in a closed circuit, and passing said slurry ultimately to calcination in a kiln.

3. The process according to claim 2, in which the classification by centrifuging produces a fraction of minus 100 mesh particles as said slurry and another fraction of plus 100 mesh material for regrinding.

4. The process according to claim 2, in which said consistency is from to 70% solids by weight.

5. The process of making cement which comprises grinding cement forming materials in the presence of water, separating coarse particles of substantially over mesh from the ner particles by means of a centrifuge capable of separating suspended solids and removing .such solids substantially as fast as separated, returning the separated coarse particles for re-grinding in a closed circuit, discharging from the centrifuge a slurry containing substantially no particles over 100 mesh, and having a dilution of substantially less than 50%, and feeding the slurry to a, kiln.

6. The process of making cement which comprises Wet grinding the cement forming materials at a consistency of dilution substantially directly suitable for calcining, classifying the ground materials substantially at said dilution in a continuous centrifuging operation by which a ne fraction slurry containing substantially only particles of a size suitable without further disintegration for burning is separated from a fraction containing substantially only coarser particles, returning the separated coarse particles for regrinding in a closed circuit, and passing on the ne fraction slurry for ultimate burning in a kiln.

7. The process according to claim l, in which the classification by centrifuging produces a ne fraction slurry containing substantially only particles of a size suitable without further disintegration for burning and another fraction containing substantially only coarser particles for regrinding.

8. The process of making cement which comprises grinding cement forming materials in the presence of Water, classifying the ground materials substantially at the dilution at which they were ground in a centrifuging operation by which a fine fraction slurry containing substantially only particles of a size suitable Without further disintegration for burning and having a dilution of substantially less than 50% is separated from a fraction containing only coarser particles, returning the separated coarse particles for regrinding in a closed circuit, and `passing the ne fraction on for ultimate burning in a kiln.

FRANK A. DOWNES.Y

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,784,462 Miller et al Dec. 9, 1930 2,090,363 Wendeborn Aug. 17, 1937 2,144,254 Breerwood Jan. 1'7, 1939 1,985,868 Maust Dec. 25, 1934 1,842,372 Allison Jan. 26, 1932 2,149,220 Krebs Feb. 28, 1939 2,125,663 Wuensch Aug. 2, 1938 2,151,175 Wuensch Mar. 21, 1939 FOREIGN PATENTS Number Country Date Great Britain 1938

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