US2214717A - Cement manufacture - Google Patents

Description

Sept. 10, 1940. c. H. BREERWOOD CRUSHED RAW MATERIALS Hvnno SEPARATOR FINES DILUTION WATER- TURBO MIXER THICKENER CAKE TURBO MIXER BLENDING KILNs I SILIcEous EFFLUENT I MATTER TO WASTE I J i 'COMBINED REJECTS T0 WASTE FIG; 1

INVENTOR C.H.BREERWOOD BY I /guxu%l /lSZM1/w MI ATTORNEY Sept. 10, 1940. c. H. BREERWOOD 2,214,717

QEMENT MANUFACTURE Filed June 28, 1939 2 Sheets-Sheet 2 I Caususo RAW MATERIALS I Tues MILL RAKE CLASSIFIER UNPROCESSED v BLENDING q m Kins r I T CoRR'cTwEs BLENDING a To mus- DILUTION WATER- Tu RBO MlXER PRODUCT *2 RECOVERED PmMARY CAKE Pnooucfls PRIMARY Emuem \l ggw Duunon WATER- TURBO MIXER SECONDARY CAKE Duunou WATER- 1 TMCKENER TURBO SECONDARY EFFLUENT m W WA l FLOTATION T0 WASTE COMBINED Ruins T0 WASTE :E C 2 INVENTOR C.H. BREERWOO ATTORNEYS Patented Sept. 10, 1940 PATENT OFFICE CEMENT MANUFACTURE Charles H. Breerwood, Narberth, Pa, assignor to Separation Process Company, Catasauqua, Pa., acorporation of Delaware Application June as, 1939, Serial No. 281,594 In GreatBritain June 30, 1938 11 Claims.

This invention relates to cement manufacture, specifically Portland cement, its modification and other cements in which compounds of calcium and silicon are the principal constituents. More particularly, it hasto do with the beneficiation,

both chemically and physically, of available inferior cement raw materials, of a limited class identified hereinafter, by a process in which grinding, classification, centrifugal sedimentation and froth flotation, among other means, are employed in combination to effect reductions of the proportions of one or more of the constituent minerals of the natural material, whereby .the treated material may be utilized as the ultimate raw material mixture desired for burning, the 15 major proportion of the mixture, or the limebearing component of a mixture. The invention has particularly to do with the reduction of the materials and division of the ground materials into at least three fractions according to particle size, with making a principal minerals separation from the fraction of intermediate particle size range, and with the ultimate re-combination of products of steps of the process.

Throughout the specification and claims the following terms are limited by definition: Siliceous is used in generic sense to refer to all mineral compounds of silicon; fSilica is limited to mineral forms of silicon dioxide, such as quartz, and Silicate" to mineral compounds of silicon and aluminum, such as sericite mica, and also compounds of silicon and magnesium, such as talc.

In my co-pending application Serial No. 277,004, filed June 2, 1939, I have described and claimed a process for the reduction of proportions of fine silicate mineralsoccurring in argillaceous limestones, marls, chalks and siliceous components of raw material mixtures, the process employing centrifugal sedimentation, among other means. For the beneficiation of the argillaceous limestones, marlsand chalks contemplated therein, the present invention may be employed to'advantage either to effect more complete reduction of the proportion of silicate minerals, particularly for the manufacture of special cements, or to reduce the proportion of the available material that must be treated completely to provide for the correction of the entiresupply. Further, the present invention is appropriate to the treatment of other materials, not amenable to correction by the process of the said application, from which proportions of silica, silicates, or both, must be removed from particle size fractions coarser than the low orders of micron sizes. In my co-pend- 5, ing application Serial No. 281,593, filed June 28, 1939, I have described and claimed a process generally similar to the present invention, of particular advantage for the treatment of materials in which at least one of the constituents is nat- 10 urally finely divided and inwhich the materials are preferably ground to satisfactory ultimate fineness prior to correction. The present invention is appropriate for the treatment of certain of the materials of the class described in said application, but is of especial advantage for the beneficiation of materials in which the coarser orders of particle size are either largely calcite or largely calcite and silica, i. e., having a high silica ratio, the present process offering further economies both in grinding materials to satisfactory kiln feed fineness and reducing the proportion of the original materials that must be corrected further by froth flotation. I

The inferior natural raw materials contemplated herein are limited to the class consisting of argillaceous'limestones, marls and chalks in which natural crystallization is so fine, or in which the materials must be ground to such a high degree of fineness, to free the mineral bonds of a suflicient proportion of the constituent or constituents occurring in undesirable proportion, that the ground materials will contain a substantial proportion of mineral particles in the lowest orders of particle size, e. g., less than about 2 to 3 microns. These low orders of particle size may be abundant in silicates, if the latter are present in forms such as kaolin, mica andtalc, but in other cases may consist largely of calcite. The class also includes raw materials, of the types described, contaminated by impurities such as carbonaceous matter and magnesium hydroxide, in the mineral form brucite. In any case, the class is limited to raw materials, and the lime-bearing components of a raw material mixture, containing an excessive proportion of at least one siliceous mineral.

The invention is of especial advantage and is particularly directed to the processing of materials, within the class described, in which the coarser orders of particle size resulting from preliminary grinding are either largely calcite or largely calcite and silica, which may be segregated as a finished product requiring no treatment, other than additional grinding in some cases, and mixture with other recovered products, to be described.

It is among the principal purposes of the invention to correct the proportions and ratios of the mineral sources of the four essential oxides, silica, iron oxide, alumina and lime, to be combined for the formation of cement clinker, especially for the production of modern types of cement, including those of low-heat of hydration, resistant to sulphate and chloride solutions, and of limited delayed expansion in concrete. It is a further purpose, to effect the desired correction by the complete treatment of a limited proportion of the total raw material supply and to produce at least two products and preferably three, at least two of which and usually all three are of improved but different compositions, whereby they may be re-combined in various proportions and the chemist is provided with a wide range of control for the production of cements of various types. With relation to materials in which a sufficient proportion of the siliceous constituent occurring in excessive proportion can be released physically by preliminary grinding, i. e. without grinding the entire material supply initially to ultimate kiln feed fineness, it is an object to segregate and grind the coarser fractions separately to form one of the finished products, to efl'ect substantial economies in grinding, and in correctin the composition.

The purposes of the invention can best be explained by reference to complex raw materials of the class contemplated, containing proportions of the mineral sources of the four oxides essential for the ultimate raw material, but in which the minerals are present in incorrect proportions and ratios, together with one or more impurities which should be eliminated, at least in part.

' Such materials are usually deficient in calcite and silica, but contain excessive proportions of total silicon dioxide and of alumina in the form of silicates, and are frequently contaminated by undesirable proportions of magnesian silicates, carbonaceous matter and soluble alkalies. It is a principal purpose, in the treatment of such materials, to correct the proportions and ratios of the useful constituent minerals, by the elimination of quantities of one or more siliceous constituents, and to reduce the proportions of impurities, whereby the treated materials may be utilized, in various combinations, as the ultimate mixture or the major proportion'of the ultimate mixture, and when the natural composition will permit, to subject only a limited proportion of the available materials to the complete treatment to be described, whereby one or more products derived from the treated proportion may be recombined with the other proportion, untreated except for grinding, and sometimes further grinding, to form the desired mixture.

It is also a principal purpose to effect the physical correction of the mixture at the same time and partly by the same steps, the physical corrections including both reduction of the materials to satisfactory kiln feed fineness, with relation to the largest mineral particles in the ultimate mixture, and the elimination, from the portion to be completely processed, of the, extremely fine orders of particle size which react more readily in burning to clinker and at lower temperatures than the majority of the particles of the ultimate mixture.

Expressed more fully, the principal purposes are usually to reduce the proportion of alumina, to enable the production of cements low in tricalcium aluminate; to reduce the proportion of magnesia, when present in the mineral forms described, a dangerous impurity or adulterant frequently present in materials of the class contemplated, and to retain the calcite and crystalline or crypto-crystalline forms of silica, such as quartz, flint, chert, etc., either to .correct the relation between lime and silica and to increase the sillca ratio, or to change the proportion of at'least one silicate mineral constituent! at least to a degree that will permit a final correction of the ultimate composition by the addition of correctives such as sand, iron oxide, or both. In certain cases, an example of which will be given hereinafter, it is a purpose to produce a plurality of products, one of which may be used alone, or with quantities of other products, or correctives, to produce a cement of a specific type, the remaining products being utilized to produce cement of other composition.

If an available material, of the class described, is to serve as the lime-bearing component of a two-component mixture, in which the othercomponent is to be a clay or shale, the purpose is to process the lime-bearing component ordinarily to reduce the proportion of the siliceous matter, the principal reduction being made in an intermediate range of particle sizes. The siliceous matter to be reduced in proportion may consist of a single mineral but frequently includes various silicates, such as the aluminum silicates, usually kaolin, sericite mica and feldspars, and magnesian silicates such as the magnesian micasand talc. For example, the component may consist largely of calcite but with aproportion of silica solarge that the natural, material cannot be combined with the available clay or shale. In such a case, the purpose is to reduce the proportion of silica in an economical manner to a degree such that the combination can be made to provide the desired ultimate mixture, the com bination likewise sometimes including the addition of a corrective, such as iron oxide.

It is a further purpose to reduce the proportion of alkalies including that combined in the silicates and the soluble alkalies, when present in the material, to provide for uniformly accurate setting time of the cement, to avoid eiliorescence in concrete and to decrease delayed expansion of concrete. It is one of the principal purposes, in efiecting the physical correction to eliminate almost entirely, from the treated portion, and so far as practicable, mineral particle size ranges less than about 2 microns, and .ifsilicates are present, to eliminate the silicate particles up to about 5 or 6 microns in diameter, because I believe that these low orders of'particle size, and especially the silicates, are eutectic and are over-burned at the temperatures required to combine the coarser constituents, As the proportion of silicates, when present, is usually relatively great in the minus 2 micron fraction, and because nascentzsilica is extremely active, this fraction should be eliminated from the final mixture, together with larger ranges in particle size of micaceous matter, if present. Even if these low orders of size are largely calcite, it is preferable to eliminate them for these reasons from the'ultimate mixture, rather than to recombine the separated fractions with the recovered product or products. It is a further purpose to eliminate these low orders of particle size early inthe ofthematerialtomakeitpossi ble to obtain more complete separations and higher weight recoveries of the desired or useful constituents, in view of their tendency to promote natural flocculation and their effect upon the constituents to be concentrated by froth flotation, their removal usually considerably reducing the proportion of the supply of materials requiring complete treatment. This preliminary removal considerably reduces the amount and capacity of flotation equipment and the quantity of reagents necessary to effect complete correction. Likewise this elimination limits the necessary capacity of the thickening or filtering equipment and its operating cost, and also permits dewatering of the products to a greater degree, whereby the slurry may be fed to the kiln with limited water content, to economize in fuel consumption in burning and to take the fullest advantage of the available kiln capacity.

It is a further purpose to employ simple but unusual combinations of grinding, centrifugal sedimentation, froth flotation and blending to effect the above described chemical and physical corrections in a practical and economical manner.

Gravity classification methods, employing the conventional types of hydraulic classifiers lack utilityfor the preliminary removal of the low orders of particle principally because even when uneconomical quantities of dispersing agents are used, .the lower limit of separation according to particle size, under favorable conditions, is at least as high as to microns. At the degree of grinding required, for most of the materials contemplated herein, a separation or "cut within this range would involve a prohibitive loss of useful constituents especially losses of fine calcite and silica in particle sizes favorable for reaction in burning to clinker, and upon which a disproportionate amount of grinding power has been expended. Specifically, with relation to the materials of the two examples, to

be given hereinafter, a separation or cut at this range would eliminate at least the major proportion of the free grains of the various minerals, i. e., those of the particle size range from which the principal separations must be made. Further, this practice would require classification and final thickening equipment of impractical size, requiring enormous volumes of water, and the classification is ineffective and involves further losses, due to the trapping of large grains in fine floccules and the incomplete separation of fine particles from the coarser fractions. The presence of collidal matter in the underfiow promotes flocculation and limits the degree of dewatering.

In general, the invention comprises reducing the materials by grinding the crushed raw materials, or at least that proportion of them that must be treated by the complete process, at least to a dgree that mineral bond breakage is sufficient to release the excessive proportion of the siliceous mineral or minerals, together with proportions of contaminating minerals called impurities herein, if present, from the remainder of the constituents. The grinding is carried out at least to a degree that this proportion is released physically from the other constituents in the finer fractions, to permit correction by the elimination of quantities of siliceous matter from the finer fraction. If silicates are present, the grinding is preferably so controlled that substantial proportions of them are concentrated in the lowest orders of particle size and without unnecessary grinding of the other constituents. If,

after grinding, to the degree described, the coarser fractions are larger than satisfactory kiln feed fineness, they are separated and ground separately to ultimate fineness to become a finished product of the processs. Ordinarily, they will be of improved composition, usually of higher silica ratio but sometimes consisting almost wholly of calcite. The first example, to be given hereinafter, will illustrate this type of material. If the natural crystallization is so fine that the materials to be treated must be ground at least to satisfactory kiln feed fineness, to free a sum-- cient proportion of the siliceous matter present in excess, the coarser fractions are likewise segregated to avoid. unnecessary and undesirable further reduction to free the minerals, to avoid an excessive load upon the principal separating apparatus and to provide a product especially suitable for burning to clinker. As will be apparent from the description of the material of the second example, the higher orders of particle size are largely calcite and silica, and of increased silica ratio. Mineral bond breakage is incomplete, the silica being included in grains containing calcite also. No attempt should be made to grind and separate the lime and silica of such composite particles because both constituents are essential and, in my opinion, these particles are especially suitable for burning because the constituents are in perfect contact.

A slurry of the remainder of the material, consisting of fine portion, is then subjected to centrifugal sedimentation, so correlated as to the speed of operation and the rate of introduction and dilution of the material that the lowest orders of particle size are cut and separated as an eiiluent. I have discovered that micaceous matter has a relatively low sedimentation rate, the rate being about half that of other mineral particles, of the maximum diameter, due to the characteristic particle shape, and that advantage of this can be taken not only to accomplish the ultimate removal of these particles without excessive grinding, but principally to avoid neces sary reduction of calcite and quartz, with consequent losses thereof in the eilluent to be discarded as a waste product. Ordinarily, the separation or cut is satisfactory for the present purposes at about minus 2 to 3 microns, and it will be understood, for brevity, that reference herein to particle size fractions minus 2 or 3 microns is intended to include the oversized mica. particles referred to above, when present in the feed to the centrifuge. If soluble alkalies are present in the material sent to the centrifuge, they are largely carried away in the eiiiuent, as the latter includes all but a. small proportion of the water fed to the machine.

I have discovered that centrifugal sedimentation machines of the type commonly employed for tie-watering slurries and sometimes for the classificationof solids, can be operated in a novel manner to make an eflicient separation of as small a proportion of the weight of the feed as the particle size fraction below 2 to 3 microns, such machines being the only practical apparatus that I have discovered capable of separating at such low orders of particle size. Satisfactory machines of this type-include the Bird centri fuge," disclosed for example, in Piper Patent No.

1,962,461, and the F. L. Smidth & Company cen- The operation of the centrifugal separator for the purposes contemplated herein,- diifersessentially from its use in the de-watering of slurries by the overloading of the machine to such a degree that the fine fractions described are carried away as an eiiiuent. By controlling the rate of feeding and the speed of the bowl ordrum, and to a lesser degree the dilution of the feed, particle size fractions as flne as 2 microns and under, can 'be separated efliciently, these fractions representing sometimes proportions as low as 5 to 6% of the total weight of the material, and if fine silicates including micas, are present, this practice effects the removal of a substantial proportion of siliceous matter.

If natural flocculation is pronounced, and persistent in the feed to the centrifuge, the separation of the low orders of particle size is improved by electrophoresis, or dispersing agents may be employed. Although dispersing agents were not used in the pulps of the examples to be described hereinafter, substantial flocculation is characteristic of such pulps, and I have found that dispersion is complete, at least with relation to the particle sizes to be retained, following the introduction and agitation with approximately 2 pounds of calcium lignin sulphonate or sodium lignin sulphonate per ton of solids and that about the same dispersion can be obtained with 1 pound of either agent used with 1 pound of soda ash, as more particularly described and claimed in my Patent No. 2,162,525.

The relatively coarser fractions are discharged from the centrifuge as a "cake and comprise the intermediate range of particle size between the coarser fraction first separated as a finished product and the eflluent described above. Within this intermediate particle size range, the mineral constituents are free, or at least mineral bond breakage is s ufliciently complete to permit separation of the undesirable proportion of the siliceous constituent or constituents, or the remainder of the silicates occurring in undesirable proportion and not carried away in the efliuent, by froth flotation. The centrifuge cake, and which usually comprises a minor proportion of the weight of the original material, is then subjected to froth flotation for the recovery of the useful constituent or constituents thereof. In view of the fineness of the mineral particles, flotation is preferably, and usually must be, carried out in a stage-oiling circuit, usually with cleaning of the concentrate.

If the constituent to be discarded is largely silica, the flotation step is preferably carried out in the presence of the usual frothing agents and the usual calcite collecting reagents, satisfactory collecting reagents being oleic acid and the reagent described in my Patent No. 2,144,254. The calcite concentrate is thickened, the thickened product being pumped to blending silos, to form a second finished product, which may be withdrawn from storage and proportioned with the coarser fraction previously described, to form the mixture, or the major proportion thereof.

If, however, the materials are deficient in both calcite and silica, but excessive in silicates, the flotation steps may be carried out in accordance with the procedure described above, and more fully in the said patent, the flotation tailings, instead of forming a second waste product of the process, being subjected to an additional parr ticle size classification to separate a fraction highin silicates and principally micaceous, from a coarse fraction low in silicates and containing some useful quantities of calcite not recovered in the concentrate, largely due to incomplete bond breakage between the calcite and silica. This latter separation is preferably carried out in a centrifugalseparator to avoid losses of useful constituents, as this apparatus can be controlled to make an accurate division between the high silica ratio and the low silica ratio fractions. The efliuent of this centrifuge, and the first eiiiuent described, form the waste products of the process. The cake discharged from the last mentioned centrifuge is diluted to proper moisture content and conveyed to the blending silos from which it is withdrawn and proportioned and mixed with the other products of the process.

As an alternative to the separation of the tailings of the calcite concentration by means of the additional centrifuge, these tailings may be subjected to a second flotation concentration in the presence of a "positive ion or "cationic" flotation reagent to concentrate the free silicates, especially the mica, in the froth.- A satisfactory reagent for the purpose is dodecyl amine hydrochloride, or reagents of similar characteristics, the reagents useful for the purpose, and their method of employment in both stages of flotation, being more fully described and claimed in Breerwood and Williams Patent No. 2,161,011. This alternative may be employed when it is desired to eflect a greater reduction in total silicates and with an increased recovery of the useful constituents of the tailings of the first step of flotation. In the practice of this alternative, the tailings will be generally equivalent in composition to that of the coarse fractions recovered from the tailings of calcite concentration by the additional centrifuge, but will be of somewhat higher silica ratio. The micaceous concentrate will take the place of the efliuent of said centrifuge as a waste product of the process. As a secondalternative, but somewhat more expensive to practice, due to the comparatively high maining in the cake, the concentrate of this flotation being the waste product of the flotation step, the tailings beingthe recovered product corresponding to a combination of the calciteconcentrate and'the coarser fractions of the other constituents recovered from the tailings of calcite concentration. The procedure of this step of flotation is more fully described and claimed in Breerwood and Williams Patent No. 2,161,010.

For a better understanding of the invention, reference is made to the accompanying drawings, in which:

Fig. 1 illustrates the preferred flow diagram for the processing of a complex material of the class described, in which the coarser fractions of preliminary grinding are of increased silica ratio, with respect to that of the natural material, the coarser fraction requiring further reduction to make it satisfactory for burning to clinker, and

Fig. 2 illustrates the preferred flow diagram for the processing of a similar material, but which must be. ground to such a degree of fineness, to make possible the chemical corrections described above,'that the coarser fractions of grinding are of satisfactory size for burning to clinker, the

figure also illustrating the production of a product that may be utilized separately as an ultimate mixture, or the major proportion thereof, for the manufacture of a special cement, or used alternatively in mixture with proportions of other products of the process. 1

Referring to the drawi gs. and first to Fig. 1, it will be seen that the fiow diagram illustrates the essential and minimum number of steps to effect the desired chemical and physical corrections. The flow diagram, and particularly the nature of each step and its purposes, can best be explained by reference to the treatment of an appropriate material, as follows:

Example I The material selected for the example, andprocessed in accordance with the flow diagram of Fig. 1, may be described as a consolidated argillaceous marl, deficient in calcite, excessive in total silicon dioxide and alumina, both because of an excessive proportion of silicates, occurring as finely divided clay minerals and mica, satisfactory in mineral silica and iron oxide for the production of cements of high quality, but slightly deficient in the latter constituents for the use of the recovered products for the manufacture of true sulphate resisting cements without the addition of quantities of silica and iron oxide, as correctives. The term satisfactory is used with relation to the recovery of silica and iron oxide that can be made, rather than the natural proportions, which are incorrect. The silicates, including the mica, are naturally finely divided and increase considerably in proportion in the finer fractions of preliminary grinding. A substantial proportion of the calcite and silica occurs in fine crystals, but the major proportion is relatively coarse, with some recrystallization to readily visible crystals.

The purposes, in processing the material described, were to recover continuously at least two products which could be re-combined for the manufacture of Portland cement of high quality, low in tri-calcium aluminate, and to do so at low cost in equipment required, and consumption of power and flotation reagents. The test results, given hereinafter, are typical of large scale experiments made as a preliminary to the design of a raw material processing plant for a cement mill. The crushed raw materials were subjected to preliminary grinding in a ball mill, in which the charge of grinding media. was designed to effect a differential grinding, i. e. the physical release of the silicates from the other constituents with a minimum reduction of the calcite and silica. In View of grinding experiments and examinations, the grinding was carried out to a degree that 40% of the original weight of the material could be separated as a fine fraction, minus 50 microns.

The groundmaterials were subjected to hydiaulic classification in a hydro-separator operated to make the cut or separation at 50 microns The overflow comprised the particles minus 50 microns and carried 40% f the weight of the material. The underfiow, or plus 50 micron particles, represented 60% of the weight of the material.

Experiments demonstrated that this prelimi-v nary classification can be made much more efliciently, and in the end more economically, by a centrifugal separator, particularly with reference to the removal of the fines present in considerable abundance in the underflow of a gravity type separator. The cost of the centrifuge and its \installation is lower, but the centrifuge requires slightly more total power. A hydro-separator is indicated in the drawings and was used in the example to avoid confusion with the essential use of the centrifuge in the step to be described hereinafter, in which gravity type separators are useless.

The coarse fraction, or underfiow, was subjected to secondary grinding in a tube mill to satisfactory fineness for burning and was then reserved as a finished product to be blended with the product derived from the fine fraction. This product. was of increased silica ratio and the calcite content was increased from 12% in the original material, to 75%, as will appear in the table to be given hereinafter.

The hydro-separator overflow, or fine fraction, is preferably thickened to reduce the load on the centrifuge, as the capacity of a centrifuge is proportional to the rate of feed in gallons. The thickener underflow was delivered to a turbomixer and dilution water was added to provide a suspended slurry of about 30% solids, the turbomixer also serving to provide a constant head for the feed -to the centrifuge.

The centrifuge was fed at the rate of 17 gallons a minute and operated at a speed suflicient to produce a separating force equal to about 750 times gravity, the separation or out being made at 2 microns. The efiluen-t, containing the particles minus 2 microns, carried away a substantial proportion of the excessive quantity of siliceous matter, largely silicates, and was a waste product of the process.

The centrifuge cake, comprising only 28% of the weight of the raw material, and referred to herein as the intermediate fraction according to particle size, i. e., plus 2 to minus 50 microns, had a calcite content of 75%, as compared with the feed to the centrifuge which was 67.5%.

The centrifuge cake was diluted with water in a turbo-mixer to form a suspension of 20% solids and was then subjected to froth flotation in the presence of 0.06 pound per ton of resinate, used as a frothing agent, and 0.9% of collecting reagent, the reagent used being a saponified waste product resulting from the distillation of impure coconut oil fatty acid, a reagent more fully described in Vogel-Jorgensen application Serial No. 230,342, filed September 16, 1938.

The flotation was carried out in a stage-oiling rougher circuit, the calcite concentrate being the second finished product of the process. The concentrates were not cleaned, in view of their high calcite content, and were thickened to normal slurry moisture content suitable for burning and then blended with the ground separator underflow to form the ultimate mixture.

The tailings of rougher flotation were subjected to scavenging flotation, the concentrates of this step being returned to the feed of rougher flotation, as a middling, the scavenger tailings being the second waste product of the process.

The results of the processing, including the calcite content, and the weight recoveries and the grain size in microns of the products of each of the principal steps are given-in the following table:

. By way of comparison, a test was made in which the hydro-separator overflow was subjected to the flotation step described, but without removing the minus 2 micron fraction by means of the centrifuge. The grade of the concentrate was 80% CaCOa and that of the reject 45% @1003,

' as compared with the flotation of the centrifuge cake which produced grades of 86% and 28.3% respectively. Further, owing to the presence of the minus 2 micron particles, the reagent consumption was increased almost exactly 4 times, to produce the same weight recoveries, but at the lower grade. The comparative tests also indicated that almost three times as much cell capacity would be'required to make the concentration described from the hydro-separator overflow, and that the concentrates of the present process could be de-watered to a lower degree and in a thickener of only about half the diameter of that required for the concentrates of the comparative test. Two mixtures were made by proportioning and blending quantities of the hydrounderflow, after regrinding, with the flotation concentrate, and these mixtures were burned to cement clinker in a laboratory kiln.

The complete analyses of the two products so combined are given as perceht in the following table:

Ground Concentrate hydro-undernow (I) from cells Percent Percent 3. l5 1. 80 l. 2! l. 13 42. 6 47. 9

Coarse material Concentrate Iron' ore (lhnomte) Both raw mixtures were easily burned and the clinker-contained less than 0.5% free lime. The clinker was ground with 3%% gypsum to a fineness corresponding to 5% on the 170 mesh screen.

The two cements had-the following analyses:

Percent Percent SiO; 223 22. see a. 2. 84 4. 67.4 66. 0. 5 0. 0.6 0.

Total 100. 100.

B0: 2. 0 2. Silica ratio 3. l0 2.

The composition calculated according to the Bogue formula was: 1

I II

. I Percent Percent Calcium sulphate (Iss0; 3.4 3.4 Tetra calcium aluminate ferrite C4AF 8. 7 l3. 2 Tri calcium eluminate 01A 6. 8 2. 9 'Iri calcium silicate C18 64. 3 63. 9 Di calcium silicate C18- 15. l5. 2 Free lime 0. 34 0. 33

The two cements were tested and showed the 3 hours, minutes.-.

3 hours, 15 minutes. 6 hours, minutes- 6 hours, 40 minutes.

Tensile strength of 1:3 standard mortar;

I II

Lbnhlrl.

Lbal .in.

3 days 7 days It will be seen that both of the cements produced were of excellent quality and strength, and of low-heat of hydration. Although both would have unusual resistance to sulphate and chloride solutions, neither corresponds to the recent Federal specification for true sulphate-resisting cement, but it will be seen that by changing the proportion of the two products, by increasing the relative proportion of the concentrate, and by the addition of silica, such as sand, 9. true sulalo w QUIOgSlphate-resisting cement of special quality could be made. Further, this special type could also be made by increasing the proportion of the original material to be treated by the complete process, relative to the coarser fractions processed only by further grinding.

Referring to Fig.2, which illustrates a preferred flow diagram for the processing of a similar material, but one which must be ground to a i0 greater degree of fineness, to make possible the chemical corrections described, it will be seen that additional steps were employed, and an additional product produced, for reasons that can best be explained by an example of the treatment of an appropriate material.

Example II The material selected for this example, and processed in accordance with the flow diagram of Fig. 2, was a cement-rock" or.Bluestone" of the Lehigh Valley district of Pennsylvania. This raw material, being the most complex of the materials of the class described, will serve to illustrate the complete practice of the method, and to show how it may be applied to best advantage for the beneficiation of inferior materials of similar composition.

This material may be described as 9. Jacksonburg limestone, almost invariably, with respect to any type of ultimate cement raw material mixture, deficient in calcite; excessive in siliceous matter, but deficient in mineral forms of silica;

excessive in alumina; about satisfactory in iron' oxide, but deficient in iron for some types of cement; contaminated with magnesia, ordinarily to a dangerous degree; contaminated by graphitic carbon, probably colloidal, and contaminated with alkalies, both combined and soluble.

The limestone selected for the examples is particularly suitable for purposes of illustration, as it is apparently the most difficult to beneficiate in accordance with the practice of the present method, the principal difficulty having to do with the unusually fine state of natural crystallization, and the wide distribution of the various constituents and the almost complete dispersion of colloidal or near-colloidal graphitic carbon. It is intermediate in composition between limestone and shale, the color and general appearance more nearly resembling slate, but calcite is the most abundant mineral. The other principal mineral constituents are quartz; mica, mostly of the sericite variety, but magnesian micas, probably phlogopite are believed to be present; talc; kaolin; iron, principally as the hydroxide, limonite; and dolomite, which contributes a substantial portion of the magnesia. The fineness and limited degree of crystallization make accurate identification by petrographic methods d-ifllcult, the difficulty being increased by the presence and dispersion of the colloidal carbon, but it is believed from analytical determinations and examinations of related metamorphosed limestones from the same locality, that finely divided clay minerals such as kaolin and talc are present and contribute to the proportions of alumina and magnesia, respectively. Thin sections of the rock reveal distinct layers of fine calcite grains alternating with thin layers of fine scaly and fibrous sericite. There are occasional elongate lenses and elongated isolated grains of quartz. The quartz is widely distributed, usually as fine grains in excess of ten microns in diameter. The carbonaceous matter is dispersed throughout the rock in intimate contact with the various constituents, and constitutes about one-half of 1% of the total mineral weight. Total alkalies are usually less than 1%, the normal range of the material in the locality being from 0.4 to 1.5%, depending particularly upon the proportion of micaceous matter.

As will appear more fully from the chemical analyses of the specimens referred to in the example to be given hereinafter, this material is unsuitable for cement manufacture without substantial correction. The siliceous matter and alumina are too high, and the proportions of calcite and pure silica (quartz) are too low. In view of the abundance of alumina, correction of the lime-silica ratio by the usual addition of high grade limestone does not result in a mixture suitable for modern highway cement, for example, and this practice not only involves incomplete correction particularly with relation to alumina, but is enormously expensive because of the lack of reserves of high grade limestones in this producing district. The proportion of alumina also makes the material impractical, and sometimes useless, for the production of cement of moderateheat of hydration, and as the analyses will make obvious, additional correctives such as silica and ironoxide cannot be added as correctives to produce mixtures suitable for the manufacture of low-heat, and sulphate resisting types of cement. The proportion of alkalies is not dangerous but reductions are obviously desirable. The graphltic" carbon is eliminated in burning, but has the disadvantage of contributing substantially to natural flocculation, thereby limiting the degree of dewatering.

The elimination of a part of the mica, together with fine clay minerals believed to be present, particularly kaolin and talc, will correct the relation between lime and total silica, and of more importance will increase the silica ratio, 1. e., total silica divided by the sum of alumina and iron oxide, andwill make a useful reduction of magnesia. This elimination may be carried either to a degree such that the proportions of the constituents are satisfactory for a desired ultimate mixture for the manufacture of a commercial cement, or to a degree that will permit final correction by the addition of correctives, as will appear more fully hereinafter.

The test results, given hereinafter, are typical of experiments made as a preliminary to the design of a process plant for a proposed cement mill in the Lehigh Valley. Petrographic examinations of thin sections of the rock and ground specimens thereof, showed that the feed to the several minerals separating steps had to be ground to such fineness, to free a suflicient proportion of the silicates to be eliminated, that the entire feed required grinding to a degree suitable for burning to clinker. The examinations also indicated that mineral bond breakage was complete, or substantially complete, only in the particle size fractions minus 20 microns. It was, therefore, necessary to grind the materials to about minus 325 mesh sieve and the entire raw material supply was, accordingly, ground to this degree.

The raw rock, after crushing, was subjected to preliminary grinding in a Huntington mill, the product then being ground to ultimate fineness in a tube mill in closed circuit with a rake classifier, the circulating load being heavy to cause the ground materials to overflow without unnecessary reduction of the calcite and silica. The fineness of the overflow is given in the first table appearing hereinafter, together with that of the cake of the first centrifuge, its efiluent, and the effluent of the second centrifuge.

The rake classifier overflow was thickened to normal slurry moisture content, the thickener underflow then being divided, without regardto particle size, to provide finished Product No. l, unprocessed except for grinding and ultimate blending with products to be described, and a portion to serve as the feed to the separating step. The feed was withdrawn at a uniform rate and delivered to a turbo-mixer with sufficient dilution water to increase the dilution to 35.1%

solids. The turbo-mixer served to suspend the material and to provide a constant head in feeding the first centrifuge.

A centrifuge was employed, instead or a gravity type classifier, as the separation or out should preferably be made efiiciently at 20 microns, a division point below the efliclent and practical limit of gravity concentration apparatus. The separation was made in a laboratory type Bird Centrifuge, fed at the rate of 20 gallons per minute and operated at a speed sufilcient to produce a separating force equal to 191 times gravity. As shown in the table of particle size, only 4.4% of the primary eflluent was plus 20 microns.

The primary cake, with relation to the feed, comprised 55% of the weight and was increased from 73.66% to 77.94% CaCOa, and the silica ratio was increased from 1.91 to 2.38. It was diluted to normal slurry moisture content and reserved as finished Product No. 2.

The eflluent of the first centrifuge, or primary efliuent was thickened, to reduce the load on the second centrifuge, the thickener underfiow being diluted to 20.2% solids, because of the extreme fineness of the mineral particles, in a turbo-mixer, which provided a constant head in feeding the second centrifuge. The centrifuge was fed at the rate of 8.9 gallons per minute and operated at a speed to produce a separating force equal to 650 times gravity. The separation or cut was made at about 5 microns, as the coarser sillcates are almost exclusively micaceous, the particles in the eilluent plus 5 microns being largely oversized mica particles. The efiluent contained a large proportion of the carbonaceous matter and it will be seen from the table of analyses, to be given later, that the siliceous matter was almost exclusively silicate minerals, as indicated by the relative proportion of alumina and silica expressed as oxides. The secondary eflluent represented 11.5% of the original feed and was the greater in weight of the two waste products of the process.

ondary cake correspond generally to the hydroseparator underfiow and the centrifuge cake of Example I. i

The secondary cake was diluted with water to 11% solids and subjected to froth flotation in a laboratory Fagergren flotation machine, in the equivalent of a stage-oiling circuit, rougher concentration of calcite being completed in 4 minutes, in the presence of 0.037 pound per ton of an .alcohol frother, which may be described as a mixture of branched and straight chain aliphatic monohydric alcohols boiling between about 152 C. and about 162 C., obtainable along with methanol by .the catalytic hydrogenization of carbon oxides. The quantity of carbonaceous matter removed in the eilluent of the second centrifuge made preliminary carbon concentration, or depression of the carbon by means of an agent such as calcium lignin sulphonate, unnecessary. The collecting reagent consumption was 0.'7 pound per ton of the saponifled waste products of the distillation of impure coconut oil fatty acid, described in the previous example. The rougher concentrates were cleaned by fiotation in 2 minutes with the addition of 0.1 pound per ton of the same collecting reagent, the cleaner tailings being returned to the feed of rougher flotation as a middling. The rejects of flotation were the second waste product of the process. In view of the small weight of the rejects and the very limited proportion of silica therein, 'no

eflort was made to recover the quartz by further classification or flotation in the presence of a cationic reagent to concentrate the silicates as the waste products of flotation. The flotation concentrates were readily thickened to slurry moisture. content and were recovered as finished Product No. 3.

The relative weight, by reference to the feed, and not to the total raw material supply, and the analyses of the products at each of the principal steps of the ing table:

P t Analyses as per cent S ercen ca Pmduc weight ratio s10: Felol A1101 C1100; M co.

Feed 100.0 13. as 1.83 5. 4s 73. ea 4. 31 1.91 Primary cake .55. 0 13. 24 1. 7a a. s1 11. 94 3. 9s. 2. 3a 2 Primary elfiuent.. 41s. 0 14. as 2. z: 1. 37 00.13 4. 71 152 Secondary cake...- a3. 6 13. a0 1. 95 5. 95 .73. as a so 1. 72 Secondary eifluent ll. 5 18. 08 2 59 11. 71 :57. 85 5. 57 l. 26 Concentrates 25. a0 4. 3s 1. 12 a. a2 88.92 a. 01 o. as a Cleaner tails as 27. e4 3. s1 12. 31 4a to 8.10 1.10 Flotation rejects a 82 5s. 72 a. 24 1a 10. a2 a as 2. 53

The products above described were lowing fineness:

Cumulative percent weight plus size in microns of the fol- Prim Primary Secondary Sue m mlcmns Feed oak eflluent efliuent The secondary cake comprised 33.5% of the feed and the chemical composition was generally similar but of somewhat lower silica ratio. It will be realized that-the primary cake and seetion of magnesia in the flotation rejects and the 1 reduced proportion in the concentrate may be attributed to the elimination of magnesian silicates, as'it would be expected that the dolomite would be concentrated with the calcite.

The composition of the primary cake, finished Product No. 2, is especially to be noted. It will process are given in the followbe seen that the calcite content is considerably above composition, the alumina has been substantially reduced and the silica ratio raised to 2.38. Mineral bond breakage in this product is incomplete, the individual particles being calcite and composite particles of two or more constituents. These particles are, in my opinion, especially suitable for burning to clinker as the constituents are in perfect contact. No attempt should be made to grind them for this reason, and further grinding would be very expensive in power and would increase the losses of useful constituents in the secondary eiiluent and considerably increase the burden on the flotation cells.

Product No. 2 can, therefore, either be used in mixture with the other recovered products, or it can be utilized alone as a mixture for the production of a special cement by the addition of minor quantities of 'correctives. Thus, if 100 parts of the product were mixed with 2 parts of sand and 1 part of iron oxide, in the form of roll scale, for example, the mixture when burned to clinker and ground to cement with 4% of gypsum, would produce a cement of the following oxide composition, allowance having been made for coal ash contamination: SiOz 21.9%; F8203 4.1%; A120: 5.6%; CaO 63.0%; MgO 2.7%; S03 1.7%, and ignition loss of 0.8%. This cement would satisfy the specification for moderate heat cement, the New York City Water Board, and the New York and New Jersey State Highway specifications. 1

If the original division of the raw material was made in almost exactly equal parts, continuous mixture or re-combination of Products Nos. 1 and 2, derived from an original material of the calcite content given, could be directly re-combined for the production of an ordinary cement, but one of low silica ratio, Product No. 2 being a by-product to be used separately. However, for the production of improved types, combinations of all three products should be made and the deficiency in silica corrected by the addition of sand or other high silica ratio materials. In like manner, increasing the relative proportion of the feed, to the unprocessed proportion, would also permit re-combination of Products Nos. 1 and 2, with increasing additions of silica, to effect the ultimate correction.

With reference to both of the examples given above, it will be understood that the flotation operation can be carried out, at least in part, in the presence of cationic reagents, such as dodecyl amine hydrochloride, and reagents of similar characteristics, to increase the degree of separation of the siliceous matter, and especially the silicates, by concentrating these minerals in the froth. The siliceous concentrate would then take the place of the flotation tailing as the waste product of flotation. This type of flotation may be employed to treat the tailings of calcite flotation in which at least a limited proportion of the calcite has been concentrated, this concentrate together with the tailings of the second or cationic flotation step forming the equivalent of Product No. 3. Satisfactory cationic reagents and their method of use are more particularly described in Breerwood and Williams Patent No. 2,161,011.

As a second alternative, the centrifuge cake may be subjected to flotation exclusively in the presence of a cationic reagent and a frothing agent, to concentrate the silicates, in accordance with the method of Breerwood and Williams Patent No. 2,161,010, but in view of the limited weight losses resulting from calcite flotation and the relatively low cost of calcite collecting reagents, the .1 6 01 the cationic reagents is not essential in the treatment of materials of the typeand composition described.

I claim:

1. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, I each containing an undesirable proportion of at least one siliceous mineral which proportion can be released physically in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding at least a portion of the material to a degree sufficient to free a proportion of said siliceous mineral at least equal to the undesirable proportion in the finer fractions of particle size, separating the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to speed of operation and rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated from a remainder of the material of intermediate size range, subjecting at least a portion of the particles of intermediate size range to froth flotation to separate said undesirable proportion of siliceous mineral, and burning the remainder of said portion as at least a substantial proportion of the ultimate cement raw material mixture.

2. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral which proportion can be released physically in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding at least a portion of the material to a degree sufficient to free a proportion of said siliceous matter at least equal to the undesirable proportion in the finer fractions of particle size and to a degree sufficient to increase the proportion of the silicate mineral in the low orders of particle size, separating the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to the speed of operation and rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry including a substantial proportion of said silicate mineral are cut and separated as an efiluent, subjecting at least a portion of a remainder of said material of intermediate particle size range so separated to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, and burning the remainder of said portion as at least a substantial proportion of the ultimate cement raw material mixture.

3. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral which proportion can be released physically in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding at least a portion of the material to a degree sufiicient to free a proportion of said siliceous matter at least equal to the undesirable proportion in the finer fractions of particle size and to a degree sufiicient to increase the proportion of the silicate mineral in the low orders of particle size, separating the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to the speed of operation and rate of introduction and dilution of the material that the orders of particle size minus about 2 to 3 microns are cut and separated as an effluent including a substantial proportion of said silicate mineral, subjecting at least a portion of the remainder of said material of intermediate particle size range so separated to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, and burning the remainder of said portion as at least a substantial proportion of the ultimate cement raw material mixture.

4. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral which proportion can be released physically and increased in proportion to the other mineral constituents in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding at least a portion of the material to a degree suficient to free a proportion of said siliceous matter at least equal to the undesirable proportion in the finer frac-- tions of particle size and to a degree suflicient to increase the proportion of the silicate mineral in the low orders of particle size, separating andrecovering the coarser fractions of decreased silicate mineral content from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to the speed of operation and the rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated as an effluent including a substantial proportion of said silicate mineral from a remainder of the material of intermediate size range, subjecting at least a portion of said remainder of the material to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, and burning the remainder of said portion as at least a substantial proportion of the ultimate cement raw material mixture.

5. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of at least one siliceous mineral which proportion can be removed from a portion of the total raw material supply, and which portion, at least, must be ground to such a high degree of fineness to free the mineral bonds of said undesirable proportion that the ground material contains a substantial proportion of mineral particles in the low orders of particle size and the coarser fractions of grinding are physically suitable for burning; the improvement which comprises grinding at least a sufficient portion of the total raw material supply to permit removal of said undesirable proportion of siliceous mineral to a degree sufficient to free phys ically a proportion of siliceous mineral particles at least equal to the undesirable proportion in the finer fractions of particle size, separating and recovering the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to speed of operation and the rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated from a remainder of the material of intermediate size range, subjecting at least a portion of the particles of intermediate size range to froth flotation to separate said undesirable proportion of siliceous mineral, blending the remainder of said proportion with proportions of the other portion of the total raw material supply, and burning the blended material as at least the major proportion of the ultimate cement raw material mixture.

6. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of at least one siliceous mineral which proportion can be removed from a portion of the total raw material supply, and which portion, at least, must be ground to such a high degree of fineness to free the mineral bonds of said undesirable proportion that the ground material contains a substantial proportion of mineral particles in the low orders of particle size and the cnarserfractions of grinding are physically suitable for'burning; the improvement which comprises grinding at least a sufiicient portion of the total raw material supply to permit removal of said undesirable proportion of siliceous mineral to a degree sufiicient to free physically a proportion of siliceous mineral particles at least equal to the undesirable proportion in the finer fractions of particle size, separating and recovering the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to speed of operation and the rate of introduction and dilution of the material that the orders of particle size minus 2 to 3 microns are cut and separated from a remainder of the material of intermediate size range, subjecting at least a portion of the particles of intermediate size range to froth flotation to separate said undesirable proportion of siliceous mineral, blending the remainder of said portion with proportions of the other portion of the total raw material supply, and burning the blended material as at least the major proportion of the ultimate cement raw material mixture.

7. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of at least one siliceous mineral which proportion can be removed from a portion of the total raw material supply, and which portion, at least, must be ground to such a high degree of fineness to free the mineral bonds of said undesirable proportion that the ground material contains a substantial proportion of mineral particles in the low orders of particle size and the coarser fractions of grinding are physically suitable for burning; the improvement which comprises grinding at least a sufficient portion of the total raw material supply to permit removal of said undesirable proportion of siliceous mineral to a degree sufficient to free physically a proportion of siliceous mineral particles at least equal to the undesirable proportion in the finer fractions of particle size, separating and recovering the coarser fractions from the finer fractions, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to speed of operation and therate of introduction and dilution of the material that the orders of particle size minus 2 to 3 microns are cut and separated from a remainder of the material of intermediate size range, subjecting at least a portion of the particles of intermediate size range to froth flotation to separate a calcite concentrate from a tailing including said undesirable proportion of siliceous mineral, blending at least the calcite concentrate with proportions of the other portion of the total raw material supply, and burning the blended material as at least the major proportion of the ultimate cement raw material mixture.

8. In a method of manufacturing cement clinker in which an available inferior material is utilized, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral, which proportion can be released physically and increased in proportion to the other mineral constituents in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding the material to a degree sufiicient to free a proportion of said siliceous mineral at least equal to the undesirable proportion in the finer fractions of particle size, separating the coarser fractions of decreased silicate mineral content from the finer fraction, grinding said coarser fractions to satisfactory kiln feed fineness, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to speed of operation rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated as an effluent from a remainder of the material of intermediate size range, subjecting at least a portion of said remainder of the material to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, blending the remainder of said portion with the ground coarser fractions first separated and burning the blended material as at least the major proportion of the ultimate cement raw material mixture.

9. In a method of manufacturing cement clinker in which an available inferior material is utilized and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirablev proportion of siliceous matter comprising at least one silicate mineral which proportion can be released physically and increased in proportion to the" other mineral constituents in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding at least a portion of the material to a degree sufficient to free a proportion of said siliceous matter at least equal to the undesirable proportion in the finer fractions of particle size and to a degree suflicient to increase the portion of the silicate mineral in the low orders of particle size, separating and recovering the coarser fractions of decreased silicate mineral content from the finer fractions, grinding the coarser fractions to satisfactory kiln feed fineness, subjecting a slurry of the finer fractions to centrifugal sedimentation so correlated as to the speed of operation and the rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated as an efiiuent including a substantial proportionof said silicate mineral from a remainder of the material of intermediate particle size range, subjecting at least a portion of said remainder of the material to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, blending the remainder of said portion in suitable proportions with the ground coarser fractions, and burning the blended material as at least the major proportion of the ultimate cement raw material mixture.

10. In a method of manufacturing cement clinker in which an available inferior material is utilized as a source of a plurality of products of different chemical composition that may be utilized in the preparation of satisfactory ultimate mixtures, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral and which proportion can be released physically and increased in proportion to the other mineral constituents in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding the material, dividing the ground material into one portion to be used as a first finished product from another portion including said undesirable proportion of siliceous matter, separating the coarser fractions of decreased silicate mineral content as a second finished product, subjecting a slurry of the finer fractions so separated to centrifugal sedimentation so correlated as to the speed of operation and the rate of introduction and dilution of the material that the low orders of particle size comprising a minor proportion of the weight of said slurry are cut and separated as an eflluent including a substantial proportion of said silicate mineral from a remainder of the material of intermediate size range, subjecting at least a portion of said remainder of the material to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, recovering .at least a part of the remainder of said portion for use as a third finished product, blending proportions of at least two of said finished products, and burning the blended material as at least the major proportion of an ultimate cement raw material mixture.

11. In a method of manufacturing cement clinker in which an available inferior material is utilized as a source of a plurality of products of different chemical composition that may be utilized in the preparation of satisfactory ultimate mixtures, and in which the material is a member of the class consisting of argillaceous limestones, marls and chalks, each containing an undesirable proportion of siliceous matter comprising at least one silicate mineral and which proportion can be released physically and increased in proportion to the other mineral constituents in the finer fractions by grinding the material to a degree that the ground material contains a substantial proportion of mineral particles in the low orders of particle size; the improvement which comprises grinding the material, dividing the ground material into one portion to be used as a first finished product from another portion including said undesirable proportion of siliceous matter, separating the coarser fractions of decreased silicate mineral content as a second finished product which may be used separately as at least the major proportion of an ultimate mixture, subjecting a slurry of the finer fractions so separated to centrifugal sedimentation so correlated as to the speed of operation and the rate of introduction and dilution of the material that the low orders of particle sizecomprising a minor proportion of the weight of said slurry are cut and separated as an efiluent including a substantial proportion of said silicate mineral from a remainder of the material of intermediate size range, subjecting at least a portion of said remainder of the material to froth flotation to separate the remainder of the undesirable proportion of siliceous matter, re-

- covering at least a part of the remainder of said portion for use as a third finished product, blending proportions of at least the first and third fini'shed products, and burning the blended material as at least the major proportion of an ultimate cement raw material mixture.

CHARLES H. BREERWOOD.

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