The milling of talc, as is the case with many non-metallic minerals, until recently, has not received adequate technical consideration, for the talc industry has become of importance only within the last decade. At first, talc was used only in the massive form, for foot warmers, griddles, and so on, and milling methods were unnecessary. As the demand for ground talc increased, producers adopted the machinery used in the milling of flour; in fact, many talc mills were rebuilt flour mills. Improvement has been slow, but today several types of grinding and separating machinery are in use; many of them, however, are still inadequate and inefficient.
To determine the best methods of milling talc, it is necessary to understand the essential properties a talc must possess to fit it for a particular use. For toilet purposes, whiteness, freedom from lime and grit, fineness of grain, and good “slip” are essential. As a paper filler or coating, the talc must be white, uniform, and fine grained, have a good slip, and be free from grit and iron; freedom from lime is a disputed point. Fibrous talc is supposed to be superior to massive on account of the interlocking of grains in the paper, thus increasing its strength; this point, though, is in doubt. In general, talc for the paper industry need not be of as high quality or as fine grained as that used for toilet purposes.
Talc is usually bought by sample, for which reason it is difficult to state the essential properties for use in paint, rubber, roofing, and so on. The adoption of standard screen tests and standard grades by producers is necessary before an accurate basis for manufacturing or selling standards can be established. Off-color talcs might be utilized, as in Germany and Austria, by the establishment of standard grades of colored talcs.
The machinery to be used depends on whether the talc is fibrous, foliated, or massive; hard or soft; of uniform or variable grade; pure or impure. In some cases, it is possible to change the mining practice or to utilize different sections of the deposit in order to vary some of these factors, but in many cases these factors are fixed, so the milling must be designed to suit the conditions.
PROCESSES AND MACHINES NOW IN USE
Crude talc from the mine, in many cases, is too moist and sticky to permit of efficient milling without first being dried. Sometimes it is dried in heaps exposed to the air; sometimes on a steam-heated, iron, drying floor; and sometimes in rotary, direct- or indirect- heat dryers fired with coal, coke, or steam. If direct-heat dryers are used, coke must be the fuel so that the color of the talc may not be impaired by smoke and soot. Where drying is necessary, rotary dryers are preferable.
Dryers may be essential in some mills; in others, they may be used only occasionally or they may be omitted entirely, especially if proper precautions are taken in mining.
The machinery for the primary reduction of talc rock does not differ essentially in various parts of the country. Crushing to about 1½ in. is usually done in jaw crushers of the Blake type, although for large production gyratory crushers might be used to advantage. Rolls or rotary crushers reduce these lumps to about ½ in. Between the primary and the secondary crushers there should be a rotary screen, which will remove material fine enough to pass the discharge opening of the secondary crusher.
The next step is accomplished by a variety of machines. The principal types used are the Raymond roller mill, the Fuller-Lehigh mill, various types of vertical and horizontal emery and burr mills, pulverizers of the swing-hammer and other types, so-called disintegrators, the Hardinge conical mill, and other continuous and intermittent types of ball and tube mills. These machines are followed by inclined vibrating screens, rotary screens, silk-bolting reels, or various systems of air separation; or the product may be bagged direct. Where screening or air separation is used, the product is conveyed to bins and then bagged. The product is carried from one stage of reduction to the next by bucket elevators, and belt or screw conveyors.
In the selection of all crushing, grinding, separation, and handling machinery care should be taken not only to provide sufficient capacity but some reserve; also that the capacities of the various machines are in proper proportion to one another. In one mill, the secondary crusher was too small for the rest of the plant, so that the mill capacity was lowered to the capacity of that crusher. Another mistake is to gage the capacity of a mill by the rated capacity of each machine running full time; it is impossible to maintain the maximum rated capacity for all machines simultaneously for any length of time.
The coarse and secondary crushing units are usually fairly efficient, but the full value of such machines is often not utilized. It is a fundamental principle that material already crushed fine enough should be removed before passing through the next reduction unit. Feed to a crusher making a 1½-in. product should be freed from all material finer than 1½ in. by screening. If allowed to go through the crusher, it not only reduces the capacity of the crusher for doing useful work but it tends to cushion the crushing effect on the coarser material. When belt conveyors are used to feed crushers, a magnetic pulley at the discharge end is advisable. This removes all tramp iron, and not only protects the machines from injury but keeps out fine iron which may contaminate the finished product.
Opinions have differed widely as to the most efficient type of fine grinding and separating machinery, but certain fundamental principles should be recognized. First, the feed to a machine grinding to 200 mesh should not be over ¾ in., and ½ to 3/8 in. or finer is to be preferred. The very fine grinding of coarse material in one stage is not efficient, no matter what type of machine is used. Second, continuous operation is to be sought instead of intermittent, as in the case of certain types of pebble mills now in use.
Screening or bolting of material finer than 100 mesh is usually expensive, slow, and inefficient, though good results have occasionally been obtained, under careful competent supervision. Where dry grinding is used, some system of air separation is the cheapest and most efficient method of grading very finely ground talc. It has not been proved that fibrous talc from New York State, which contains fibers, flakes, and rounded grains, can be successfully separated by air, for, without fairly close sizing, large thin flakes settle in air at the same rate as small rounded grains. But here the problem may be solved by selective mining, close sizing before air separation, improvement in the design of air separation machinery, or the use of more than one separator in a closed circuit with the fine-grinding machinery.
The efficiency of ball or pebble mills for the fine grinding of talc is in dispute, but certain facts seem to be established. Pebble mills of the intermittent type, or “dump cylinders,” are not economical; they are slow and have a small capacity. The finished product is not uniform and its grade is dependent on the opinion of the man in charge, as it is not mechanically controlled. As the grinding progresses, the inefficiency increases: the most active grinding is done in the first hour, for the fine talc gradually cushions the impact of the pebbles and little grinding is done at the end. In one mill 5000 lb. of pebbles are used to one ton of talc in a mill revolving for 5 to 7 hr. at 22 r.p.m. In roller mills, which are continuous in operation, equipped with air separation, when the talc is ground sufficiently fine it is automatically removed, so that the full crushing effect is continuously applied to the coarser material. Such mills may have a capacity of about 5 tons per hour when producing a 200-mesh product.
Tube or pebble mills with continuous feed and discharge may be necessary in the milling of fibrous talc or talc mixed with tremolite; they may also be economical with certain other talcs, but the use of three or four large tube mills in tandem with no screening or separation between the mills nor for the sizing of the finished product is neither efficient nor economical. A good product is made by one talc producer by the use of short tube mills, but each mill is followed by bolting reels, which return the oversize for regrinding. In a plant being erected, a Hardinge mill and a large tube mill will be used with an air separation between them and another at the end of the tube mill; the coarse material from each air separator will be returned for regrinding. One plant practising close sizing between mills consumes 61 hp. per ton per hour; a plant that neither sizes nor separates between mills consumes over 375 hp. per ton per hour. Though there is some difference in hardness between the talcs ground in these cases, it is not enough to account for this difference in power consumption.
One argument given for the use of tube mills is that no iron produced by attrition of grinding surfaces can contaminate the product of a pebble mill lined with siliceous brick or pebbles. But iron can enter the pebble mill from the primary and secondary crushers. Furthermore, grinding machines in the most modern plants manufacturing the highest grades of toilet powders have all-iron grinding surfaces. It is stated that 0.5 per cent, of iron is not injurious in talc used for filling paper; that is, in milling a talc naturally free from iron, 10 lb. of metallic iron per ton of talc may be added without injury. In a mill producing 5 tons per hour, this would amount to 50 lb. of iron per hour, an amount of attrition that no mill could possibly produce and still be of practical use.
Vertical Roller Mills
Where a large capacity at uniform fineness is desired, a vertical roller mill is probably the most efficient for soft talcs. Such a machine, with automatic feed, in which the product is continuously removed by a current of air introduced from outside, is used in some of the best designed plants. For smaller capacities of fine material, swing-hammer and similar types of pulverizers and disintegrators and vertical emery mills equipped with air separation are used. Some types of these machines are suitable only for coarse material, 40 to 100 mesh, but when equipped with air separation, these mills may be used to produce a certain percentage of 200-mesh product. The tailings from the air separator must be carefully reground or sent to screens making a coarse product. The tailings from all mills can be continuously returned for regrinding, but as there is a demand for coarser material, they are often sized and sold to the roofing industries. The best type of screen for this purpose is the flat, inclined, vibrating screen.
Horizontal Burr Mills
The inefficiency of horizontal burr mills, compared with vertical emery mills, for the grinding of most talc, seems to have been definitely established, as they have been discarded by nearly all producers.
Selection of Mill
The best type of hammer mill, pulverizer, disintegrator or emery mill for the grinding of a talc from a new deposit can be determined by a careful comparison of the physical properties of this talc with those of other talcs now successfully ground; or by actual tests made by companies selling grinding machinery. But whatever machines are used, the product from the last grinding machine should be carefully sized either by bolting or by air separation. No grinding machine can be relied on to produce continuously a fine uniform product without screening or air separation. Furthermore, the final, sized product should be tested frequently by hand screening. Probably nothing has injured the talc industry so much as the marketing of non-uniform, improperly sized product.
The finished products from all machines may be bagged by hand, but the cheapest and most efficient results are attained by the use of automatic bagging machines, provided with ample bin capacity.
Grinding 300-mesh Material
With the Raymond, the Sturtevant, or similar air separation systems, a small amount of almost impalpable dust is produced. This is usually of very high grade but it cannot be obtained in large enough quantities to warrant its exploitation. No machine now on the market can produce by air separation such fine material in large quantities at a cost that will permit its sale at a reasonable price, for with increasingly fine grinding, the machine capacity is rapidly reduced, proportionately increasing the cost per ton of product. But one plant has found that a somewhat larger production of this grade may be obtained at only a slightly increased cost, by using a secondary system of dust collectors in conjunction with the primary air separation system. If the demand for very finely ground material, that is through 300 mesh, increases it may be necessary to use wet grinding and water classification.
Electric Motors and Bins
Most of the older plants are steam driven, but there is a tendency to install electric drives in new mills and in the remodeling of old mills. Individual motor drives for each important machine have many advantages; in one mill, each motor has its own voltmeter, ammeter, and circuit breaker. This permits an accurate record to be kept of the performance of each machine and is valuable in checking efficiency and in estimating the cost of producing different grades of talc. A recording wattmeter would also be valuable.
A point in mill design often overlooked is that of providing ample bin capacity for crude talc, crushed rock, and for several grades of finished products. Most mills have adequate capacity for one of these, but rarely for all three. Bins are not expensive but they are of great value in insuring steady production when the mine is temporarily closed or when sections of the mill are shut down for a few hours. Among the objections to the use of bins for finished products are: that if through accident, improperly ground material is made a large quantity of good talc in the bin is contaminated; and that certain talcs when finely ground are very sticky and will not flow freely from a bin. The first objection may be removed by using a bin divided into compartments and frequently checking the product by screen tests. The use of several compartments is advantageous, in any circumstances, in order that several grades of product may be made. Frequent screen testing is likewise desirable in order that a high quality may be maintained; in several mills screen tests are made every hour. The second objection may be removed by heating the inside of the bins with exhaust steam, or by installing some system of mechanical stirring, or agitation. Finely ground material if dry and warm will often flow freely, when it will not flow at all if cold and slightly moist.
There are no definite specifications for the selling of talc and I have not found a consumer who could tell exactly what he wanted. He tries a sample and if that is satisfactory he uses that talc. In fact, the tests are so poor that consumers have refused one kind of talc and accepted a talc made in just the same way, saying that it was satisfactory.
The first thing needed is to devise specifications. Certain consumers demand a talc with a slip, but no one can define or measure slip. There are no specifications whatever for those tests. So, first, the tests must be devised. The idea that fibrous talc must be used in the paper, paint, and other industries was probably originally fostered by some producers of that material. As a matter of fact, the fibrous talc is not so well fitted for paper as the granular talc, for unless it is carefully prepared the fibers are apt to pierce the paper and make a bad spot. Less fibrous than granular talc is now used in paper. Some paint men object to fibrous talc because the fibers, if long, are apt to upend themselves in a film of paint, and be brushed off, leaving an opening. The talc men have not taught the consumers the uses of the various varieties of talc.
To what extent is it possible to vary the amount of grit in the finished product by a selection of grinding machinery? Also, what results have been obtained with water flotation?
The grinding test referred to has not been completed. All the screen tests have been made. The silica determinations have not been made, but from microscopic examinations of material remaining on the screens it seems probable that a method can be devised whereby probably 80 per cent, of the silica can be removed. The talc in question was foliated and when ground broke up into fine, thin scales, whereas the quartz was in little, rounded grains. I believe it is possible to grind that talc in either a Raymond roller mill, by which the coarser particles can be removed continuously from the inner cone, or by some form of a hammer mill equipped to throw out the heavier particles. I think that by one of these methods most of the grit can be eliminated.
If the talc is granular, I doubt if this could be done because the specific gravities of talc and silica or quartz are about the same, and when ground probably the grains would be about the same size. It would be difficult to separate grit or silica from talc in the case of a granular talc. Wet grinding and water flotation have not been practiced at all. Some experiments have been made, but no definite results have been obtained. I think that eventually the finer grades of talc, possibly the 300 mesh and finer, will have to be made by this method. I understand, however, that by air separation they are able to make a product of 350 mesh; but that would not apply to all talcs.
If talc is fibrous it can undoubtedly be floated; we have succeeded in floating fibrous material, such as asbestos, with excellent results.