Table of Contents
Until the advent of the porphyry coppers and the introduction of flotation which soon followed, crushing and grinding for many years proceeded along somewhat stereotyped lines, without important alteration in type of machinery. For the finer crushing and grinding, stamps, rolls, and various patterns of Huntington and Chilean mills were in general use. Ball mills were in use abroad but owing to their small capacity and the high cost of screens and steel, they never obtained much footing in the United States.
Ball Mill Crushing by tube-mills were first introduced into the crushing departments of cyanide plants when it was found that for crushing finer than 30-mesh other types of crushing machinery were not efficient. In order to crush with one pass, these mills were made 18 to 22 ft. (5.5 to 6.7 m.) in length. Pebbles were used as a grinding medium and the mills were lined with either silex blocks or fragments of quartz or flint pebbles set in cement. While the cylindrical tube pebble-mill had been used to some extent in small installations of concentration mills, the first instance of its use on a large scale was at the concentrating plant of the Miami Copper Co., where 8-ft. by 22-in. (2.5-m. by 55.9-cm.) Hardinge mills were tried out against Chileans. The results of these experiments led to the gradual replacement of the Chilean by the Hardinge mill. Robert Franke published an article, describing the work done, comparing the two types of mills. At the same time, according to Franke, a test was run using a 6-ft. (1.8-m.) Hardinge ball-mill as a substitute for rolls for intermediate crushing on ½-in. material. According to Franke, this mill was soon discarded, since it was found that to obtain the desired product the mill must be limited in capacity. The Miami Copper Co. made no test with other types of ball- or pebble-mills. Following this example, others installed the conical type; and when the Inspiration Consolidated Copper Co. decided to build a concentrator, it adopted the conical mill. As the requirements for mills would be very large, it purchased manufacturing rights in the State of Arizona from the
Hardinge company, believing it would be cheaper to do this than to pay the manufacturer’s profit.
The Inspiration company built a 500-ton test plant to work out the final details of the grinding and flotation problem, its previous tests having already indicated the advantage of flotation. The regrinding mills in this 500-ton test plant consisted of 10-ft. by 28-in., 8-ft. by 36-in., 8-ft. by 44-in., 8-ft. by 72-in. Hardinge mills and a 6 by 20-ft. Chalmers & Williams tube-mill. These mills were equipped with pebbles as a grinding medium. During the operation of this test plant, the concentrator building was erected, of the same size and dimensions as that of the Miami Copper Co., having an estimated capacity of 7500 to 10,000 tons per day.
The general results of the regrinding mills at the Inspiration test plant showed that the 10-ft. (3-m.) Hardinge mill was the most unsatisfactory of all, due to the excessive pebble consumption and the power required to operate it. The mill of this type that gave the best results was the one that approached the cylindrical shape, having a cylindrical portion 72 in. in length (182.9 cm.). The mill that seemed to give equal results, as to power and pebble consumption, was the Chalmers & Williams tube-mill, although it required considerably more space than the 8-ft. by 72-in. Hardinge.
Introduction of Marcy Mill
Later, the company installed a Marcy ball mill 8 ft. (2.5 m.) in diameter and 5 ft. (1.5 m.) in length. This mill was experimental, an entirely new design, being the first mill in which the entire discharge end was fitted with a grizzly or screen. This grate was intended to deliver a maximum size of 1/8-in. and between the grate and the discharge end were lifters to discharge the undersize from the mill. It was claimed that by keeping a minimum of undersize in the mill the relative weight and efficiency of the balls was considerably increased.
By allowing the Marcy mill to take 3-in. feed and discharge a product below 1/8-in., capacity of the Hardinge pebble-mill was greatly increased and the general extraction was improved; it was found that an installation of Marcy mills followed by conical pebble-mills could readily treat 10,000 tons per day. As a result of the test in October, the company made arrangements to manufacture its own Marcy mills from the designs of the one that was in operation, modified by the results of their own experience.
At this time, it was suggested that it might be possible to do all the crushing in a Marcy mill arranged in closed circuit with a mechanical classifier; that is, to take the 3-in. feed and crush it to flotation size in one operation. Experiments with the Marcy mill and a new classifier were conducted, but were not altogether satisfactory. In December, at the suggestion of Mr. Hardinge, the conical mills equipped with steel balls were tried in a similar manner, but with inferior results. Afterwards, since the Marcy mill had demonstrated that it could do the work in one operation, it was purchased by the Inspiration company and manufacturing proceeded.
The new concentration plant was started in the summer, equipped entirely with No, 86 Marcy mills, 8 ft. in diameter and 6 ft. in length, in closed circuit with Dorr classifiers, the product going to flotation machines and the sands from these machines to concentration tables. On starting, the shells of the Marcy mills were found to be defective, due partly to the light design and partly to the fact that the manufacturer did not have time under his contract to make and anneal the castings properly.
Test of Hardinge vs. Marcy Mill
Later yet, the Inspiration company decided to add two sections to its mill, and an offer by Mr. Hardinge to equip a section with two of his mills without cost to the company was accepted. These mills were expected to do the work of the same number of Marcy mills, with much less power. The fact that a joint test between Hardinge and Marcy mills was to be run by the Inspiration company was widely advertised and created considerable interest among those interested in crushing; since the expense of crushing is greater than that of any other milling operation, these tests were of considerable importance. A statement by David Cole covers this very well and is therefore repeated verbatim:
The comparison of work done, based upon the scientific theory of Stadler, Gates, Kick, et al., is beautiful on paper, but there are a lot of us who hesitate to accept the theory as “law.” We are inclined to regard a direct comparison of grinders arranged side by side, getting feed from a common source through a mechanical distributor, and making a product that affords as nearly as may be the same screen measure, and at any rate affording an equal metallurgical opportunity for the subsequent treatment, as the Supreme Court in these grinding matters. The Marcy versus Hardinge ball- mill controversy is soon to have this kind of a hearing at the Inspiration plant, and the results will be watched with great interest.
As no data have yet been given out regarding this test, I have thought this a good opportunity to publish the facts.
A section comprising two 8-ft. Marcy mills equipped with 225-hp. motors, which had been in continuous operation, was used in comparison with a section comprising two 8-ft. Hardinge mills equipped with 150-hp. motors. Each of the Marcy mills took the coarse feed from the bin and, in closed circuit with a 6-ft. Dorr classifier, made a finished product.
The Hardinge mills were first arranged in tandem, the first mill taking all of the coarse feed from the bin, its product going to a Dorr classifier, the sands from which passed to the second Hardinge mill working in closed circuit with the second Dorr classifier. Each of the sections was equipped with an automatic scale so that the total or the hourly tonnage could be recorded and noted. The crushed product, the overflow from the Dorr classifiers, was carefully sampled in each case by automatic samplers. The daily report sheets of the finished product showed some variation from the desired 2 per cent, on 48-mesh with both types of mills, but by applying a correction factor the final results, as tabulated, could be reduced to the basis of 2 per cent, on 48-mesh. This correction factor was derived by Dr. Gahl from actual operating experience. The results are as shown in Tables 1 and 2.
The daily reports show that various ball charges and various sizes of balls were used in the Hardinge mills; that the speed of the Hardinge was changed a number of times; various types of scoop feeders were used; the delays due to overloading the Hardinge mill, changing balls, etc., as mentioned, were very great. The Marcy mill continued with its ball load unchanged and practically without delays.
The record shows that the capacity of the Marcy mill was 130.5 per cent, greater than the Hardinge, and the Marcy saving in power over the Hardinge was 34.04 per cent. At times the motors of both types of mills were slightly overloaded. As the power was measured by integrating wattmeters, this does not affect the results and comparisons given.
Mr. Hardinge said he wished to experiment with his mills so as to do better work. The results show, however, that he did not increase his capacity nor decrease his kilowatt-hours per ton. The
Hardinge mill results from May 15 to June 11, when the contest ended, were not so good as shown in the data given. The figures showing daily tonnage and kilowatt-hours per ton are averaged from the daily report sheets issued by the Inspiration management. These figures were accepted by the manufacturers of both the Hardinge and the Marcy mill, and there is no doubt as to their correctness.
No ball consumption was given out by the Inspiration company on the Hardinge mills because many changes had been made in the ball load. The operation of the Marcy mills was in charge of the regular mill crew, while that of the Hardinge section was under the supervision of Mr. Hardinge and his assistants, who were at the plant when the test was discontinued. The ball consumption of the Marcy mill in the entire plant is 1.7 lb. of steel for each ton of ore crushed. The speed of the Hardinge mills was faster than the Marcys; the. ball load was greater, and from the tabulated reports, the tonnage was less than one-half. From this, it would appear that the ball consumption in the Hardinge mill would be nearly double, as the total daily ball consumption depends upon the speed and number of balls used in the mill rather than upon the amount of ore crushed.
The ball-mill floor in this plant is equipped with a traveling crane capable of picking up a mill and its load of balls. When a mill needs relining, the bearing caps are removed, the mill is picked up by the crane, and a relined mill with its load of balls is placed in the same bearings. This saves the time that would be lost if the mills were lined in place, so that the actual loss of time due to ball-mills in the entire Inspiration plant averages less than 0.4 per cent.
Fine Crushing
The Anaconda Copper Mining Co. purchased about 50 Hardinge mills when it decided to install flotation. The mills were 10 ft. (3 m.) in diameter with a 60° cone on the feed end and a 40° cone on the discharge end, and with the cylindrical portion 48 in. (121.9 cm.) in length. This was about the size of one of the Hardinge mills used in the Inspiration plant and, as heretofore pointed out, was the most undesirable.
The Anaconda mills were equipped with 225-hp. motors, so that balls could be used. It was found that the pebble consumption was from 12 to 15 lb., which was prohibitive, and when steel balls were used the motors were not of sufficient capacity, for which reason, it was necessary to lag up the mills with wooden blocks. The cylindrical portion is now 7 ft. 6 in. (2.29 m.) in diameter, and about the same length, and the mills, due to the 40° discharge end, are practically cylindrical mills. The fifty 10-ft. Hardinge mills of the original installation have all been rebuilt to the above size and are operated at 15 r.p.m. The effect of converting these into cylindrical mills and reducing the speed has been a great improvement in cost and character of operation, as compared with the original recommendations.
The Calumet & Hecla Co. has installed sixty-four 8-ft. by 16-in. (2.44-m. by 40.64-cm.) Hardinge mills in its crushing plant at Lake Linden. These mills use pebbles and crush about 45 tons per day each, taking feed at below 3/16 in. and reducing it to about 30-mesh. A new crushing plant of this company, however, will consist of 8-ft. mills having a cylindrical portion 72 in. in length, which will make their inside dimensions practically the same as those of the Anaconda Copper Mining Co. The Calumet & Hecla Co., in running a test with a 5 by 20-ft. (1.5 by 6-m.) tube-mill and a Hardinge mill, found the tube-mill equally efficient, but it required too much space.
Analyzing the mill on the assumption that the greatest diameter is to produce the greatest effect in crushing, we find that the weight of crushing pebbles is proportional to the square of the diameter (machine half full); that the energy per unit pebble weight is something nearer the square than the first power of the diameter; and that the velocity with which the ore or pulp being crushed passes through the mill is inversely proportional to the square of the diameter. The result is that the energy applied per pound of pulp at various points along the cone is inversely proportional to about the sixth power of the diameter. This means that half way toward the apex of the cone, only 1/64 as much work is done as at the cylindrical portion, while three- fourths of the way toward the apex, only 1/4000 is done.
At Anaconda, and at the Calumet & Hecla mill, it has been found that a lengthening of the cylindrical portion increases the efficiency and capacity of the Hardinge mill. Undoubtedly, in the Hardinge mill there is a tendency for the smaller balls and pebbles to segregate in the conical portion. Taggart has shown, however, that the segregation decreases the efficiency of the mill. He says:
A ball charge composed of 5-in. balls makes a greater reduction in the size of particle at one passage through the mill than a mixed charge composed of 5-in., 4-in. and 3-in. balls.
The reason for the greater reduction in the size of the particles is that the smaller balls tend to segregate in the conical portion of the mill and cut down its efficiency, both on account of the small size of the cone and the small size of the balls themselves. There is not sufficient energy to do the work.
Reversing Feed to Hardinge Mill
I carried on experiments with the Hardinge mill in the laboratory of the Engineering Co., to determine its efficiency. It would appear that if the feed were introduced into the so-called discharge end there would be a marked difference between the resulting product and that produced when the feed is put into the mill through the feed end, in the regular way. To conduct this simple experiment, I operated a 36-in. (91.44-cm.) Hardinge pebble-mill, feeding first in the regular way, into the short cone. After running this test, I placed the scoop feeder on the long, or discharge end, and ran a second test in this maimer. The entire product in each case was caught in a tank, then mixed and sampled. My tests were carefully run on samples of quartz gravel, using about 1 ton of gravel to each test, with particular care to maintain uniformity of operating conditions for both tests. These samples, both feed and discharge, were carefully mixed and a portion cut out for screen analysis. All slimes were first washed out of the samples to be screened, through a 200-mesh sieve, and dried and weighed. The sands were then sized on Tyler standard sieves, using a Rotap machine, with results shown in Table 3.
Two similar tests were made at the University of Utah, using a mill of the same diameter, 3 ft., but with a shorter cylindrical portion, which, therefore, did not crush so rapidly. The result of these two tests confirmed the data observed in the first test, except slightly greater reduction in average size when operated the reverse way. This work was checked and reviewed by Prof. Robert S. Lewis. The power instruments were connected to the motor by the electrical department for the purpose of ascertaining whether the motor requirements differed when the scoop feeder was changed from feed to discharge end. From the averages, no differences could be determined. On account of the light motor load and heavy friction load, no attempt was made to determine the efficiency of the mill by measurement of power.
I will draw no conclusions from my own experiments, but desire only to say that I believe they are of sufficient importance to be repeated with a large conical mill. When operating a Hardinge mill at a very reduced tonnage, it is possible to make a fairly uniform product in one pass, just as it is with a cylindrical overflow mill with a reverse screw in the trunnion; but when operating with a large circulating load, according to modern practice, the shape of the conical mill is a disadvantage. It is suggested that the conical mill is strong because of its truss shape; but it seems unnecessary to build a truss over a long span when a tubular construction can do better work within less space and is equally strong. For instance, the conical mills at Inspiration were 16 in. longer between the bearings than the Marcys, with less than half the capacity.
The Marathon, or rod-mill, has not been adopted as quickly as one would expect. Undoubtedly it requires more care than a ball-mill, and its mechanical troubles offset its power efficiency in some degree. If the rods become bent its great advantage is lost. Its particular field is in fine crushing where slimes are considered undesirable.
Recent Tests of Ball Mill Crushing BY C. T. VAN WINKLE