The Colburn mill, in the Cripple Creek district, has been turned over to the operators, to be tuned up as a preliminary to steady running. It will shortly be in operation. Inasmuch as this mill marks a new departure in the treatment of the Cripple Creek ores, it is being closely watched by local operators as well as by mining men throughout the state.
In view of this appreciation I gladly avail myself of the invitation of the editor of Mining Science to give some authoritative information as to the reasons for building this mill and some notes as to the construction and process used.
The Ajax mill was constructed by Judge E. A. Colburn, president of the Ajax Gold Mining Co., with his own resources and under a working arrangement with the Ajax Gold Mining Co. For some years past the Ajax Gold Mining Co., in spite of making a large production, has not earned a net income adequate to its production.
A careful study of the situation showed that with the grade of mineral produced there were too many intermediaries in the handling of the ore. Further investigation showed that the only solution of the problem was to build a mill as close to the shaft house as possible and to cut out as many handlings of the ore as possible.
About a year ago Judge Colburn and his son, who is local manager of the mine, seriously investigated the treatment of the Ajax ores, and as a result finally adopted as a basis of work the proposals of J. C. Clancy to use the calcium cyanamide process in conjunction with electrolysis.
Mr. Clancy's proposals have been modified, and after a year's experimentation it is believed that a thorough workable metallurgical process has been evolved. The Clancy patents are controlled by the Moore Filter Co. of New York.
It may be of interest to note that in experimenting upon the Ajax ores average samples were not used. There has been some discussion as to the use of the plural in speaking of the ores yielded by a mine. T. A. Rickard, for example, objects to the use of the plural in speaking of tailings, concentrates, etc.
Inferentially he objects to the use of the word "ores" in speaking of the minerals produced by a mine. Yet careful study of the Ajax mine has clearly demonstrated that while the mineral produced is a gold ore, yet metallurgically the mine produces from its different veins ores of such widely differing metallurgical character that the plural seems to be more correct than the singular.
To experiment upon the average grade of ore might lead to some false conclusions, and hence a large number of characteristic samples were prepared from different levels and different veins for the purpose of testing; in all about 20 characteristic lots of ore were tested. It may be of interest to note that since these ores differed widely in their characteristics they will be kept segregated in the mill almost to the last operation. It is hoped by this method to secure the best metallurgical results.
It will be remembered that the original proposals of Mr. Clancy were to prepare a complex solution consisting of calcium cyanamide, potassium cyanide, potassium sulpho cyanide and potassium iodide, and to pass a current of electricity with a density of 50 amperes to the square foot through the solution, thereby generating, among other compounds, the halogen cyanide necessary to break up the telluride minerals. The solution had to be kept at or near the neutral point. The ore was to be "slimed" and agitated with the chemical solvents.
It early became apparent that these proposals would have to be modified in several important particulars, notably in the question of the density of the electrical current. It was proposed to use 90 amperes per ton of daily capacity. It was found that to force this amount of current through a solution with a density of 50 amperes per square foot at least 50 volts of effective pressure would have to be used, or in the neighborhood of 4½ kW. per ton of ore treated.
An agitation of 24 hours was necessary and the cost of electricity would have been prohibitive. It was found that by using a current density of about five to seven amperes per square foot from five to seven volts would do the work. Moreover, from 15 to 20 amperes per ton of ore treated, or from 105 to 140 Watts, was all that was necessary.
The process was, therefore, brought within the range of commercial possibility. Next it was found that it was impossible to maintain the solutions near the neutral point. This was due to the fact that in electrolizing potassium sulphocyanide, which is necessary to the process, the solution rapidly became acid and the cyanide was therefore rapidly attacked. The solutions should be kept rather alkaline.
It developed that in spite of the alkalinity of the solution as a whole in the neighborhood of the anodes the solution was always acid and, therefore, the halogen cyanogen could be formed. Iodo-cyanogen is rather slowly attacked by a solution containing 3 or 4 lbs. of alkalinity per ton, and hence the effect of that compound upon telluride minerals could be obtained.
As a preliminary to working out a mill process a very thorough study was made into the reactions which take place, and it was definitely proved that the process as developed rests upon sound scientific foundations.
Early in the year S. A. Worcester, mechanical engineer of Cripple Creek, was engaged to draw the plan for a 200-ton plant, making arrangement, however, so that it could be enlarged to at least 400 tons of daily capacity. The exigencies of the local conditions demanded a great deal of ingenuity in order to bring all the material together at the only possible millsite.
The mill had to be placed close to the mine but above the dump. Moreover, two lines of railroad track—the Florence & Cripple Creek narrow gauge and the Midland Terminal standard gauge (Short Line trackage)—further complicated the situation, and Mr. Worcester, together with E. A. Colburn, Jr., have worked out a very simple and ingenious method of assembling all the ores and dump material together at the crushing plant. The description of these plans, however, will not be attempted in this article.
The mill is divided into two parts: the crushing and sampling mill and the ore treatment plant proper. In order to get the ore treatment plant coordinated with the crushing plant the former had to be placed much higher than the latter.
The material is delivered to the chemical plant by means of a conveyor belt 250 ft. between centers. The crushing plant breaks the ore down to about a 10-mesh screen, the ore being delivered to the mill by a 6-ton ore car operated by two 5 h. p. crane motors, and receiving its electric power through a third rail.
The material is dumped directly from the car into a 5-C Symons crusher, which breaks the ore to about 1¼ in. From the crusher the ore is elevated and delivered to two conical sheet-iron ore bins, one of which is provided with a Fairbanks scale with a capacity of 35 tons. This weighing bin is for the purpose of weighing lessee ore.
From the ore bins the material is passed through a magnet and is elevated to a set of Colorado Iron Works impact screens. After screening the ore is delivered to three sets of Allis-Chalmers rolls, 16 by 36 in. The screens used are ⅝, ¼ and 10 mesh. All the material that passes through a ¼-mesh screen passes over a 27-in. Snyder sampler which takes a cut of 5 per cent. The sampler cut is delivered to a sampling plant which presents no novel features.
The finely crushed material is delivered to a 16-in. rubber conveyor belt, already referred to, and is carried up a 20 degree slope to a vertical elevator which delivers the crushed ore to three large, round steel ore bins situated at the head of the chemical treatment plant. The combined capacity of these ore bins amounts to 600 tons. Each bin will store a different character of material, and they are arranged so that they will each feed a separate classifier.
The classifiers are of simple design, merely consisting of a trough, in the bottom of which a 12-in. screw is operating.
The fine slimes washed out by the classifiers are delivered immediately to the agitator system and the over-size, which is diluted to a proper degree of thickness, feed a Stearns-Roger spiral fed trunnion tube mill 16 ft. long and 5 ft. in diameter. There are three tube mills, each with its own closed cycle and each crushing to a different degree of fineness adapted to the character of the ore which each tube mill is receiving.
The crushed pulp is elevated by air lifts of special design both from the delivery end of tube mill and from the slime discharge of the classifier. The ore is crushed in solution.
The slime goes to a special design agitator. There are five agitators, each of a capacity of from 80 to 100 tons of dry slime. The pulp will have a ratio of one ton of ore to 1.5 tons of solution. In the agitators there are 20 electric baskets for the purpose of electrolizing the solution.
Each electrode basket has an anode surface of about 4 sq. ft. and with a capacity of 35 amperes each. The electrodes are carried on heavy steel rail bus bars. The current is furnished by a General Electric 15-kw generator arranged to deliver 3,000 amperes at 5 volts, or 1,500 amperes at 10 Volts.
By means of field resistances the voltage of the generator can be regulated to anything between five and ten, with corresponding amperages. The delivery of the current is regulated to each agitator by means of a Westinghouse switchboard.
The ore which will be crushed to 150-mesh screen approximately is agitated for 24 hours with low-pressure air delivered by the mill compressor, although the air system is connected with the mine compressor giving air at 100 lbs. in case of emergencies.
After agitation the pulp is delivered directly to the loading tank of the Moore filter plant with a filtering area of about 9,500 sq. ft. and a capacity of about 200 tons of dry slime daily.
The Moore filter plant offers some novelty in that the crane is entirely operated by hydraulic power, both for lateral and vertical motion. The hydraulic cylinders of the crane work at a pressure of from 250 to 300 lbs. Water is forced into a 4 by 12-ft. accumulator by means of a belt driven Deane triplex pump with plungers 2¾-in. and making a 6-in. stroke.
This pump consumes about 5 h. p., which suffices for the operation of the crane. The power consumed by this filter plant compares favorably with any installation now in existence. The use of the equalized load furnished by a hydraulic pump compares favorably with electrical cranes both for economy and ease of operation.
The clear gold solution delivered by the filter system is precipitated by zinc dust fed intermittently by means of a tripping arrangement operated by the flow of gold solution. The precipitated gold is collected by two Stearns Roger special filter presses with 24 by 30-in. plates. Each press contains from 24 to 30 plates.
The slime delivery from the Moore filter plant will be made by a screw conveyor working in the bottom of the discharge tank. The slime will be conveyed from the mill in a large bucket carried on a cable-way and moved by a small electric hoist.
On the cable-way trips are provided so that automatically the bucket will dump in a different place at every trip. By this arrangement it is hoped that the material will have a chance to spread and dry before another bucketful is dumped upon it. This is of importance to prevent large slides in the slime piles.
The chief points of interest in the plant from a mechanical point of view are the extremely simple arrangement of the various parts of the mill, the use of air lifts everywhere to handle solutions and crushed pulp, the special agitators, the zinc dust precipitation, the hydraulic operated filter crane and, lastly, the method of discharge of the slime.
It should have been noted that this mill is entirely operated by electric motors, all of the General Electric make. The principle has been adopted, as far as possible, of driving every important unit by its own special motor, thus making for great flexibility combined with low power costs.
The electric current is furnished by the Colorado Light and Power Co. from Canon City, and is stepped down in one of the best arranged and mechanically perfect transformer houses in the Cripple Creek district. The transformer house is entirely constructed of Portland cement and the general arrangement reflects great credit upon J. H. Hitchcock, the electrician at the plant.
In the chemical treatment part of the mill is situated the laboratory from which the metallurgical operations are controlled. Many tests have been made as to the probable results which may be expected in the plant. At this time it is unnecessary to go into the questions of extraction, chemical consumption and costs, inasmuch as the mill itself can only satisfactorily settle these matters.
It suffices to say, however, that the expectation is, when the mill becomes settled down to steady work, the extraction process will leave probably less than $1 per ton in the tailings, with a chemical consumption of about 12 cents per ton and a cost of operation of $1.50 per ton.
It may be some months before these figures will be obtained, but several hundred tests have established the extreme probability that they will be reached eventually.
The plant is in charge of E. A. Colburn, Jr., as local manager; W. A. Kunkle, mill superintendent; J. A. Hitchcock, electrician; W. A. Gilbert, chemist; A. C. McKeehan, master mechanic.
It is not too much to say that the successful operation of the Ajax mill means a great deal to the Cripple Creek district. There are in that district quantities of medium grade ore which cannot now be handled or treated at a profit. At present no mine can earn a profit on $10 ore.
If the Cripple Creek district is to progress it is absolutely essential that some relief must be found for this situation. It is not improbable also that the successful solution of the problem of treating low-grade ores may result in shipment to custom mills and smelters of a much larger quantity of high-grade ore than is now the case.
It is unquestionably true that when mining the low-grade, bodies of high-grade ore will be encountered, and it may prove more profitable to ship this high-grade ore rather than to treat it locally. But, in order to reach the smaller bodies of high-grade ore it is necessary that the mine should at least pay all its expenses out of the low-grade material.
In the Cripple Creek district there is a very large amount of material handled in proportion to the tonnage of ore shipped. The many dumps of the district are mute but irrefutable witnesses to the accuracy of this statement. If all this material could be made to bear its quota of mine expense a new era of activity would be inaugurated in the Cripple Creek district, and one of greater prosperity than it has yet seen.