Show modern gold milling practice BY S V TRENT the general tendency of modern gold milling may be stated as being towards a concentration in capacity and a segregation of departments that is to say larger units are coming into vogue and it is now aimed to concentrate and separate various branches of the work such as the coarse and intermediate crushing the fine grinding etc and also to a more marked degree to separate the amalgamation process enair ly from the milling or grinding of the rock this last mentioned departure in particular is being very extensively followed in the transvaal region of south africa and the newest and largest recently constructed or now projected are nearly all based upon the crushing and grinding in one department and locating the work of amalgamation in an en entirely firely separate building experience peri ence shows that this segregation tends toward greater efficiency and also greater economy of work especially in the direction of amalgamation this practice is desirable as by its means it is much easier to offset off set the loss of amalgam etc which is usually such a difficult factor in most gold milling plants one cause for this tendency is the importance of limiting the amount of space occupied by mills the average size of which grows larger and larger every year for it is found that the concentrated plants are much more easily supervised and labor economized fine pulping or grinding Re the merit in general of fine grinding as applied to milling gold ores by amalgams tion is also well established but now that m copt os t g gold ores demand the use of cyanide leaching IF achin as a finishing I 1 treatment and as in most cases the most perfect results in cyaniding cyan iding aye are also attained by fine grinding the process is becoming now very general in carrying fine grinding to an extreme it is for the above reason that the tube mill or grinding pan is so generally rc sorted resorted to in modern plants for it is cound that in tamp milling the attempt to carry the g grinding or crushing to the necessary fineness results in a rapid drop in the capacity of the stamps by the time thirty or forty mesh screens are reached the reduction in output with stamps when the grinding in r has to go beyond that stage is so very marked that it makes the operation altogether too costly the result is usually the addition of tube mills to a plant representing in a 1 heavy item of additional cost as well as a large increase in the total power required qui red combination of crushing and sliming almost any kind hind of gold ore will be better treated when the crushing process can be accompanied by sliming but there are many cases of ores of so low a grade that added cost of the sliming operation makes 1 it prohibitive especially when it has to be carried out by separate process and in dif afferent machines for these latter cases at least the mill mans groat great desideratum will be a type of crushing or grinding machine which will in one operation reduce the quartz to a sufficient degree of fineness with only a small loss in normal capacity large units vs small units the above stated tendency towards using units of large capacity rather than small units which was formerly so much favored in gold milling is pretty closely restricted so far as the work is confined to stamps it is true that gravity stamps have been constantly increasing in size and weight until stamps of from 1500 to 1800 pounds are becoming frequent however even this gain in capacity does not help matters very much in plants of large tonnage it is in these operations that a machine such as the mill looms up as affording great rel relief lefas as the logical solution of the situation for instance the 6 foot mill of which there are a large number in use will perform at least as much crushing as a 20 stamp mill of 1000 pound stamp and in some cases it will easily equal thirty stamps the 7 foot is the full equivalent of at least thirty such stamps and generally speaking ng will keep pace with forty the 8 foot which is now in process of manufacture will displace fully sixty of such stamps on any character of crushing the 60 giamp mill if placed in a continuous line as is usually done will occupy a length of building of feet these stamps alone without reference to feeders or other connecting pieces will cover square feet of floor surface they will have bearings or three bearings for each camshaft and three for the reducing or counter shaft the amount of power required to obern operate te these stamps will be horse power the 8 foot on the other hand hafid will vill have two bearings its length will be twenty feet its floor space square feet and the average power required will be 50 horse power in the other item of attendance it will be safe to say that not more than one half or probably bly one quarter of that needed for the stamps will be required by the single 8 foot mill |