Table of Contents
To compare properly two different methods of mining, it is not wise to attempt to calculate the total cost of each, but rather to compare the costs of those operations that are differently executed in the two methods under consideration. When any method is in operation the proper way to reduce the total cost is to dissect it into its different operations and see if the cost of one or all of these cannot be reduced. Keen competition has forced large manufacturing and other industrial enterprises to give careful attention to cost accounting and efficiency engineering. This calls for the investigation of the smallest details; conditions are adjusted so that a workman will not lose time in going after a tool or in walking from one machine to another. An increase in the efficiency of a number of small operations means a corresponding decrease in the total cost of production. The reasons why efficiency engineering has been so generally adopted are two: First, it attacks its problem in a scientific manner and, second, it brings results in dollars and cents. Mine managers are not excusable from applying its principles just because the ore may be high enough in grade to pay dividends even under lax conditions.
The different operations of mining can be separated in different ways. Fig. 8 is a diagram which shows a logical arrangement and the smallest operations shown in it could be subdivided into yet smaller details ad infinitum. In determining upon a new method or in attempting to reduce the cost of one already established, these operations must be carefully studied. If modifying a method will increase the efficiency of one operation without affecting the rest, the problem is a simple one, but usually more than one operation is affected, some adversely, and some to advantage, in which case to determine which is the cheaper method it is necessary to apply to each operation its cost under the different conditions.
Costs of Drilling and Blasting
Comparison of Stoping Methods
As an appendix to this article I have collected a number of costs of mining as published in the reports of mining companies.
Obviously but few of these reports show the costs of the operations given in Fig. 8, and in order to bring out their detailed costs it is necessary to make certain assumptions. To bring out the costs of the different operations of drilling and blasting, consider the data on North Star, Alaska-Treadwell, and the Rand as given in the appendix, with general data on Cripple Creek and the Michigan copper mines. All are straight overhand stoping methods with the exception of the Rand, and, although not so stated, I assume that these data refer to underhand stoping. The first consideration is drilling. Up to the present time for underhand work, mounted machines were the only ones available, but it is probable that in the future they will be largely replaced by light unmounted machines of the Jackhamer type. For overhand work, the hammer stoping drill, unmounted, has demonstrated its superiority in reduced labor cost, increased ease of handling, and greater drilling speed in rock that is not too hard. While actually at work machines will drill from 0.25 to 15 in. a minute, usually 1 to 3 in. At the North Star mine, evidently, 25 to 30 ft. are drilled for each machine shift. Light stoping drills are used and break 7½ tons in a 4-ft. stope. In Cripple Creek the average of several mines is 60 ft. for each machine shift. Light stoping drills are used and the machine runner is using the machine about 6 out of every 8 hr. In a 5-ft. stope 14 4½-ft. holes will break about 15 tons. In the Lake Superior copper region, in the amygdaloid lodes, machines drill from 30 to 47 ft. a shift in different mines. In the conglomerate, on account of more massive copper and higher quartz content, about 24 ft. for each machine shift is the average (9-hr. shift) and seldom is more than 28 ft. drilled. Holes in both cases are usually 8 ft. deep and a burden of 2 to 3 ft. is placed on them. In the conglomerate, although harder to drill, 54 tons are broken for each machine shift, whereas in the amygdaloid only about 38 tons are broken. The stopes are 10 to 30 ft. wide. At the Alaska-Treadwell each machine in the stopes drills 28 to 34 ft. and breaks 32½ to 45 tons. Heavy piston drills are used and the stopes are about 60 ft. wide. On the Rand large machines break 7.2 tons in a 55-in. stope, 10 tons in a 6-ft. stope, 19½ tons in a 7-ft. stope, and small machines break 3.7 tons in a 4-ft. stope. To arrive at the actual cost of drilling it is also necessary to consider the cost of compressed air, machine labor, repairs, and drill steel and sharpening. The cost of compressed air varies from $0.40 to $2.50 for each machine shift. It depends on the cost of power, size and efficiency of the compressor, size and efficiency of machine drill, and the conditions of the drill. The accompanying table taken from Ore
Mining Methods, by W. R. Crane, gives the cost of operating a 3-in. drill in granite.
At the North Star mines machine men cost $3 a shift or 43c. a ton; machine power, 40c. a shift or 6c. a ton; repairs and lubrication, 42c. a shift or 6c. a ton; drill steel and sharpening, 75c. a shift or 10c. a ton; total for drilling, $4.57 a shift or 65c. a ton. To this must be added 25c. a ton for powder, 4c. for fuse and lc. for caps, which makes a grand total of 95c. a ton. Allowing for rock left in the stopes this would be reduced about 15 per cent.
At Cripple Creek machine men cost $3.50 a shift or 24c. a ton; air, $2 a shift or 13c. a ton; repairs and lubrication, 25c. a shift or 2c. a ton; drill steel and sharpening, 50c. a shift or 3½c. a ton. Total for drilling, $6.25 a shift or 42½c. a ton. To this add 24c. a ton for powder, 3½c. for fuse, and 1c. for caps, which gives a grand total of 71c.
At the Alaska-Treadwell machine men and helpers average $6.85 a shift or 21c. a ton; air, 75c. a shift or 2c. a ton; repairs, 50c. a shift or 1½c. a ton; drill steel and sharpening, 45c. a shift or 1½c. a ton. Total for drilling, $8.55 or 26c. a ton. Adding 15c. for powder, 1c. for fuse, and ½c. for caps gives 42½c., to which must be added an additional 6c. for extra labor which includes powder men. This gives a grand total of 48½c.
The data given in the appendix on operations on the Rand are too incomplete to analyze as the others are analyzed. From other data which I have, I assume that the costs are about as follows: Machine labor. $4.50 a shift; air, 75c.; repairs, 50c. ; drill steel and sharpening, 50c. This is a total of $6.25. An average of 12 tons were broken each shift, which gives a cost of 52c. a ton for drilling. To this must be added 25c. a ton for explosives, which gives a grand total of 77c.
On the Rand, despite the greater stoping width, the cost of explosives is nearly as much as at North Star or Cripple Creek and the drilling cost per shift is much greater. The influence of the width of the stope on the tonnage broken is well illustrated in the Alaska-Treadwell and the added cost of machine labor is minimized. The long holes used and the character of the ore, no doubt, preclude the possibility of using one-man machines.
From the above, it is seen that an item worthy of consideration is that of drill steel and sharpening. It varies greatly but in any case is a factor worthy of attention. Not only the cost of new steel and the wages of the blacksmith, but the delivery of sharp drills to the stope and the removal of dull steel, should be considered. By comparing cost items occurring in the reports, which are given in the appendix, other features influencing drilling and blasting might be shown, but the greatest value results in making comparison when conditions are similar and are known with exactness.
Underground Mine Lighting
It is stated in one of the mining books that the cost of lighting for each ton of ore mined is about the same in all mines. This is not the case. The cost varies according to the light used and the number of men underground. The entire subject of mine lighting has been well covered by Frederick H. Morley in the Mining and Scientific Press of April 11, 1914. The following table taken from this article gives the cost of acetylene lighting in 10 of the large metal mines.
Timbering and Handling Ore in Stopes
Square-Set vs. Top-Slicing Methods
There is often a question as to whether square setting or top slicing is more economical. For instance, in massive deposits of heavy sulphides the ore is easily drilled and breaks well and if overhand stoping with square sets is used the weight of the ore necessitates very heavy timbering. In order to compare the two methods and also to show cost data on timbering and handling ore in stopes, assume the methods applied to a block of ground 50 ft. square and assume that the development work is the same in each case. The sets to be 6 by 6 by 8 ft. In the top-slicing method a drift one set wide is started at the top of the raise and run to the boundary of the block and then across the end of the block. This will give 10 sets of drift. It will take about eight holes, each 4½ ft. deep, to break a round, or 12 holes to break a set. All work in top slicing is breast work, and the drilling must be done with a drill mounted on a column. A mounted drill will drill slower and require more time for moving than a light, unmounted, stoping drill, which can be used in square-set mining. . The cost of each set for the. first 10 sets of drift in top slicing will be about as follows:
Each set contains 288 cu. ft. or approximately 28 tons; this makes the cost of drilling and blasting 48c. a ton. The distance to tram the ore to the raise will average 50 ft. and the cost for mucking and tramming may be estimated as follows: A man will shovel this ore at the rate of about 2½ tons an hour, or 1 ton in 24 min. A wheelbarrow will hold about 1/7 ton and to wheel seven barrow loads 50 ft. will require from 10 to 15 min., say 11 min; this makes the time consumed in shoveling and wheeling 1 ton 35 min. or 22c. a ton if shovelers, wages are 37½c. an hour. This is equal to 14 tons handled for each 8-hr. shift and a cost for each set of $6. Exclusive of the raise there will be a total of 63 sets in a slice, of which 10 sets are mined for 70c. a ton. (48c. for drilling and blasting plus 22c. for mucking). The remaining 53 sets have two free faces to break to. The cost of breaking will be 30 per cent, less or 35c. a ton. The average distance from the raise will be less, which will reduce the cost of shoveling, say to 20c., which gives a cost of 55c. for the remaining 53 sets or an average cost of 57c. a ton to mine the whole slice. On one slice there will be required 72 posts, 64 caps, and 72 girts. A post 8 by 8 in. by 8 ft. will contain 42 ft. B.M. and costs 84c. if lumber is figured at $20 per M. Caps and girts 8 by 8 in. by 6 ft. will cost 64c. each, figured on the same basis. Framing the posts will cost about 10c. each by hand or 5c. each by machines, say 6c. Framing the caps and girts will cost about the same. This gives:
which is $2.55 a set. The cost of taking the timber into the stope and placing it will amount to from $1 to $2 a set, say $1,50. Other timber for lagging and blocking will cost about; 85c. a set, which gives a total for timbering of $4.90 a set or 17c. a ton. This added to the cost of breaking and mucking gives 74c. a ton.
To mine this same block of ground with square sets, it would be necessary to divide it into two stopes four sets wide and eight sets long. The ore has been assumed to be heavy and it would be impossible to carry a wide stope. As soon as a stope is mined it should be filled. For drilling, a light stoping drill can be used which, under the assumed conditions, will drill easily 50 per cent, more footage than the machine on the slice. One-third less explosives per ton will be required, so the reduction in cost of breaking will be at least 30 per cent, or to 30c. a ton. A flooring of plank will have to be laid on the top set and about 75 per cent, of the broken ore will not fall into the chutes but must be shoveled. A shoveler will handle 3 tons an hour at a cost of 12½c. a ton for 75 per cent, of the ore. This is equivalent to 9c. for the total tonnage. We have, therefore, reduced the cost of breaking and shoveling from 57c. in top slicing to 39c. in overhand stoping with square sets, a saving of 18c. a ton.
Consider the timbering. It is not necessary to go again into detail but first assume that the ground can be held up by square sets of 10 by 10 in. timber. A post 10 by 10 in. by 10 ft. long contains 66 ft. B.M. and at $20 per M. will cost $1.32. Since it is heavy timber and framed on both ends, the framing will cost about 12c., making its total cost $1.44. Caps and girts will cost $1 each plus 8c. for framing, or $1.08 total. This makes a total of $3.60 for timbers in the square set instead of $2.30 in the set used in top slicing, an increase of $1.30. There will also be an added cost of placing of about 50c., which makes a total additional cost for timbering in the square-set method of $1.80 per set or 6½c. per ton.
These figures show very plainly that the timbering used in square set must be very heavy to make the increased cost over top slicing greater than is the saving in breaking. Here the saving in the latter is 18c. but the increased cost of timber is only 6½c., leaving a balance of 11½c. in favor of square-set mining. This is in accord with experience, but when timbers will not support the ground and many braces are required, or filling must be resorted to, top slicing becomes cheaper. Filling will cost at least 20c. a ton and probably 50c. One mine that I have had in mind in preparing these figures has a cost of timbering and filling of more than $1 a ton. At the Esperanza mine, Mexico, in 1907, 35 ft. B.M. of timber were used for each ton of ore, which at $20 per M. is 70c., and that for timber alone. It must be remembered, however, that in ore that does not break easily the additional cost of breaking in the top-slice method becomes much greater.
I do not want to give the impression that these figures are to indicate the total cost of mining by the different methods. There are many other items such as superintendence and ventilation that enter into the cost of mining. There are also incidental expenses in breaking and timbering, such as repairing, timbering chutes, and the like, the cost of which will enter into the total cost of any method, and the amount can be judged only by experience. The figures given bring out only the relative costs of different operations in different methods under assumed conditions. In my opinion, a few figures are necessary to aid in determining the merits of different methods or in reducing the costs of methods in use. With the figures, experience and judgment must be added. These are probably more necessary in mining than in any other business, because there are so many variable conditions.
How Much does Tramming Cost
The costs of tramming as given in published reports nearly always include not only tramming, but loading also, either from chutes or from the floor of a drift. The following interesting data are from the Engineering and Mining Journal of Mar. 8, 1913.
“At the Elkton Consolidated Mining & Milling Co., Cripple Creek, Colo., the 1911 cost of tramming was 14.6c. per car of approximately 0.7-ton capacity. The South Utah Mines & Smelters, Newhouse, Utah, reports its tramming cost for the year ended June 30, 1912, at 15.76c. per ton of-ore, which evidently includes the cost of handling waste removed. In Goldfield, Nev., tramming has averaged about 18c. per ton of ore produced from stopes and has ranged from about 14 to 25c. These figures are for actual tons trammed and do not include any shoveling in stopes. At the North Star mine, Grass Valley, Calif., observations show that a man pushes an 18-cu. ft. car about 150 ft. per minute and shovels about 3 tons per hour from a plat into car against 2 tons when shoveling off a rock bottom. According to this a shoveler’s efficiency is increased about 50 per cent, by using a plat. The Wolverine Co., Houghton, Mich., reports tramming costs at 17.4c. per ton of ore, and the Wettlaufer-Lorain, Cobalt, Ont., 21c. per ton of ore.”
Cost Breakdown of Tramming
If a man shovels from a plat at the rate of 2½ tons an hour and wages are 37½c. an hour the cost of shoveling is 15c. a ton. If a 1,000-lb. car is being used he will fill it in 12 min. If the tramming distance is 1.0 ft. and the trammer walks 200 ft. a minute the trip will require 10 min., allowing 2 min. to dump, the tramming will take the same time and cost the same as the loading, that is 15c. a ton or a total of 30c. a ton. Three things are very evident: Tramming is an important item of cost in nearly every mine, the size of the car makes a decided difference, and the resistance to traction of the car itself and the grade and condition of the track are important factors. I remember seeing one stretch of track 1,000 ft. in length over which it took two men to push a car of 1,000 lb. capacity. Figuring as above this increased the cost of tramming 15c. a ton, and in fact it was even more because the two men could push the car but slowly.
Henry Nagel, Superintendent of the Vindicator mine at Cripple Creek, has made some interesting observations in regard to tramming. He noted in one case that tramming on 800 ft. of level track cost 3c. a ton more than tramming on 800 ft. of track which had a grade of ½ per cent, in favor of the loads. In another case a man who had been shoveling and tramming in a drift with bad air, did 50 per cent, more work when a good circulation of air was secured. In his opinion the greatest efficiency is secured from a trammer when the track grade is such that he can ride the car when going in loaded. As to the latter assertion a few figures may show how it would not be true under all conditions. A car weighing 500 lb. and holding 1,200 lb. of ore will weigh 1,850 lb. when a 150-lb. man is riding. If it has good bearings and has a resistance to traction of only 20 lb. to the ton it will run on a 1 per cent, grade. If it has very poor bearings and has a friction of 60 lb. to the ton it will require a 3 per cent, grade for coasting. On this grade the empty car returning weighs 500 lb. and would require one-fourth of 120 lb. or 30 lb. force to push it. This is too much for the average man. The car with the good bearings would take only a push of 10 lb. to bring it back up the grade, which shows the importance of good bearings, as well as of good track.