Table of Contents
Only where a group of mines operates in a single district are costs comparable and then only with reservations. In general, cost systems are fairly uniform, yet in studying costs of a number of plants it is noticeable that in some cases there is a tendency to omit certain operations which are proper charges against ore dressing and treatment. These should cover the first stage of coarse crushing, whether it be underground or on the surface, as well as the disposal of the residue, the recovery of bullion, and returns from products sold and must include the cost for labor, power, supplies, repairs, and compensation.
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In 1936 when data were being compiled for “Cyanidation and Concentration of Gold and Silver Ores,” considerable published information was available on milling costs in various parts of the world. At the present time, however, it is extremely difficult to obtain reliable figures on the cost of ore treatment owing to the fact that during a period of rising prices and wages the mine managements do not consider current cost data typical of normal operation and are unwilling to release them for publication.
Another factor which applies particularly to the United States and Canada and which tends to make cost-per-ton figures unreliable is the disparity between the rated capacity of many of the mills and the actual tonnage being handled today. This is partly attributed to shortage of underground labor and partly to the fact that during the war period not only was maintenance heavier than normal but opportunities for improvements in technique were lacking.
The operating costs for straight cyanide plants show a greater uniformity than is the case for plants employing combinations of cyanidation and flotation.
Figure 97 shows the relationship between the tonnage capacity and total milling cost, per ton based on the 1939 figures for a number of typical Canadian plants. Saving in overhead and labor is the principal factor that enters into the decreasing cost per ton for the larger operations.
Considerable variation will be found in individual cases depending upon hardness of ore, fineness of grind, hours of treatment required, reagent consumption, and the situation of the property in its bearing on cost of supplies, etc.
The total cost of producing an ounce of gold in Canada increased from $22.35 in 1939 to $32.07 in 1945, according to the report of the director of the Ontario Mining Association for 1945. This represents a 43.5 per cent increase. From various other data which are available, however, it appears that milling and treatment costs (mining excluded) have probably not risen on the average over about 30 per cent. The broken line in Fig. 97 indicates estimated present (1948) average cost on the basis of this 30 per cent rise.
It is probably safe to assume that the milling costs for straight cyanide plants in Canada today (1948) will be found to be somewhere between these two lines.
Kerr Addison, for instance, is milling 2800 tons per day for a total of 72 cents per ton.
Hollinger in the 40 weeks ending Oct. 6, 1948, milled an average of 3627 tons per day at a total cost of 77.29 cents per ton, of which 37.90 cents was labor cost.
Average figures compiled from a number of plants indicate the following general distribution of costs on a percentage basis, though considerable variations are noted between individual plants.
Where a combination of flotation and cyanidation is used, the combined cost approximates this same percentage.
This includes such items as heating and lighting, sampling, assaying, experimental work, repairs, and various indirect costs, depending upon the system of cost distributions in use. It is partly because widely different methods of charging out such costs have been adopted that considerable divergence in overall cost distribution is to be found.
DIRECT CYANIDATION COST
The following typical examples of cost distribution in Canada may be of interest.
FLOTATION AND CYANIDATION COST
A typical example of cost distribution is to be found in the following 1936 figures for a 1000-ton-per-day flotation and cyanidation plant in Ontario, Canada.
FLOTATION, ROASTING, AND CYANIDATION COST
Consolidated Beattie gold mines is a good example of a large plant employing flotation, roasting, and the cyanidation of concentrates. Approximately 1300 tons per day of arsenical gold is treated for an overall cost of $1.05 per ton, distributed as shown in Table 98.
The roasting cost works out at approximately $1.22 per ton of concentrate, distributed as shown in Table 99.
At MacLeod Cockshutt Gold Mines, Ltd., the cost of roasting in 1941- 1942 was 32 cents per ton milled or $1.25 per ton of ore roasted, while at Lake Shore mines for the same year the cost was about 80 cents per ton roasted.
LOW-COST OPERATION IN THE UNITED STATES
The 700-ton mill operated by the Standard Cyanide Co. in Nevada between the years 1939 and 1942, when it was closed as a result of government order during the Second World War, succeeded in making a profit from ore carrying as little as 0.06 oz. gold per ton. Cheap, open-pit mining methods were used, and good extractions were obtained when grinding to only 3 mesh. These, among other factors, made for extremely low-cost operation.
The 596,482 tons milled yielded $1.86 per ton at a total cost of $1.18 per ton of which $0.52 was milling cost.
COSTS ON THE RAND
The distribution of costs at Randfontein Estates Gold Mining Co., which is typical of the older sand-slime type of plant, is shown in Table 100.
In the case of the more modern continuous-treatment plants, East Geduld’s total milling cost in 1946 was 36d (60 cents), of which Cyaniding accounted for 19.5 cents.
The percentage cost distribution at Marievale Consolidated Mines, Ltd., the newest plant of the Union Corporation, is shown in Table 101 for the year 1947, during which 617,000 tons was milled.
White labor includes operating and engineering, supervision, plant mechanics, foremen, and shift operators. Native labor includes cost of compound which is about 42 per cent total native costs.
Cyanidation
The power required in cyanide plants varies with type of ore, fineness of grind, etc., but in general the range is 20 to 30 kw.-hr. per ton of daily capacity.
The power distribution at Preston East Dome mines in Ontario, Canada, is shown in Table 102.
The relative distribution of power between the crushing and grinding sections will vary according to the fineness of crushing and the type of plant, but on the average these departments will together consume 60 to 70 per cent of the total power.
Flotation. The power consumption for straight single-product flotation plants varies, according to A. M. Gaudin, from 12 to 20 kw.-hr. per ton, depending on the fineness to which the ore is ground. The average percentage power costs for the various.departments of seven United States producers is given in Table 103.
POWER CONSUMPTION ON THE RAND
The power consumption at Randfontein Estates, which is milling 13,000 tons per day by the older sand-slime process, is shown in Table 104.
Distribution figures for the new 2100-ton-per-day Marievale plant are shown in Table 105.
