The larger portion of the rock hoisted from the mines of the Cripple Creek district is separated into four products: waste, ore, screenings, and slime. In a few cases a fifth product, called middling, is separated.
One or more of the large mines estimate that nearly 85% of their rock is thrown on the dump as waste after sorting, leaving but 15% ore. Hence it happens that a mine shipping 15 tons of ore daily may have to hoist 100 tons of rock, in order to maintain its output.
This explains the immense size of some of the larger waste-dumps of this district, which includes, with their immediate surroundings, the towns of Cripple Creek, Victor, Elkton, Anaconda, Goldfield, Independence, Altman, Cameron, and Gillette.
In these mines practically the entire vein is broken and sent to the ore-house for sorting, and where the vein is too narrow for a stope or drift, enough waste is shot down to give the necessary width, and all of this rock is sent through the orehouse.
In the most economical plants the ore is automatically dumped by the mine-skip through suitable deflecting spouts to any one of several grizzlies.
Beneath the grizzley, which is inclined at an angle of 45°, and has bars spaced from 1½ to 4 in. apart, is placed the screen, of ¼ in. wire-cloth with ¾ to 1 inch openings. Middling, or the product which passes the grizzley, but not the screen, is usually allowed to fall with the crude ore from the grizzley into the crude-ore bin, in which case the grizzley is used only to protect the screen from damage by large rocks, and the screen-bars are widely spaced.
In some cases a diagonal spout or trough is placed beneath the screen, running from one corner to the opposite one. The spout is very narrow and diverts a small proportion of the screenings into a bin or box. This sample is assayed, and in some cases shows such low value that the screenings are thrown on the dump, as otherwise they would be shipped at a loss to the operator.
It is characteristic of nearly all of the ores of this district that the gold is found principally in streaks of friable material lying between the harder and the barren gangue. In the operations of mining the greater part of the gold-bearing material is broken comparatively small, leaving the waste in coarser pieces. By passing the material over a screen of suitable size of opening, the richer may be separated from the leaner ore.
Considerable profits have been made by lessees by merely screening old dumps which had not been previously screened. The fine material which adheres to the gangue, and is removable after screening only by washing, or some equivalent operation, has still higher gold-tenor than the screenings.
At the Strong mine, which is an exception to the general rule, in that the ore exceeds the waste in quantity, the sorting, after screening, is done on low tables, using shovels or forks for handling the ore, and picking out the waste by hand. The waste is thrown into cars and trammed to the washer.
A primitive method of screening which is used by some of the small dump lessees, consists in shoveling the rock onto an inclined screen in the same manner in which sand is ordinarily screened for making mortar.
The second step in the process of preparing ore for shipment is by washing at some mines, and by dry-sorting at others. Where specially accurate sorting is desired, the preference seems to be for washing, for the reason that the various minerals show much more distinctly when the rock is wet, and has the dust removed from its surface.
At mines which wash only the waste, the sorting tables are placed at the gate or mouth of the crude-ore bin. The table has a steel or cast-iron top from 3 ft. square to 3 by 4 ft. in size, and from one to four men work at each table.
At the larger plants the ore and waste are raked or scraped over the edges of the tables into separate spouts leading to ore and waste-bins, or perhaps to washers.
At the Portland, Findley, and Golden Cycle mines, the ore is scraped over the front edge of the table into spouts leading to the ore-bins and the waste is scraped over the sides into spouts leading to the washers. A small piece of sheet-steel is used for a scraper.
At the Elkton, Cresson, Granite, and Abe Lincoln mines the crude ore is screened by revolving cylindrical screens, and washed by the Crane patent revolving cylinder washer. This cylinder is inclined, with its lower end below the surface of the water in the slime-tank.
The cylinder is perforated with about ½ in. holes, and has an interior spiral which carries the rock from the lower or receiving end to the upper end, and delivers it on a picking or sorting belt. This belt is from 30 to 50 ft. centre to centre, and has a number of spouts beside it, into which the sorters throw the ore, allowing waste to pass on over the end of the conveyor to the waste-bin or car.
While the ore is probably washed more thoroughly by the Crane washer than by the more commonly used spraying device, the initial cost of the Crane machine prevents it from being more generally used.
Its washing is more thorough, for the reason that it turns the rock over and over, subjecting all sides to the action of the water, while in the spraying-washer the ore merely slides through the machine, without turning over and over, and with very little of the active rubbing which occurs in the cylinder machine.
The spraying washer, in its ordinary form, consists of a spout or chute placed visually at the mouth of the crude-ore bin, but in some cases having the ore or waste brought to it by spouts. It has a steel-plate bottom with ⅜ to ½ in. perforations closely spaced and with a slope or fall of two thirds of its horizontal length.
A gate at the lower end of the washer is opened periodically by the sorter or trammer, allowing a small quantity of ore to pass through onto the sorting table, or if waste is being washed, the gate delivers it to a car, which is trammed to the waste-dump.
The slime and fine material are washed through the perforated bottom-plate into tanks, in which they are deposited. A pump returns the water to the spray-pipes with sufficient pressure to give the issuing jets considerable force, in order to make the washing effective.
Usually a rotary pump, driven by an induction motor, is used, but in some cases steam or other pumps are employed. The settlings are shoveled out of the tanks periodically and dried either by steam-heat or in pans heated by fire.
At some plants, as for instance at the El Paso mine, after equipping all crude-ore bins with spraying washers, wash only certain classes of ore, shutting off the water from one or more washers and allowing the ore to pass through dry.
The spraying washers used at the Findley and Strong mines have reciprocating perforated pans with a slight inclination. The waste rock is fed to the upper end of the pan from bin-gates, and is sprayed during its passage to the lower end. The slime passes through the perforations into settling-tanks beneath, and the washed rock drops from the lower end of the pan into the waste-bin or car.
The simplest form of washer uses an oblong box, having a perforated plate resting a few inches below the surface of the water with which the box is nearly filled. The rock is shoveled onto the plate and moved around in the water with a fork or shovel until washed, the slime passing through the perforations to the bottom of the box, which is cleaned periodically and the slime dried and sold. This crude arrangement is used, of course, only on a small scale.
In designing a surface-plant for hoisting ore and preparing it for shipment, it is well, in view of the processes which have been described, to give the orehouse ample height, so that the waste may be delivered at a sufficient elevation to allow of dumping for a considerable time.
It is also well to consider the advisability of having the mine-skip dump directly, by suitable spouts, into the ore-house, thus eliminating all expense in transporting from the shaft to the ore-house. Where a large tonnage is handled this item becomes one of considerable importance.
The objection which may be raised, that fire in the ore-house would endanger the mine, can be easily overcome, for example, by fire-proof doors.
The importance of thorough washing is illustrated by the fact that one large mine has made for some years a constant practice of washing all waste in three successive spraying washers, and saving slime of considerable value by the third washing.
A considerable number of old waste-dumps have been washed over with success, in some cases tramming the rock to a stationary washer of the spraying type, and in others using a portable machine having a cylinder revolving in a tank, similar in arrangement to the Crane washer, but small and light enough to be moved and kept within easy shoveling distance of the pile to be washed.
The rock is shoveled directly into the receiving end of the cylinder and delivered at the other end to a car, which is trammed away and dumped. Power for rotating the cylinder is furnished by a 2 or 3 hp. induction-motor.
The ore-house and head-frame are of recent design, and embody some of the best features of current practice in small plants. The skip dumps its contents on a short slope, from which they fall on the deflector. This deflector or door may be swung through an angle of 90°, so as to direct the crude ore to either one of the two grizzlies.
This arrangement contemplates the addition of a third unit, including grizzley, bins, sorting table, washer, etc., at a future time. As it will be necessary to handle waste, without sorting, when development is in progress in the mine, a waste-bin is provided, close to the head-frame, and a switch, operated by the engineer, is arranged to dump the skip-load into the waste-bin.
The skip is so designed that the box or body can be easily and quickly removed and hung up out of the way. A removable cage-floor is then placed on the cross-bars at the bottom of the skip-frame, converting it into a cage for conveniently handling men, rails, timbers, etc.
In this plant middlings are not separated, but pass over the screen and into the crude ore, and are washed and sorted. Ordinary mine cars stand on the sorting-room floor, within easy reach of the sorters, who throw the waste into the cars and tram it to the waste-dump, while ore is scraped over the edge of the table into the vertical spouts leading to the bins below.
The screens are adjustable to varying angles, so as to be effective for either dry ores or those which, being wet and sticky, require a steep pitch in order to prevent clogging. The screening-bins have long spouts passing down the corners of the ore-bins and delivering screening at the same elevation as the ore, for shipment.
The settling tank for the slime from the washers has several unusual features. Its construction is very cheap, it being merely nailed together, without any tonguing and grooving, the lumber being dressed on one side only and no rods are used. This rough and cheap construction has proved thoroughly practical for slime-tanks of moderate depth.
The arrangement of baffles for minimizing the speed of the entering and out-going water and slime and for preventing any rapid currents in the tank, makes it an effective settler and reduces the size and expense of the tank for a given capacity.
A steel-plate lining, at one side of the tank, forms a steam heater, through which the exhaust from the hoist passes, preventing freezing in winter, and giving more effective washing than can be accomplished with cold water. An exhaust steam drier, consisting of a shallow wooden steam box, covered by steel-plates, on which the slime is dried, is placed in the space under one screening bin, not occupied by the slime-tank.
Light for the sorting tables comes from the top, side, and ends of the sorting room. All bin-bottoms slope 45°, so that no shoveling or scraping is necessary in order to empty the bins. All posts rest on concrete piers, with ¾ inch dowel pins.