The mineral that is two millimetres and under in size is sent to the Hardinge mills for re-grinding. These mills are 10 ft. diam. by 4 ft. long, and are in closed circuit with simplex Dorr classifiers, one classifier to each mill, 6 mills to the section, and 8 sections in the establishment.
The overflow from the classifiers goes to the flotation division, and the classifier-sand is returned to the Hardinge mills. At present pebbles are being used in the Hardinge mills, but, in all probability, steel balls will ultimately be used. With pebbles the Forbes lining is used, but with steel balls in use, each mill will be fitted with a false wooden lining to reduce the diameter of the cylinder, and a manganese-steel lining will be placed inside of this.
Each mill has a direct-connected 225-hp. motor. The mill fitted with pebbles required from 95 to 115 hp., but the motors are made extra heavy, in anticipation of the use of steel balls.
In each section of the flotation plant there are four Minerals Separation machines, each having 15 agitators and 14 floating compartments. Below these are six Callow cells for cleaning the concentrate. The agitators tor the Minerals Separation machines are made of gun-metal and are driven by bevel-gears from the main shaft. (See Fig. 17.) Each machine is driven independently by a 150-hp. motor, running at 385 r.p.m. on full load. The speed of the agitators is reduced to about 225 r.p.m.
The product from the machines is a tailing, which goes to waste (from this tailing, fire-brick, building-brick, and acid-proof brick will be made in a new brick-plant now under construction) ; a concentrate, which is sent to the Dorr thickener-tanks, for settlement—this comes from the first three cells of the frothing-machines. The rough concentrate from the next six cells of the M. S. machines, is further cleaned in the Callow cells (See Fig. IS), making a clean concentrate; and the middling, which, with the middlings from the remaining five cells of the M. S. machines, is returned to the feed of these same machines.
Fig. 17 minerals separation frothing machines.
About 6 to S lb. of 50° B. sulphuric acid, per ton of flotation feed, is used, together with two to three pounds of kerosene-sludge acid and from one-half to one pound of wood-creosote. A portion of the wood-creosote is added to the head of the Hardinge mills, and the remainder is added at the head of the M. S. machines. The pulp is heated to 60 or 70° F. by steam introduced into the first agitating-box. The method of adding the reagents is unique. The mixer consists of a revolving disc, to the circumference of which are attached a number of cups. The disc is set vertically, so that its lower edge dips into the pan of acid, or oil; as the disc revolves the cups are filled, and later discharge their contents into a suitable launder leading to the flotation machines. The disc is driven by the friction of a wheel against another disc attached to the main drive. The wheel is run at constant speed and any variation in feed can be made by changing the point of contact between the wheel and the disc. In addition to the speed regulation, the amount of acid, or oil, fed may be varied by changing the number of cups, or the size of the cups.
On account of lack of space in the old mill, now being remodeled for flotation, an auxiliary plant has to be installed to handle the extra slime from the mill. This is now under construction and will consist of 20 M. S. machines of the same type as those now in use. Fourteen of these will be required for treating the current slime from the mill; the remaining six will treat old slime from the dump. This will be brought from the dump dry and mixed with water before treatment in the machines. The capacity for the dump-slime is about 1000 tons per day. No Callow cleaners are to be used in this plant.
The concentrate from the flotation machines is thickened to about 60% solid in Dorr tanks, 50 by 12 ft., the spigot-product of which goes to 11½-ft. diam. by 12-ft. face, Oliver filters, each having a capacity of about 150 tons daily.
The cake from the filters contains about 18% moisture and will be fed onto a conveyor carrying mill-fine concentrate to the new roasting-plant now in process of construction. In this way the filter- cake will be fairly well mixed with the concentrate, so that a generally uniform feed will be obtained for the roasters.
At the present writing two sections (out of eight) of the old mill have been re-modeled to use flotation; and a third section is now nearly ready. The mill is being re-modeled a section at a time by the company’s engineering department. Some remarkably rapid work has been accomplished in changing over from one system to the other.
Fig. 18- Callow Cleaner, Showing the Mineral Froth
For instance, the No. 2 section of the mill was in operation on the 26th day of May for ordinary water-concentration; but was wholly re-constructed and in operation, using the flotation process, on the 26th day of June. Each section has a capacity of 2000 tons of original feed daily. At the present rate of re-construction the entire plant will be re-modeled by January 1, 1916.