This machine is of the rotary drum filter type, but the filtering medium is applied on the interior of the drum which acts as its own container. The inner surface of the drum is divided into a series of fabric-covered panels, each forming a face of a polygon inscribed within the cylinder. The compartments thus formed connect individually with an automatic valve, which, in turn, applies vacuum and compressed air in proper sequence. The closed end of the drum is carried by a trunnion and bearing, and the open end is equipped with a steel tire which runs on rollers. An annular ring forms an internal flange at the open end, serving as a dam for retaining the pulp being filtered. Feed may be introduced by a pipe or by a trough passing through the open end. Cake is discharged by compressed air into an internal hopper, equipped for either chute discharge or discharge on to a conveyor belt or screw conveyor.
Side and end elevation drawings of the Dorrco filter are shown in Fig. 34. Oliver United Filters makes these machines in six diameters, from 4 to 14 ft., and up to 18 ft. in length.
At the Pioneer mill, British Columbia, the practice in maintaining the Dorrco filter was to treat the filter cloth—a No. 26 twill—with acid every 21 days. The procedure consisted of hosing out the filter, dumping in a dilute solution of hydrochloric acid, and rotating the filter drum for an hour; the filter was then drained, again hosed out, and put into service. There was no scrubbing of the cloth, the pores being kept open easily throughout its life. To keep the cloth in best condition, low-pressure air was blown through it for several seconds after the cake had been discharged. The filter cloth was changed after 108 days of service, two men making the change in about 8 hr. Each filter had 371 sq. ft. of canvas area, which, on the basis of washing 200 tons of solids per day, showed a capacity of 1078 lb. per sq. ft. canvas per 24 hr.
Rotary Drum Filter Manufacturing
Rotary Drum Filters are of all-metal construction, including cast filter segments. There is no loss of vacuum due to warping or deterioration of filter drum. Each segment consists of a correctly machined metal casting. The segments have the drains so sloped that the filtrate flows rapidly to the pipe connections at one end. These metal segments are bolted and doweled to two heavy steel end plates which in turn are bolted to cast iron hubs. The entire assembly is then mounted in a large lathe and accurately turned. Acid resisting metals can be furnished.
Rotary Drum Filters will successfully dewater finely ground materials such as flotation concentrates and cyanide tailings, so as to secure the best washing and drying of the filter cake. The capacity, moisture content, and the nature of the cake depend upon the physical characteristics of the material handled These factors can best be determined by laboratory testing.
Rotary Drum Filters are of all metal construction with cast iron filter segments bolted to heavy steel ends so that the leakage and warping, so common in wooden types, is eliminated. The segments have the drains so sloped that the filtrate flows rapidly to the pipe connections at the end. An oscillating agitator in the bottom of the tank keeps the pulp in suspension, and a repulper can be furnished for the discharged cake.
The valve can be arranged for drawing off two solutions and is provided with a port for a short, quick blast of air to loosen the cake before scraping, and also a port for a hard blast of air after scraping to open the pores and clean the cloth or other filter medium used.
Pipe connections to the valve and segments are exposed on the outside of the drum and are readily accessible. Simplified all metal construction permits economical transportation, quick installation, and low operating cost. For granular types of feed high capacities can be obtained by use of a fine wire screen as the filter medium—using oversize piping and valve ports and increased vacuum capacity. For industrial or special uses acid resisting metals can be furnished.
Special emphasis has been given to all operating features and to simplicity of operation. At the same time unnecessary refinements have been eliminated without sacrificing mechanical efficiency or metallurgical performance.
Rotary Drum Filter Capacity
There is a definite need for this unit in all plants. . . for small tonnages of concentrates, either table or flotation; for cyanide clarification; and especially for experimental work. Because of the gravity method of filtration, a larger tonnage per square foot can be treated than by the straight vacuum filter where the material is not brought into contact with the filter medium by the force of gravity.
The duplex machine is preferred where more than one product is handled simultaneously or where simulating continuous filtration of a single product, as one pan can be filled while the second pan is finishing filtration and discharging the filtered material.
Chemical works, test plants, pilot plants and cyanide mills all can use this simple, inexpensive, dependable filter.
Oliver Rotary Drum Filter
The Oliver Rotary Drum Filter is illustrated in Fig. 64. It consists of a hollow drum revolving on a horizontal axis and partially submerged in a tank containing the concentrate pulp. The drum is carried by a longitudinal shaft to which are attached its two end plates (drum heads), each of which is fitted with a trunnion, and one or more intermediate spiders to support the shell, if required. The trunnion runs in bearings fixed to each end of the tank. The shell is built up of wooden staves or sheet metal, and is divided into narrow longitudinal compartments each consisting of a shallow box, the top of which is covered with a wooden drainage grid. One or more drainage pipes connect every compartment with an automatic control valve bolted to one of the trunnions. The filter cloth, usually cotton twill, rests on the grids and covers the whole of the outside of the drum ; it is kept in place by an evenly-spaced wire winding, which, encircling the drum spirally from one end to the other, not only holds the fabric but also protects it from wear by the scraper. The operation of putting a cloth on the drum is a somewhat lengthy one and involves emptying the tank.
The automatic control valve consists of two parts—a valve seat, which is bolted to the trunnion and revolves with it, and a valve chamber, which is kept in contact with the seat by a compression spring but does not rotate. The drainage pipes of the drum compartments are connected to ports in the face of the valve seat. Ports in the face of the valve chamber are connected either to a vacuum pump or to a blower, and register with those in the valve seat. They are so arranged that the compartments are either under vacuum or slight air pressure according to their positions in the filtering cycle.
The tank is kept filled with pulp. As each compartment of the revolving drum becomes submerged, the valve places it under vacuum, with the result that a thin cake of concentrate, ½ to ¾ in. thick, collects on the cloth covering it. As the compartment emerges from the pulp, the water in the cake drains through the grid and is drawn off through the drainage pipes and control valve by the vacuum pump, which then draws through air, drying the concentrate by evaporation. Just before the compartment reaches the top edge of the other side of the tank, the valve cuts off the vacuum and admits compressed air, loosening the cake sufficiently to permit of its removal by a scraper placed there for the purpose. Every compartment in turn goes through a similar cycle of operations.
The drum is rotated by a motor through a worm reducing gear, the gear-wheel being fixed to one of the trunnions. Its speed is, on the average, about one revolution in ten minutes. An agitating mechanism is suspended from the trunnions and is rocked by means of crank arms from the driving gear. It consists of a framework which passes under the drum and carries three or more rakes extending the length of the tank, the movement of which keeps the solids from settling.
Oliver Filters are made with drums of the following diameters, exclusive of laboratory and pilot plant sizes :—
The smaller sizes are made in lengths which range from once to twice the diameter of the drum ; those of the larger sizes range from 1 to 1½ times the diameter.
The drum filter, which is illustrated in Fig. 67, is of the rotary drum type with the filtering surface inside instead of outside the shell. It has no tank, the pulp being contained in the drum itself. The shell is made of sheet metal. One end is closed and carries a trunnion ; the other end is open except for a circumferential flange of sufficient width to allow the filter cloth to be covered with pulp to the required depth. The trunnion is supported in a bearing and the open end of the drum runs on riding rollers. It is driven at an average speed of about one revolution in six minutes through a worm engaging with a gear-wheel mounted on the trunnion, which also carries an automatic control valve of the same type as that fitted to the other filters. The shell is divided into compartments similar to those of the Oliver Filter, each compartment being connected by a drainage pipe to a separate port in the valve seat. A heavy metal screen covers the compartments and serves to support the filter cloth, which is fastened into grooves at the ends of the drum and along the partitions separating the compartments. Should the cloth become damaged, it can be changed without undue delay.
The pulp is usually brought in at the open end of the drum, but it can, if desired, be introduced by a pipe through the trunnion. Formation of the cake is rapid, as the heavier particles settle at once on the filter cloth and form a bed of granular material which tends to prevent slime from reaching the pores of the fabric. The cycle of operations is the same as in the other types of filters, and the dried cake is discharged when it is just past the highest point reached by the compartment. No scraper or roller is employed, but two or three pulsations of low-pressure air are applied by the valve instead of a single blow ; these are sufficient to cause the material to fall into a hopper placed inside the drum to receive it, whence it is discharged by chute or conveyor belt. When a belt is used, the trunnion is made hollow to enable the tail pulley to be outside the drum so that it is accessible for inspection and adjustment. Dorrco Filters are made in the following sizes:
The length of the drum ranges from about ½ to 1½ times its diameter.
Rotary Drum Filter
This machine consists of a cylindrical drum divided at its periphery into a number of individual compartments and covered with a cotton material, held in place by a winding of wire. The lower portion of the drum is immersed in a tank filled with thickened pulp to be filtered. The pulp is fed to the tanks continuously and is maintained in suspension by reciprocating stirrer bars. The drum is supported by two trunnion bearings and is revolved slowly through a worm and gear drive. The interior of each compartment of the filter shell communicates‘through a separate conduit to a rotary port valve mounted at one end of the drum. Through this valve either suction or positive air pressure is applied to the different compartments in the proper sequence and for the desired period. The slime cake is discharged from the drum by a stationary scraper blade which barely touches the cloth at the point of discharge. Compressed air is admitted just ahead of this position to loosen the cake. Sprays or drip pipes for washing the cake may be applied through nozzles mounted on the frame. A wash net riding on the surface of the cake and upon which the drip falls serves to distribute the water evenly over the cake. Figure 35 shows some Oliver filters at work.
The Oliver-type or drum filter is now more generally used than any other type in cyanide plants for washing and final dewatering.
At Wright-Hargreaves in order to prevent high dissolved loss, the filters must be scrubbed and acid-treated about every second day. The scrubbing was formerly done by two men and required more than 2 hr. to make a good job of scrubbing and acid-treating. A machine was developed on the property to do this work, and with its use one man can now do the job in about 30 min. Before scrubbing is started, the filter is reversed, and the scrubbing machine laid against the back at an angle of about 5 deg. As the scrubbing proceeds, weak hydrochloric acid is sprayed ahead of the brushes. The average life of the covers is a little more than 2 months.
Rotary Drum Filters have firmly established their place in many industries wherever it is necessary to remove liquids from solids for the recovery of valuable liquids such as cyanide solutions, water for re-use and dewatering solids such as flotation concentrates.