The Vacuum Disk Filter consists essentially of a slowly revolving drum, the lower half of whose circumference is submerged in the pulp to be filtered. The filtering medium is applied all around the periphery, which is divided up into a number of separate segments, more or less isolated from one another.
The vacuum is applied through the axle bearing by means of a header valve which is so arranged that at a certain point in the revolution of the drum the vacuum connection is severed and compressed air admitted under the ceramic filter medium which serves to detach the cake and allow it to drop onto a metal deflector or scraper, whence it slides down to the residue discharge launder. As the newly formed and adherent cake rises from the bath of slime pulp it comes under the action of a number of atomising spray nozzles by means of which the wash liquor is distributed over the surface and is at once absorbed, tending to act as a wash for displacing the value-bearing solution.
Disk Filters consists of a number of parallel, individual, cloth-covered disks mounted on a hollow shaft through which suction and compressed air may be applied. The lower halves of the disks are submerged in the slime tank. The disks are divided into independent and removable sectors. Wash water may be applied from spray nozzles. The assembled disk and shaft section is shown in Fig. 32 (see also Fig. 33).
The American disk filter is a continuous machine of mechanical simplicity. It occupies small floor space and presents a larger filter area or surface in proportion to space filled than any other filter. Inflation of the filter bags during each discharge period keeps the cloth in good condition, and the cake discharged has low moisture content. The changing of filter cloths and the sectors is done easily and quickly. No pulp agitator is needed in the tank. This filter is made by the Oliver United Filters in four sizes: 4, 6, 8½, and 12½ ft. in diameter with 1 to 12 disks.
In operation the thickness of the cake ranges from 1/8 to ¾ in., depending on the material being filtered. As the disk slowly revolves, each sector in sequence rises from the pulp with vacuum still applied. Fine sprays of solution or water then are applied to the cake, thereby displacing the original liquid left in the pores of the cake. After a sector passes out of the spray zone, vacuum still applied, a large portion of the wash that has replaced the original liquid is also drawn out. Now as dry as it can be made, the cake is in proper condition for discharging. This is done by cutting off the vacuum and introducing a small quantity of 2 to 3 oz. air, which slightly inflates the bag. The cake will be so loose that it can either drop off or be scraped or rolled off easily. In the metallurgical field the American filter finds its principal use today in dewatering of flotation concentrates.
Disk Filters Plans
The Disc Filter is of rugged construction with each segment so made that it can be removed separately from others constituting the disc. A heavy duty central shaft, running in totally enclosed bearings, carries the predetermined number of discs, the whole being mounted on a welded steel tank, which, in turn, is supported on a heavy steel frame.
The tank has dividers welded to it, for cake discharge, and can be completely divided into two or more compartments, to handle separate pulps, as conditions dictate.
Rotary Disk Filters Drawings
The filter proper is rotated by means of a totally enclosed worm gear drive which may be driven by belt, “V” belt, or direct connected motor through a speed reducer.
Cast iron hubs are keyed to the central shaft, one for each disc, and respective segments are attached to these hubs with an effective drainage connection fastened with suitable, quickly removable clamps. The segments are of wood, grooved on a diagonal, for immediate and total drainage.
Disc Filter is an alternate to the Rotary Drum Filter and has the advantage of being able to filter two or more products simultaneously with one unit.
It consists of a predetermined number of discs rotating in a tank, on a common shaft. The discs are divided into segments and covered with a suitable filter medium and are connected through an appropriate valve assembly to a vacuum system, causing the proper segments to be under suction at the right time. The exclusive gravity drainage feature of this filter gives the driest possible product. All filtrate drains by gravity down the grooves in the disc segment to the drainage pipe and out the valve, before discharge of the filter cake.
The capacity of a filter depends largely on the nature of the material to be filtered, particularly the amount of fine material and slimes and the percent solids. The following are Disk Filter capacities, on average:
Copper flotation concentrates: 150 to 300 lbs. per sq. ft. per 24 hrs.
Lead and zinc flotation concentrates: 300 to 630 lbs. per sq. ft. per 24 hrs.
Cyanide tails: 300 to 600 lbs. per sq. ft. per 24 hrs.
Coal flotation concentrates: 1500 lbs. per sq. ft. per 24 hrs.
Barite: 3000 lbs. per sq. ft. per 24 hrs.
The foregoing figures indicate the large variation in filter capacity, depending upon the material. Laboratory leaf tests should always be made to determine the filtering rate of a particular material, the size filter required for a given tonnage, the optimum speed, amount of moisture to be expected in cake, and the size vacuum equipment required.
Disk Filter Capacity
Disk Rotary Vacuum Filter Operating Variables
Rotary Vacuum Filter production depends upon balancing four operating variables, DENSITY of the feed, SPEED of the DISC, condition of the FILTER SECTORS and the INCHES OF VACUUM. (Vacuum is measured in inches that a vacuum can lift a column of mercury) the density of the feed is controlled by the source it comes from (thickener or holding tank).
As the operation of a thickener was discussed a little earlier you already know how to operate it. The holding tank is another story. What is there to operating a tank? It’s only function is to provide storage room for the filter feed! Unfortunately that depends upon the type of equipment available and how easy the concentrate is to filter.
The problem, that is encountered, is too much water trapped in the circuit without a means of escape. This situation stems from either having a pump, which has uncontrolled gland water, in the circuit, or the feed to the tank having too low of density. This may mean that there is more water in the concentrate than the filter can remove. If you have difficulty filtering the concentrate the circuit will have to run longer to filter the same amount of mineral. The extra gland water and the lowered density to the filter boot will mean that filter will not be effective in removing the water or the concentrate. The concentrate that is returned to the tank by way of the overflow, line will reduce the storage tank’s capacity.
The correction of these problems require balancing the four operating variables to the problem. To know how to balance the variables it is a good idea to look at the method that the filter uses to filter the material. When the concentrate is pumped into the boot it has a density. A determined percentage of concentrate with in a set volume of slurry. The filter sectors as they enter the slurry subject the slurry to a vacuum, this vacuum only effects the concentrate particles within a set distance from the filter sectors. The effects of the vacuum become less the farther away from the sectors you get. When you filter the material the density quickly drops to start with, this is due to a lot of mineral close to the sector and-easily picked up. The size of the particle will also dictate the speed that it is filtered. The larger material presents a larger mass for the vacuum to act on. Considering both the large particle and the smaller particle are in suspension in water the resistance to movement is small. This makes the larger particles move easier as the vacuum effects them the greatest.
As the density drops, the distance between the particles becomes greater and the resulting filter load decreases. This is due to the time that the filter sector is in the slurry isn’t long enough to pull the concentrate to the filter sector. After the initial surge in density drop, the rate of decline will level off at a much slower rate. This is due to all of the coarse material being pulled out of the slurry leaving only the fine. The fine material will also cause problems due to what is known as BLINDING. Fine material will get between the weave in the filter cloth, blanking off the opening. This will prevent the vacuum from acting on the concentrate. That is the reason some mines prefer the operators running the disc through clean water or washing them down with a hose before shutting them down. This is to clean the cloth which will help to maintain their filtering efficiency.
Let me go over the four variables once more, DENSITY, RPM of the disc, VACUUM PRESSURE, and FILTER SECTOR CONDITION, We just discussed how density effects the filters. From the discussion it is easily seen that the length of time that the filters sectors stay in the slurry is very important. There is a variable speed drive on filters which is very effective in controlling the through put of the filters. If the disc is speeded up, the CAKE, which is the concentrate that is on the sector, will be thinner as the pickup time in the slurry is lower, the amount of moisture in the filtered concentrate may change as well however. This will depend upon the drying quality of the concentrate. The thinner cake may dry easier if the quantity of coarse concentrate particles are still high. Because of the larger material there will be minute spaces between each of them.This gives the moisture a path to take during the drying portion of the vacuum cycle. Fine material will compact itself well enough to form a seal preventing the water from being pulled through the cloth.
On the other hand, a slower moving disk may attract a large cake but be unable to dry it due to the finer material, when this happens it may be wise to balance the third variable the vacuum pressure. In most cases there will be two separate vacuum lines entering the filter. One is the pick-up vacuum the other is the drying vacuum. If you control the amount of vacuum that is being used on the pick-up portion of the filters cycle you will control the thickness of the filter sector’s cake.
The vacuum on the drying portion of the cycle may receive the benefit of the extra vacuum not being used by the pick-up. This results in a drier concentrate. You then balance the tonnage and the percentage of moisture to obtain the best -results possible. The last variable is the condition of the filter sectors, with this there are three areas of concern for the operator. The SEAL, the CLOTH, and the INSTALLATION of the sector. The sector and the cloth has other names that you may hear them referred to, such as, the SECTOR LEAF or the FILTER BAG. The sector leaf itself may be constructed from metal, wood, neoprene. Each has its good and bad points.
For the operator however the main area of concern is the method of attaching the cloth to it. These bags have to be changed whenever they become worn and develop holes. If the bag becomes holed, the vacuum will drop. Not only in that one leaf but in all the others that share that portion of the manifold. The concentrate that is pulled through the hole and into the equipment by the vacuum will cause a lot of extra wear for that equipment. All of this means that the operator has to maintain the condition of the filter. Part of that job is ensuring that any holed sectors -are taken off’ and repaired. Then put back onto the filters correctly.
Where the filter sector joins the manifold there is a SEAL, a gasket of some type. This gasket will have to be checked to ensure that it isn’t damaged to prevent a vacuum leak. If that seal leaks the resulting lack of vacuum will affect the performance of the sectors. When the sector is REINSTALLED care must be taken to tighten the nuts on the radial rods evenly. If they are not tightened down with even pressure, the disc will not rotate in a straight line. This is like the spokes on a kid’s bike, if they are not tightened evenly then the wheel will wobble.
The American Disc Filter is illustrated in Fig. 65. It consists of a series of circular discs mounted on a heavy central shaft over a specially shaped tank. The shaft is carried on trunnion bearings and is driven by a worm gear in the same way as that of an Oliver Filter ; its average speed is about one revolution in eight minutes. An automatic control valve of the same type as the one already described is fixed to one of the trunnions. Every disc is composed of eight to ten similarly shaped sectors, constructed as shown in Fig. 66; each is covered with a separate filter bag of cotton twill or other suitable material.
Filter Disks Sectors
Replacement Filter Disks Sectors
Old sectors we made of wood, deeply grooved to allow rapid drainage of the water as it is drawn through the bags. Each one is mounted on a hollow cast-iron cap with a projecting nipple which fits into a hole in the central shaft communicating with the appropriate drainage channel, the joint being made air-tight with a rubber washer. Steel rods project from the central shaft like the spokes of a wheel and are so arranged that they fit into the space left between the edges of adjacent sectors. A frame clamp, designed to grip the two adjoining corners of the sectors, is passed over the end of each rod and secured by a nut. Thus, when all the clamps are in position and their nuts tightened, the sectors are held firmly in place round the shaft in the form of a complete disc. A great advantage of this construction is that a damaged filter bag can be changed in a few minutes without disturbing the other sectors. Any number of discs can be mounted on the shaft.
The interior of the central shaft is cast with the same number of longitudinal drainage channels as there are sectors in a disc, each channel being connected to a separate port in the seat of the automatic valve ; the corresponding sectors in every disc drain into the same channel. The automatic valve controls the cycle of operations—cake formation (on both sides of the disc) and drying under vacuum, and discharge under pressure—as in a drum filter. Removal of the cake is facilitated by rollers which make contact with each side of the bag when it is inflated. The tank is recessed between the discs on the discharge side so as to give the material a free fall as it leaves the rollers. No agitating mechanism is provided. If difficulty is encountered through the settlement of the heavier particles, the pulp is pumped through the bottom of the tank and allowed to overflow back to the pump, the rising current being maintained fast enough to keep the solids in suspension. As a rule, however, the movement of the discs provides sufficient agitation.
American Filters are made with discs of the following diameters :—
Any number of discs can be mounted on the shaft from one upwards, but in practice the greatest number supplied seldom exceeds the number of feet in the diameter of the disc fitted. For example, the maximum number of 8 ft. 6 in. discs fitted to a filter is eight.
For this reason we will stay with the rotary disk filter and use it as our model. The components of the filter start with the Rotary Disk Filter BOOT.
The Rotary Disk Filter boot is the portion of the filter that the concentrate is pumped to, to allow the filter access to it.
It will have a Rotary Disk Filter DISCHARGE CHUTE for dry concentrate and a FEED INLET for the wet concentrate.
Because the filtering rate is often slower than the speed of the pump delivery there is an OVERFLOW LINE provided for the material that didn’t become filtered and must be returned to its point of origin. The feed may come from a HOLDING TANK or a THICKENER. If the settling rate of the concentrate is fast, the boot may also have an AGITATOR in it to keep the solids in suspension in the liquid.
Rotary Disk Filter Boot
The Rotary disks, which there may be one or several of, revolve through the slurry. It is then that the vacuum begins to play its part in the system. A vacuum is applied to the slurry through the rotating discs. The concentrate is trapped at the surface of the disc while the water is pulled through. Each disk is made up of a series of removable Filter SECTORS.
It is these sectors that filter the concentrate. Each sector is covered with a cloth that is made from a nylon or cotton weave. Through the sector and under the cloth there are channels to ensure that the vacuum is introduced evenly through the cloth.
When the vacuum pulls the slurry, the water passes through the tight weave of the cloth while the concentrate is caught on the surface. It is then removed from the slurry and allowed to fall off of the sector when it is over the discharge chute. This process is done in a continuous cycle. The sector enters the slurry.
As it enters the vacuum (suction) is applied. This is called the PICK UP STAGE (in the filtration zone) because the vacuum on the filter sector actually picks the concentrate up and holds the mineral to the sector as it continues to revolve. When the sector that has the concentrate on it is lifted from the slurry it enters the DRYING portion of the cycle. This is where the vacuum pulls as much water as it can off of the concentrate.As the disk sector finishes the last portion of a complete revolution, a blast of air is forced into the sector causing the cloth to bulge outwards. This removes the concentrate from the sector. The blast of air is timed to happen while the sector is over the discharge chute of the filter. The sector then enters the slurry again to begin another cycle. Rotary Disk Filter Internal Piping & Manifoldhttp://www.solidliquid-separation.com/
System supply Typical vacuum filter system with filtrate pump and vacuum receiver. 1. Vacuum receiver 2. Moisture trap (Normally used for aggresive filtrate only) 3. Vacuum pump 4. Liquid separator 5. Silencer 6. Filtrate pump 7. Floor drain For plants without a filtrate pump: 8. Drain line from vacuum tank (barometric leg) 9. Water lock
Dewatering with a Disk Filter
This filter is moderate in price, its running expenses are low, and it is continuous in operation, and while it cannot yet be considered a perfect solution of the filtration problem it seems to have the elements of more permanent usefulness than any other device at present in vogue.
Its principal defects are its small washing capacity, necessitating one or more decantation washes before filtration, and the mechanical difficulties involved in keeping the pool of pulp properly mixed and agitated in cases where there is a larger proportion than usual of granular or sandy material present. It shows itself at its best when employed as a dewaterer after several decantations have been given to remove pregnant solution, and when the material fed to it approaches most nearly to the definition of a true slime.
With this, as with all other vacuum filters, a thick pulp is essential to good work, both to ensure the homogeneity of the pulp in the container and also to obtain a high capacity per unit.
The Disc Filter is comparatively new, but it appears to be doing satisfactory work where it is in use. The principle of it is sufficiently indicated in the accompanying sketch.
The third type of filter is represented by the Kelly, the Sweet- land, and the first Burt filter. The fundamental principle of these, as already stated, is the enclosing of a number of vertical filter leaves in a closed receptacle and forcing the pulp by mechanical pressure into such receptacle, with the result of building up cakes of solid on the leaves and expelling the clear liquor into a header pipe outside.
The methods of discharge vary. In the Kelly the leaves are withdrawn on a roller carriage before dumping. In the Sweetland the leaves remain stationary and the lower half of the containing shell or receptacle is removed, while in the Burt the cake is first blown off and then the end of the shell is removed: the latter being erected at a steep angle, the residue cake slides out as soon as the door is removed.
The later Burt filter is rather different from any of the foregoing. It consists of a revolving horizontal tube somewhat like a tube mill, and has a complete lining of filter material. The pulp is fed to it under mechanical pressure and a cake is built up in the form of a uniform lining inside the tube from 1 to 4 or 5 inches in thickness. When the supply of pulp is cut off, the pressure is maintained with compressed air so that filtration can be carried to a finish and no removal of excess pulp is necessary. For dumping, one end of the cylinder is removed and the revolving motion being restarted, the cake automatically falls off and discharges itself into the tailing launder.
This filter is said to be especially applicable in the case of exceptionally granular and porous pulps, and is in successful use at one or more mills in the El Oro district of Mexico, though it has not obtained much vogue elsewhere. The idea, however, seems capable of wider application.
Thickener & Vacuum Filters Layout Arrangement
Vacuum pump, filtrate water receiver tank, thickener, wash water.