Ore pulp dewatering system

Sinonine think that dewatering is the separation of pulp into two parts, one is relatively solid-free and the other is relatively liquid free, with respect to original pulp. As most of the mineral beneficiation operations are conducted by the use of substantial quantities of water, the products are obtained in the form of pulp. Hence water has to be removed from the pulp to get the final product in dry form. The removal of water from the pulp is called dewatering. If the solid particles in the pulp are relatively coarser, screening the pulp results in removal of water by passing the water through apertures and retaining the solid particles on the screen. Draining is a fairly effective method to remove water from coarse sands, but if fine sands and slimes are present, they tend to run off with

the water.

The pulps containing fine sands and slimes, flotation pulps for example, are

dewatered usually in the following three stages:

1 Sedimentation.

2 Filtration.

3 Drying.

Sedimentation means gravity settling or subsidence of solids suspended in liquid. This operation, coupled with continuous overflow of water and withdrawal of partially dewatered solids (thick pulp) from the bottom, is called thickening. Much of the water is first removed by the thickening operation and the thick pulp so obtained is then filtered to produce moist filter cake. The filter cake, in many cases, is subjected to further processing directly. When it is required to eliminate the moisture, the filter cake is dried thermally to about 95% solids by weight. Removal of water by drying is the most expensive operation. Hence it is usual practice to eliminate as much water as possible by filtration, leaving only the moisture contained in the filter cake for removal by thermal drying.

1 THICKENING

Thickening is an operation of concentrating a relatively dilute slimy pulp into a thick pulp by allowing solid particles to settle under the influence of gravity force. Thickening is also considered as a classification with a difference that in thickening all the solid particles are allowed to settle whereas in classification only certain solid particlesare allowed to settle.

Thickening is a complex phenomenon to analyse and its rate is difficult to predict as a large number of factors are involved. As the process of sedimentation (or thickening) proceeds, the slurry becomes more and more concentrated and as a result, the rate of settling of particles decreases. Sedimentation is thus a time-dependent phenomenon and its rate decreases with time. A preliminary study of the phenomenon, known as batch sedimentation test (Figure1), is usually performed in the laboratory on a sample of the slurry. A sample of the slurry is taken in a graduated cylinder and is kept under observation. At the outset, the particles settle at their maximum hindered settling velocity and the rate of sedimentation will be essentially constant. Soon a zone of clear liquid (A) develops at the top, below it appears the thickening zone (B) of essentially uniform concentration of solids more or less similar to that of the original slurry and the heavy sludge or sediment that accumulates at the bottom constitutes the compression zone (D). A transition zone (C) may also appear between the thickening zone and compression zone. As settling proceeds, the interface between the thickening zone and the clear liquid zone descends down, decreasing the height of the thickening zone and increasing that of the clear liquid and compression zones. As the process is in progress, the density and viscosity of the suspension increases and the sedimentation rate gradually decreases. It reaches a critical point where the thickening zone (B) and the transition zone (C) disappears and only two zones, thickening and compression zones, exists. Further sedimentation results in compaction of the compression zone D by oozing out some liquid into clear liquid zone due to the weight of the clear liquid in zone A. This process of removal of some liquid from compression zone due to weight of clear liquid is known as exudation. The exudation process continues till the ultimate height of the compression zone is reached. As very fine particles of a few microns in size settle extremely slowly, the thickening operation is very slow and cannot produce clarified liquid. In order to increase the settling rate, these fine solid particles are agglomerated or flocculated. Flocculation is defined as clustering, coagulation or coalescence of fine particles to form floccules or flocs in a liquid medium. Flocculation may be achieved by the addition of certain reagents known as flocculating agents or flocculants such as lime, starch, glue, gelatine, alum, gypsum, sulphuric acid, copper sulphate etc. to the pulp. Molecules of the flocculant act as the bridges between separate suspended fine particles and form the flocs. These flocs settle rapidly leaving the clear liquid at the top.

 Batch sedimentation test.


Figure 1 Batch sedimentation test


The thickening operation is usually carried out in large tanks called thickeners.Thickener consists of a large cylindrical tank of 3 to 30 metres in diameter and 2.5 to 3 metres in depth with a very short conical bottom. Dorr thickener is one of such conventional thickeners and is shown in Figure 2. The feed is introduced continuously through a central semi-submerged feed well at a rate that allows the solid particles to settle at a safe distance below the overflow level. Thus clear water overflows into the top peripheral launder. The settled solids are swept by a slowly revolving raking mechanism attached to the central rotating shaft and positioned slightly above the tank bottom. Slow agitation of the slurry helps in reducing the apparent viscosity of the suspension. The sludge swept by the rakes is directed towards the center to facilitate easy removal by a suitable pump such as diaphragm pump.

In a thickener, the surface area must be large enough that the upward velocity of liquid is always lower than the settling velocity of the slowest-settling particle which is to be recovered. The degree of thickening produced is controlled by the residence time of the particles and hence by the thickener depth. Hence the diameter of the

thickener is usually large compared with the depth. The diameter of the thickener is reduced by using Multi-tray thickener where a series of unit thickeners mounted vertically above one another. They operate as separate units, but a common central shaft is utilized to drive the sets of rakes. The high rate thickener and lamella thickener are the two thickeners used to save the floor space at the same time as increasing the settling rate.

In the thickening operation, the feed pulp containing about 15–30% solids is

thickened to a pulp of 55–65% solids. The main aim of thickening is to obtain thick pulp. If it is desired to obtain clear liquid rather than the thick pulp, the thickening operation is called clarification. This operation involves a dilute pulp of 1–5% solids.

The unit used for clarification is similar to that of the thickener in design but called as clarifier. However, the clarifier cannot achieve 100% removal of solids in a reasonable length of time; although some operations in chemical plants produce overflows containing 3–4 ppm of solids. The treatment of tailing is done to obtain clarified water for recirculation.

thickener.

Figure 2 Dorr thickener


Thickening can also be performed in a centrifugal field of force either by hydrocyclones or centrifuges. Thickening by hydrocyclone is simple and cheap but less efficient at very fine particle sizes.


2 FILTRATION

Filtration is the separation of finely divided solid particles from a fluid by driving the pulp to a membrane or septum(commonly called as the filter medium), porous to the fluid but impervious to the solid, through which the fluid called filtrate passes. The volume of filtrate collected per unit time is termed as the rate of filtration. As the filtration process proceeds, solid particles accumulate on the filter medium forming a packed bed of solids called filter cake. Pressure difference between two sides of filter medium, more pressure at the upstream side and less pressure at the downstream side,is the driving force.

The simplest type of filter consists of a tube of small bore through which the fluid is sucked while the solid particles accumulate at the entrance. As the device is operated, solids at first pass through the tube, but they quickly arch or bridge across the opening, allowing only clear liquid to pass afterwards as shown in Figure 3.

For bridging of a pore by solids, the diameter of the coarsest solid particles must exceed a certain minimum size; this minimum size is one-third of the pore opening if dealing with coarse material, but it may range to a small fraction of this ratio if the particles are fine. After bridging, solid particles continue to accumulate on the

filter medium. The thickness of the cake thus increases as filtration continues and it offers more and more resistance to the flow of filtrate. The role of the filter medium is merely to act as a framework on which the bed of solids is supported. Although the primary purpose of the medium is to retain solids, other factors are significant. A good filter medium should be able to bridge solid particles across the pores, have a low resistance to filtrate flow, resist chemical attack, have sufficient strength to withstand the filtration pressure and mechanical wear and should allow efficient discharge of the cake.

Mechanism of filtration


Figure 3 Mechanism of filtration.

1 Filtering area.

2 Pressure difference across the cake and filter medium.

3 Average cross section of pores within the filter cake.

4 Number of pores per unit area of the filter medium.

5 Thickness of the filter cake.

6 Size range of the particles.

7 Degree of flocculation.

8 Pulp temperature.

3.Types of filters

Cake filters are most frequently used in mineral beneficiation plants where the recovery of large amounts of solids is the requirement. Cake filters are of two types: Pressure filters and Vacuum filters. Pressure filters are used where higher flow rates and better washing and drying are required. Vacuum filters are the most widely used filters in mineral beneficiation plants and they are of either drum type or disc type.

The  Rotary Vacuum Drum Filter   (Figure 4) consists of a cast cylinder mounted horizontally and rotated at a very low rpm. The cylinder has a perforated surface over which the filter medium is wrapped tightly. The periphery of the drum is divided into compartments. Each compartment is provided with a number of drain lines passing through the inside of the drum and connected to a rotary valve assembly on the central drum shaft. The drum is immersed to the required depth in the slurry which is kept agitated to prevent settling of solids. By the action of automatic rotary valve on the drum shaft, vacuum is applied to the immersed compartments and results in cake buildup on the filter medium surface. As the drum rotates, the cake is raised above the slurry level and wash water is sprayed on the surface if required. On further rotation, the cake is dried. The vacuum is continuously applied till the end of drying stage. Air is admitted in to the compartment before it re-enters in to the slurry, thus permitting the cake to be blown away from the filtering surface where it is removed by scraper

on to a belt conveyor. The heart of the filter is the valve assembly which at a predetermined position causes a change from vacuum to pressure (or blowback).

vacuum drum filte

Figure 4 Rotary vacuum drum filter


The Rotary Vacuum Disc Filter(Figure 5) is similar to the drum filter except that instead of a cylindrical drum it consists of a number of circular discs mounted on a horizontal tubular shaft and spaced by means of hubs. A line of holes is drilled from the outside of the shaft into each of the tubes for the insertion of the sector nipple. Each disc consists of ten sectors (Figure 6). Filter medium consists of cloth bags that cover over the sector and fastened to the nipple. The completed sector is joined to the central shaft by screwing the nipple into the holes of the shaft.

Vacuum Disc Filter

Figure 5 Rotary vacuum disc filter.

fig6.jpg

Figure 6 Sector.


The cycle of operation is similar to that of a drum filter. The build up of cake takes place on both sides of the disc in the disc filter whereas in the drum filter cake build up is on the periphery of the drum. Hence the filtering area in the disc filter is more for the same floor area. It can also filter several products in single unit. Worn out cloth can be replaced without interrupting the process for long time.

Ceramic Filters: Ceramic filter discs utilize microporous ceramic sectors instead of conventional filter cloth. Suction is created through capillary action. The microporous filter medium allows only liquid to flow through and no air penetrates the filter medium. Consequently, Ceramic filters require only a small vacuum pump to transfer filtrate from the discs to the filtrate receiver. Filter cake is removed from the ceramic discs by scraper, eliminating the need for compressed air for blow off.

The capillary action dewatering system is exceptionally versatile for many applications. The disc material is inert, resistant to almost all chemicals and slurry temperatures, and has a long operational life. Ceramic filters are best suited to filter feed slurries with consistent, high solids concentration, and to solids with a d80 coarser than 40 microns.

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