Mineral Processing Laboratory Manual 1 92 CHAPTER 16 DEWATERING 16.1 Theory Most mineral processing operations, with few exceptions of dry processing, involve the use of substantial quantities of water. For example, grinding involves 45 - 60% by volume water, and classification and wet concentration methods typically involve 80 - 85% by volume water. At some stage it is necessary to separate the water from the solids either partially so as to prepare the feed for subsequent processes, or substantially to produce a relatively dry concentrate for shipment. Solidliquid separation, as generally understood in the mineral processing industry, is the separation of insoluble solid particles from liquids (mostly water) by some mechanical means with the aim of recovering the valuable solids (mineral concentrates), or recovering the liquid (leach liquor), or recovering both the solids and the liquid. Dewatering methods can be broadly classified into three main groups: Dewatering Sedimentation Gravity Sedimentation Centrifugal Sedimentation Filtration Thermal Drying Chemical Sedimentation Coagulation Pressure Filtration Vacuum Filtration Flocculation Sedimentation is most efficient method when there is a large density difference between liquid and solid. Sedimentation cannot always be applied in hydrometallurgical processes, however, because in some cases the carrier liquid may be a high grade leach liquor having a density approaching that of the solids. In such cases, filtration may be necessary. Mineral Processing Laboratory Manual 2 93 Complete dewatering is normally a combination of the below methods. Concentrate 30‐40 % solids Sedimentation 50‐60 % solids Filtration80‐ 90 % solids Water Thermal Drying > 95 % solids Water Final Concentrate 0‐5 % water Water 16.1.1 Sedimentation Sedimentation process produces a clarified liquid, which can be decanted, leaving thickened slurry, which may require further dewatering by filtration. Stokes or Newton’s laws, depending on the particle size, govern the settling rate of particles in a fluid. Very fine particles, of only a few microns diameter, settle extremely slowly by gravity alone, and centrifugal sedimentation may have to be performed. Alternatively, the particles may be agglomerated or flocculated into a relatively large lumps, called flocs, that settle out more rapidly. 16.1.1.1 Gravity Sedimentation (Thickening) Gravity sedimentation or thickening is the most applied dewatering method in mineral processing, and it is a relatively cheap, high-capacity process, which involves very low shear forces, thus providing good conditions for flocculation of fine particles. Thickening is defined as removing a portion of the liquid from a slurry or suspension, thereby concentrating the solid particles into the remainder. In gravitational thickening, it is achieved by allowing the solids to settle under the influence of gravity in a sedimentation basin. Zone A: Clear solution zone Zone B: Hindered Settling zone Zone C: Transition zone Zone D: Compression zone Figure 1. Conventional thickener Mineral Processing Laboratory Manual 3 94 The method developed by Coe and Clevenger, is commonly employed to determine surface area of a thickener when the material settles with a definite interface. If F is the liquid to solid ratio by weight at any region within the thickener, D is the liquid to solid ratio of the thickener discharge and W (t/h) of dry solids feed to the thickener, then (F-D)*W (t/h) of liquid moves upwards to region from discharge. The velocity of this liquid current is thus; (F − D)xW AxS where; A is the thickener area (m2) and S is the density of the liquid (t/m3). Because this upward velocity must not exceed the settling rate of the solids in this region, at equilibrium, (F − D)xW =R AxS where R is the settling rate (m/h). The required thickener area is therefore; A= (F − D)xW RxS From a complete set of R and F values the area required for various dilutions may be found by recording the initial settling rate of materials with dilutions ranging from that of the feed to the discharge. The dilution, corresponding to the maximum values of A represents the minimum solidhandling capacity and is the critical dilution. In using this method the initial constant sedimentation rate is found through tests in graduated cylinders using dilutions ranging from feed dilution to the underflow dilution, the rate of fall of the interface between the thickened pulp and clarified solution being timed. Once the required surface area is established, it is necessary to apply a safety factor to the calculated area. The Coe and Clevenger method requires multiple batch tests at different arbitrary pulp densities before an acceptable unit area can be selected. The Kynch model offers a way of obtaining the required area from a single batch-settling curve and is the basis of several thickening theories, which have been compressively reviewed by Pearse. The Talmage and Fitch method applies Kynch’s mathematical model to the problem of thickener design. The results of a batch-settling test are plotted linearly as mudline (interface between settled pulp and clear water) height against time. A simplified version of the Talmage and Fitch method is offered by determining the point on the settling curve where the solids go into compression this point corresponds to the limiting settling conditions and controls the area of thickener required. In Figure 2., C is the compression point and a Mineral Processing Laboratory Manual 4 95 tangent is drawn to the curve at this point, interesting the ordinate at H. A line is drawn parallel to the abscissa, cutting the ordinate at Hu, Hu corresponds to the intersection on the ordinate of a tangent from a point Cu on the curve, where Cu is the solids concentration of the thickener underflow. The tangent from C intersects this line at a time corresponding to tu. W * (1/ C − 1/ Cu) The required thickener area is: A= (H − Hu) / tu where; (H-Hu)/tu is the gradient of the tangent at point C, i.e. the settling rate of the particles at the compression point concentration. Figure 2. Modified Talmage and Fitch construction Since C*H = Co* Ho A= W * (H / Co * H o ) − (H u / Co * H o ) (H − H u ) / tu =W tu Co * H o where A is the thickener area (m2), tu is the time (h), Co is the original feed solids concentration (t/m3), Ho is the original mudline height (m) and W is the capacity of plant (t/h). In most cases, the compression point concentration will be less than that of the underflow concentration. In cases where this is not so, then the tangent construction is not necessary and tu is the point where the underflow line crosses the settling curve. In many cases, the point of compression on the curve is clear, but when this is not so, a variety of methods have been suggested for its determination. 16.1.1.2 Centrifugal Sedimentation Centrifugal sedimentation can be regarded as an extension of gravity separation, as the settling rates of particles are increased under the influence of centrifugal force. Centrifugal separation can be Mineral Processing Laboratory Manual 5 96 performed either by hydrocyclones or centrifuges. The simplicity and cheapness of the hydrocyclone make it very attractive in mineral processing. 16.1.1.3 Chemical Sedimentation (Coagulation and Flocculation) In a suspension system there are two main forces between fine particles, i) attraction forces, known as London-Van de Waal’s forces, which are effective only at very close range and causes particles to adhere directly to each other. The adhesion due to the attraction forces is prevented by the presence around each particle of an electrically charged atmosphere, which generates ii) repulsion forces between particles approaching each other. Therefore, there is a balance between the attraction forces and the repulsion forces present at the solid-liquid interface. All same particles in any given system will have the same electrical surface charge and so the repulsion forces between particles both prevent agglomeration of the particles and retard their settlement by keeping them in constant motion. Coagulants are electrolytes having an opposite charge to the particles, thus causing charge neutralization when dispersed in the system, allowing the particle to come to contact and adhere as a result of molecular forces (attraction forces). Inorganic salts have been used for this purpose, and as counter-ions in aqueous systems are most frequently positively charged, salts containing higly charged cations, such as Al+++, Fe+++ and Ca are mainly used. Flocculation involves the formation of much more open agglomerates than those resulting from coagulation and relies upon molecules of reagent acting as bridge between separate suspended particles. The reagent used to form the bridges are long chain organic polymers, which were formerly natural materials, such as starch, glue, gelatine and guar gum, but which are now increasingly synthetic materials, loosely termed polyelectrolytes. The maximum effect of a flocculant is achieved at an optimum dosage rate; excess polymer can cause dispersion of the particles due to floc breakdown. Polyelectrolytes are normally made up to stock solutions of about 0,5-1 % which are diluted solution to about 0,01 % before adding the slurry. 16.1.2 Filtration Filtration is the process of separating solids from liquid by means of a porous medium, which retains the solid but allows the liquid to pass. The conditions under which filtration are carried out are many and varied and the choice of the most suitable type of equipment will depend on a large number of factors. Whatever type of equipment is used, a filter cake gradually builds up on the medium and the resistance to flow progressively increases throughout the operation. Factors affecting the rate of filtration include: Mineral Processing Laboratory Manual 6 - The pressure drops from the feed to the far side of the filter medium. This is achieved in 97 pressure filters by applying a positive pressure at the feed end and in vacuum filters by applying a vacuum to the far side of the medium, the feed side being at atmospheric pressure. - The area of the filtering surface - The viscosity of the filtrate - The resistance of the filter cake - The resistance of the filter medium and initial layers of cake Filtration in mineral processing applications normally follows thickening. The thickened pulp may be fed to storage agitators from where it is drawn off at a uniform rate to the filters. 16.1.3 Thermal Drying The drying of concentrates prior to shipping is the last operation performed in the mineral processing plant. It reduces the cost of transportation and is usually aimed at reducing the moisture content to about 5 % by weight. Dust losses are often a problem if the moisture content is lower. Rotary thermal dryers are often used. These consist of a relatively long cylindrical shell mounted on rollers and driven at a speed of up to 25 rev/min. The shell is at a slight slope, so that material moves from the feed to discharge end under gravity. Hot gases or air are fed in either at the feed end to give parallel flow or at the discharge to give counter-current flow. 16.2 Objectives - To understand the purpose and importance of dewatering operations - To learn the effect of coagulant and flocculant - To calculate the dimensions of a thickener practically 16.3 Materials - Water - Samples for gravity sedimentation and chemical sedimentation 16.4 Equipment - Clock - Graduated cylinder Mineral Processing Laboratory Manual 7 16.5 Procedure 1. Fill 1 liter graduated cylinder with water and sample (Pulp density should be around 20 % 98 solid). 2. Shake well to obtain homogenous pulp. 3. Leave the cylinder to rest and mark the mudline height against time. 4. Plot your curve as mudline (interface between settled pulp and clear water) height against time. 16.6 Data Analysis and Discussion 1. Using sedimentation data draw a curve; clear water level vs time. 2. Draw a tangent line at compression point, C. 3. Find H, intersection point at ordinate. 4. Draw a line parallel to the abscissa which cuts the ordinate at Hu. 5. Find tu, intersection point of tangent line to C, compression point, and parallel line to the abscissa at compression zone. 6. Calculate unit thickener area for this sample by the formula; 7. Calculate the thickener area and thickener diameter for W (ton/hour) capacity plant. 8. Discuss the function of thickener in mineral processing briefly in terms of capacity, advantages, initial and operating cost.
