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Thickener waste management in mineral processing to prevent environmental pollution

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Thickener waste management in mineral processing to prevent environmental pollution

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Water plays a vital role in mineral processing and about 2-3 tons of water is used for the treatment of one ton of ore. The objective of water recovery in thickener is to increase the solids concentration at the underflow to obtain clear water at the overflow. The particle size distribution, that follows the Rosin- Rammler equation, is considered as the most important factors affecting thickener choosing and waste water treatment. If solids below 200mesh include 8% or more of weight of feed, flocculant should be used to increase the sedimentation rate and the water clarity. Increasing the concentration of solids in the feed (up to 25 wt %) reduces the size and cost of the equipments required for separating. If a high concentration of solids in the feed is used with flocculant, thickener overflow can dilute feed and can increase sedimentation rate and clarity. An extra depth should be added to thickener depth due to the space lost by the turbulence of the fluid resistance force.

Water plays a vital role in mineral processing and about 2-3 tons of water is used for the treatment of one ton of ore. The objective of water recovery in thickener is to increase the solids concentration at the underflow to obtain clear water at the overflow. The particle size distribution, that follows the Rosin- Rammler equation, is considered as the most important factors affecting thickener choosing and waste water treatment. If solids below 200mesh include 8% or more of weight of feed, flocculant should be used to increase the sedimentation rate and the water clarity. Increasing the concentration of solids in the feed (up to 25 wt %) reduces the size and cost of the equipments required for separating. If a high concentration of solids in the feed is used with flocculant, thickener overflow can dilute feed and can increase sedimentation rate and clarity. An extra depth should be added to thickener depth due to the space lost by the turbulence of the fluid resistance force.

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Thickener waste management in mineral processing to prevent environmental pollution

  1. 1. International Journal For Research & Development in Technology Volume: 2, Issue: 3, Sept-2014 ISSN (Online):- 2349-3585 5 Copyright 2014- IJRDT www.ijrdt.org Thickener Waste Management in Mineral Processing To Prevent Environmental Pollution Marzieh Hosseini Nasab1 1 Department of Mining Engineering 1 University of Sistan and Baluchestan 1 Zahedan, Iran Abstract— Water plays a vital role in mineral processing and about 2-3 tons of water is used for the treatment of one ton of ore. The objective of water recovery in thickener is to increase the solids concentration at the underflow to obtain clear water at the overflow. The particle size distribution, that follows the Rosin- Rammler equation, is considered as the most important factors affecting thickener choosing and waste water treatment. If solids below 200mesh include 8% or more of weight of feed, flocculant should be used to increase the sedimentation rate and the water clarity. Increasing the concentration of solids in the feed (up to 25 wt %) reduces the size and cost of the equipments required for separating. If a high concentration of solids in the feed is used with flocculant, thickener overflow can dilute feed and can increase sedimentation rate and clarity. An extra depth should be added to thickener depth due to the space lost by the turbulence of the fluid resistance force. Index Terms— thickener, pulp, settling velocity, Rosin- Rammler equation, flocculant. I.INTRODUCTION Approximately 80-90% of the tonnages of minerals are processed using water. It has more efficient, higher recovery and lower costs per unit of a valuable product and no air pollution (Antonio, et al., 2009; Jaroslaw, 2012). In most cases, the concentration of suspended solids is high and hindered settling occurs. Thickeners are continuous or semi- continuous units with relatively shallow reservoirs in which the transparent fluid is taken up and the concentrated product is collected at the bottom. The difference between thickener and other types of sedimentary basins such as classifiers is that suspended solids in the feed pulp in some kinds of floculant stick together. The first forces in the separation process are gravity, buoyancy and friction. Some of the factors that influence these forces are the liquid density, solid density, particle size and shape, floculation of particles and concentration of solids. Settling of solids and separation velocity may be related to these factors (Muralidhara, 1986; Roberts, 1949; Svarovsky, 2001). Continuous thickener dimensions can be determined from previous experiments, pilot-scale continuous tests or batch settling tests. But often due to time constraints, stirring restriction and lack of access to sampling materials, batch experiments are the only method based on reality for determining the size of the thickener. Before 1916, concentrated operations were considered an art, but in 1916, with the publication of Coe and Clevenger article, concentrated operations were changed to an engineering knowledge. The factors preventing possible changes in feed properties, using pressure depth or increasing thickener area to have lower pressure depth, appropriate choice of volume for the thickener, all these are factors that convert the concentration from an art into a real science (Couche and Goldney, 1959; Gaudin and Fuerstenau, 1962; Moncrieff, 1964). The concentrated radical theory began by work of Coe and Clevenger (1916). To determine the area of the thickener settling, they suggested that a series of batch settling tests should be performed at different initial concentrations. Then, carrying capacity of the solids at each concentration and the lowest capacity (maximum level) to determine the dimensions of the thickener are obtained (Coe and Clevenger, 1917; Fitch, 1962).Kynch (1952) developed a mathematical analysis by which the relationship between solids concentration and sedimentation rate is achieved by having just one settling curve. Fitch (1962, 1966, 1971), Fitch and Talmage (1955) proposed some methods for determining the dimensions of a thickener. The method of drawing the tangent, used as a standard method for the determination of thickener area by many companies, was compared to the actual results of continuous concentration. The results showed that even when the scale factor equals to 1.33, the predicted area for low concentrations of underflow is very large and for high concentrations is very small. Thus, selecting an appropriate method to determine the dimensions of the thickener is of great importance (Talmage and Fitch, 1955; Fitch, 1971; Fitch, 1966; Wilhelm and Naide, 1981). 1.1- Why dewatering is required? Dewatering is meant to separate water from solids. Humidity inparticles decreases benefits of separating valuable solids from gangue minerals (especially coal). Valuable products with excess water lead to additional costs in transportation, heating water for evaporation and blockage of transport carriers. For example, costs for a 1% increase in the moisture content of coal is equivalent to the benefit of a 4.5% reduction in coal ash. Mechanica dewatering is divided into two major phases: 1. Sedimentation: fluid arrestes to particles move freely. 2. Filtration: particles with a means been kept to fluid flow from between them (Ebrahimi, 1999; Banisi, 2007). 2- Materials and Methods 2.1 - Introduction The gravity thickener separates suspended solids from a mixture using a gravity settling. The purpose of thickening is
  2. 2. International Journal For Research & Development in Technology Paper Title:- Thickener Waste management in Mineral Processing To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585 6 Copyright 2014- IJRDT www.ijrdt.org to increase the concentration of solid particles in underflow. Most operations are performed in cylindrical sedimentation tanks with conical bottom. Thickener feed is continuously enrolled, and pulp is concentrated in the floor of the thickener and then clear liquid comes out of the overflow. Thick pulp is driven by shoveling mechanism (inward or outward) to a center output or radial multiple outputs. Shoveling mechanism outwards usually is used when the vessel diameter is large and thereby material moving toward the outlet is difficult if the outlet is located at the center. In a deep thickener there is no need to use the paddles because the cone inside angle is steep enough so that the concentrated pulp moves only with gravity to the location of the center output (Muralidhara, 1986; Chandler, 1986; Dahlstrom, 1986; Johnston and Simic, 1996). The first step for improving thickener performance is to find the optimal concentration of solids in the feed based on standard settling tests. Next, the amount of water needed to dilute the solids concentration in the feed factory is calculated and delivered to the desired concentration. The amount of this water is easily obtains by using feed flow rate and the appropriate percentage of solids it. The amount of water required for feed dilution is provided by thickener overflow. For continuity of operation, it is necessary to have a transparent water tank to store the required water for diluting. In Figure 1, the thickener and feed dilution system are shown. There is a lateral in the figure, to regulate the added water flow rate to the feed line. Diluted water flow rate can be manually set by any change in the concentration of solids in the feed using a valve located on the lateral (Banisi and Yahyaei, 2007). Feed (25 t/h) 10% Solids 26.5% Solids 0.1% Solids Clear water reservoir for feed dilution 4% Solids Folcculant 30 g/t ab c Folcculant addition points a: 40% b: 40% c: 20% Bypass Figure 1: A designed thickener for the recovery of coal tailing in a coal processing plant (Banisi and Yahyaei, 2007) 2.2 How to do a batch settling test? A series of samples with the same volume of the feed pulp in different solids concentration is prepared. After preparing the desired concentration of solids in the feed, floculant is added with additive rates to see starting of floculation. Levels of settling are recorded at different times. Once the sedimentation rate remains stable in several experiments, the experiment current of the sample is terminated and the subsequent settling experiment for a new concentration of solids in the feed is started. In general, as the amount of concentration of solids in the feed increases, a greater amount of floculant is need to see the level of sedimentation. During the tests, the flocc structure, trapping small particles and transparent in liquid phase is observed and recorded (Laros, et al., 2002; Scott and Alderton, 1966; Scott, 1968; Discusions, 1969; Scott, 1969). 3- Results and Discussion 3.1- The main factors affecting selection of solid – liquid separation equipments In metallurgy science, a pulp contains water, fine particles and colloidal material (Bonnier, 1989). This water is either ordinary water or water that contains dissolved chemicals. Chemical substances in the water can affect the behavior of settling pulp, known as electrolytes. Electrolyte has a property which can make colloidal part of the slurry congregation known as the floc. Floccs typically include water, colloidal materials and fine-grained materials that are trapped. Floccs in the aquatic environments are deposited according to certain rules (Yalcin, 1988; Mandersloot and Scott, 1986; Frank and Charles, 1986; Willey & Sons, 1982). According to the results of tests conducted, the key factors affecting the selection of solid – liquid separation equipments are summarized below. 3.1.1- Effects of particles size distribution For organic solids such substances found in domestic and industrial wastes, accelerators and measuring particle size in a logical way are impossible (Pearse, 1977). In these cases, measuring the total suspended solids and solids separated within 30 min after starting slow sedimentation are useful. The particle size analysis should determine the largest range of particle sizes (typically 98% smaller than the size), the percentage of particles finer than 200 mesh and possibly the percentage of those finer than 10 microns (the percentage of colloidal particles in the largest dimension). It is always wise to draw the distribution curve and to determine whether the particle size distribution obeys the Rosin- Rammler formula or not. If there is a large deviation from the formula, significant changes should be made in the selection of equipments. In general, sedimentation and filtration velocities of the small particles are low. Increasing the lateral surface of the particles decreases the concentration of underflow and increases the moisture of filter cake. For example, for solids with a density of 2.7, the maximum size of the thickener feed has about 60mesh (to protect thickener). The percentage of solids under 10 microns determines whether flocculant is necessary or not. However, the concentration of fine solids in the underflow of the thickener and the moisture content of filter cake are not affected by this percentage. In fact, when the solids below 200mesh include 8% or more of weight of feed, all the unit Areas required are related to the solids below 200mesh. It should be noted that some of the analyses of particle size may determine particles sizes only to a certain extent (2 microns or larger). Thus, although these analyses may report zero percents for the amounts less than the minimum dimensions, there may be existed percentages of solids that are significant.
  3. 3. International Journal For Research & Development in Technology Paper Title:- Thickener Waste management in Mineral Processing To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585 7 Copyright 2014- IJRDT www.ijrdt.org If we only use the screen analysis and the percentage of particles below 200mesh is larger than the 8 to 10, the screen analysis using wet screening methods should be carried out. 3.1.2-The role of particle characterization The nature of solids, that crystal material is amorphous or organic, affects the solid- liquid separation process. Although crystalline or granular solids are relatively fine, they are easily separated by all the equipments. The shape and surface of the particles can be also important in separation. Flat or disk-shaped lenses settle slower than others and filtration may be considered as an obstacle to fluid flow through the filtered cake. Usually, the optimal shape of the particles is spherical with non-porous. If solids are porous, due to higher moisture content in dewatered solids, the specific surface area can be considerably increased. So, regarding particle characteristics, even if they are colloidal particles (bentonite clays are flat lenses and they have low filtration rates, although they are easily flocculated) it is almost always better to consider particle shape and porosity. Moreover, chemical analysis of solids should be performed if possible, especially when the solids are gross. As density of particles increases, sedimentation rates increase. 3.1.3- Fluid properties pH, the chemical composition of the fluid, and in some cases the fluid viscosity are important for separating solid- liquid. The sedimentation rate varies with changes in pH and reaches to its maximum at a certain pH. The reason seems to be related to the effect of coagulation in game pulps. By increasing the viscosity of the fluid, sedimentation and filtration rates decrease. In this condition, temperature should be always maintained higher than a certain value in the sedimentation and filtration because a higher temperature reduces the viscosity of water. But this is economically not viable. The fluid density also plays a similar role in separating. 3.1.4- Effects of the concentration of solids on the feed In many cases, as the solids concentration increases, the separation velocity of suspended solids in the feed increases. In other words, increasing the concentration typically reduces the sizes and costs of the required equipments in the solid- liquid separation process. Thickeners are generally designed according to the unit of area needed. Therefore, one should always avoid from the unnecessary dilution of the feed. Sometimes, to increase the solid separation rate, one should dilute the feed. This is the case in flocculation process of a thickener. When a high concentration of solids in the feed (25wt %) is used with flocculant, we may have a different structure of the feed that makes settling speed significantly slow. By diluting the feed with overflow we can solve this problem and increase settling velocity of solids. If particles below 200mesh in the food form more than 7% of the total weight of the pulp, the feed should be diluted. 3.1.5- Role of pulp concentration in separation As the pulp concentration increases, the sedimentation rate decreases. The first reason is that with substitution of solid particles for the liquid, the liquid moves higher than of solid particles. The relative motion between the fluid and solids makes resistance against settling of solids. Secondly, if the concentration increases, the pulp density increases and this can increase buoyancy force and as a result, the driving force for the sedimentation of solid particles decreases. 3.1.6- Effects of differences between the solid – liquid densities Differences between the solid – liquid densities are important factors in classification, centrifugation and sedimentation. However, to determine the concentration of solids in the feed and content of dewatering product moisture, apart from the weight percentage, the volume percentage should also be considered. 3.1.7- Effects of the amount of compressive force on the compression zone (the depth of the compression zone) As the compressive force of the upper layers to the lower layers increases (or as the height of the compression zone is greater), the sedimentation rate is greater. However, increases in the pressure force make the concentration of underflow in the continuous thickeners increase. 3.1.8- Effects of the amount and types of flocculants By increasing the flocculant content up to a certain value, because of higher connection of particles together, the settling velocity increases. After this value, increases in the flocculant amount no longer change the sedimentation rate. This is related to breaking chains due to increasing its length. On the other hand, increases in the amount of flocculant, can confine considerable amount of the liquid within the flocs, it can increase floc porosity. This reduces the pulp viscosity. It is worth noting that the amount of the pulp mixed after adding flocculant affects the sedimentation rate. As the degree of mixing or stirring the pulp is higher, the deposition rate of separation decreases due to breaking the chains. Flocculant types depending on the chain length and other characteristics affect the sedimentation rate. 3.1.9- Effects of sedimentation vessel diameter If the sedimentation container diameter is more than 2 inches, the container diameter can not affect the sedimentation rate. Otherwise, the sedimentation rate in a pot with a diameter less than 2 inches is higher until it reachs the critical point, and before the critical point, the sedimentation rate is lower, compared to the sedimentation rate in container with a diameter more than 2 inches. 3.1.10- Role of operational settings If the real situation of the work is not sufficiently known, it may not be possible to achieve the desired goals. However, before finding a final solution to the problem of solid- liquid separation, one should clear the logical and optimal conditions. Sometimes it is an exaggeration to say that there is no suspended solid material in a liquid flow. Some of the solids in the refined liquid may be allowed and is not harmful. In addition, the complete removal of solids increases
  4. 4. International Journal For Research & Development in Technology Paper Title:- Thickener Waste management in Mineral Processing To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585 8 Copyright 2014- IJRDT www.ijrdt.org the required filtration area. Due to high costs of thermal drying, dewatering is often more favorable than thermal drying. Making a good tank and installing a "door" in the thickener wall, can reduce the retention time, because the loader, bulldozer or other machines can easily be imported the thickener. Moreover, the maximum residence time required and easier access to the equipments should be also considered. As a result, the solid - liquid separation process is costly in many cases, and it is need to consider different points of view so that the operational and capital costs are minimized. Conclusion The results of this study show that in the thickener choice and wastewater refinery, particles size distribution should follow the Rosin- Rammler formula.If solids below 200 Mesh include 8% or more of the feed weight, we should use flocculant to increase the sedimentation rate and the water clarity.Increases in the concentration of solids in the feed (up to 25 Wt %) reduce the size and cost of the equipments required for the separation. However, when a high concentration of solids in the feed is used with flocculant, we can use thickener overflow to dilute feed and increase sedimentation rate and water clarity. As the degree of mixing or stirring the pulp is higher, the deposition rate decreases due to breaking the chains.Regarding depth of the container, the following results are obtained:There should be enough depth for feed to avoid surface turbulence.There should be enough depth for the clear fluid avoiding the feed amount chsnges and the pulp properties of continues thickeners variation.There should be enough depth to have maximum capacity to avoid high cost and frequent over-discharge of continues thickeners. The required depth of thickener is obtained by calculating the capacity of concentrated zone, so that the stored solids is equal to the total capacity of the reservoir during the necessary time for pulp concentration that reaches the proper density in the underflow. We should add extra depth to the thickner due to the space lost by the turbulence of the fluid resistance force in the thickener. Furthermore, a depth between ft 2 1 1 and ft 2 1 2 should be considered for the feed and another depth for material storage capacity when the underflow is close. Acknowledgements The author would like to thank University of Sistan and Baluchestan for supporting this work. She is grateful to the reviewers for their useful comments. REFERENCES 1. Antonio E. C., Joaquim D., Mauro R., Rogerio C., Water in Mineral Processing: BRAZILIAN Case Studies. 23: 27-35 (2009). 2. Banisi S., mineral processing booklet. Third Edition, dewatering Chapter, Shahid Bahonar University of Kerman. (2007). 3. Banisi S., Yahyaei M., Feed Dilution Based Design of a Thickener for Refuse of a Coal Preparation Plant. Mining Engineering. (2007). 4. Bonnier A.C., Practical liquid / solids thickeners. CIM Bulletin. 82, 922: 75-76 (1989). 5. Chandler J.L., Thickening. In Advances in Solid-Liquid Separation. 79-105 (1986). 6. Coe H.S., Clevenger G.H., Methods for Determining the Capacities of slime Settling Tanks. Trans. AIME. 55: 356-384 (1917). 7. Couche R. A., Goldney L. H., The design of continuous thickeners for flocculated materials. Inst. of Min. and Met. 191: 117–139 (1959). 8. Dahlstrom D.A., Selection of Solid-Liquid Separation Equipment. In Advances in Solid-Liquid Separation. 205-239 (1986). 9. Ebrahimi A., Redesign thickener for factory make high grade copper from SARCHESHME mine. Special projects, undergraduate mining, Shahid Bahonar University of kerman. (1999). 10.Fitch B., Sedimentation Process Fundamentals. Trans. Society of Mining Engineering AIME. 223: 129-137 (1962). 11.Fitch B., Current Theory and Thickener Design. Industrial and Engineering Chemistry. 58, 10: 18-28 (1966). 12.Fitch B., Batch Tests Predict Thickener Performance. Chemical Engineering. 83-88 (1971). 13.Frank M.T, Charles S.Y, Introduction to Solid-Liquid Separation Principles and Theoretical Aspects. Advances in Solid-Liquid separation. 1-36 (1986). 14.Gaudin A.M., Fuerstenau M.C., Experimental and Mathematical Model of Thickening. Society of Mining Engineering. 122-129 (1962). 15.Jaroslaw D., Water in Mineral Processing. SME, Technology & Enginering. 371-388 (2012). 16.Johnston RRM., Simic K., Fluid Flow and Natural Dilution in Open-Type Thickener Feedwells. (1996). 17.Laros T., Slottee S., Baczek F., Testing, Sizing, and Specifying Sedimentation Equipment. 1295-1312 (2002). 18.Mandersloot W.G.B., Scott K.J., Geyer C.P., Sedimentation in the Hindered Settling Regime. Advances in Solid-Liquid separation. 63-77 (1986). 19.Moncrieff A.G., Theory of Thickener Design Based on Batch Sedimentation Tests. Institution of Mining and Metallurgy. 729-759 (1964). 20.Muralidhara H.S., Advances in Solid-Liquid Separation. Science. 191-202 (1986). 21.Pearse M.J., Factors affecting the Laboratory Sizing of Thickeners. Fine particle processing. Chapter 81, 1619-1642 (1977). 22.Roberts E.J., Sedimentation and Thickening: Thickening — art or science. Mining Engrg. 1: 61-64 (1949). 23.Scott K.J., Alderton J.L., Maximum Solids handling capacity of continuous thickeners. Mineral processing and Extractive metallurgy. 77: 201-210 (1966). 24.Scott K.J., Affecting settling rate of solids in flocculated pulps. Mineral processing and Extractive metallurgy. 77: 85- 97 (1968). 25.Author’s reply (Scott K.J.) to discussion on paper published in Theory of thickening: factors affecting settling rate of solids in flocculated pulps. Mineral processing and Extractive metallurgy. 77: 85-97 (1969). 26.Discusions and contributions on Theory of thickening: factors affecting settling rate of solids in flocculated pulps. Mineral processing and Extractive metallurgy. 116-119 (1969). 27.Svarovsky L., Solid-Liquid Separation: Thickeners. Forth Edition: 166-190 (2001). 28.Talmage W.P., Fitch E.B., Determining Thickener Unit Areas. Industrial Engineering Chemistery. 47: 38-41 (1955).
  5. 5. International Journal For Research & Development in Technology Paper Title:- Thickener Waste management in Mineral Processing To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585 9 Copyright 2014- IJRDT www.ijrdt.org 29.Wilhelm J.H., Naide Y., Sizing and Operating Continuous Thickeners. Mining Engineering. 33: 1710-1718 (1981). 30.Willey J. & Sons, Sedimentation. Chapter 17, Introduction to Mineral Processing. 327-342 (1982). 31.Yalcin T., Sedimentation characteristics of Cu-Ni mill tailings and thickener size estimation. Mineral processing, CIM Bulletin. 81, 910: 69-75 (1988).

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