I. INTRODUCTIONIn mineral processing operations, using water as part of the processes is inevitable. However, water can be one of the sources of contamination in the succeeding process. Hence, wet beneficiation of minerals necessarily requires removal of large amount of water before the processing of the concentrated ores can be carried out4. The process of removing water absorbed by the particles which increases the pulp density is called Dewatering. This process is done for a number of reasons; for easy handling and transportation, to save on the energy required in thermal drying to recycle process water used in the processing operations, and to dispose of the invaluable mineral.
Conventional dewatering can be done through dewatering screens, sedimentation, filtering and thermal drying. These techniques utilize screens, thickeners, vacuum filters, centrifuges and pressure filters to reduce product moisture to a lower value4. Sedimentation employ principles of gravity settling and applied in mineral concentrates and tailing. This technique is carried out by passing water into a large thickener or clarifier.
In these devices, the particles settle out of the slurry under the effects of gravity or centripetal forces. These are limited by the surface chemistry of the particles and the size of the particles. To aid in the sedimentation process, flocculants and coagulants are added to reduce the repulsive forces between the particles. This repulsive force is due to the double layer formed on the surface of the particles. The flocculants work by binding multiple particles together while the coagulants work by reducing the thickness of the charged layer on the outside of the particle.5The objectives of this laboratory activity were to provide visual observation of the behaviour of the solids, through its settling rate, with the aide of flocculant and to familiarize with the calculations for the area of a thickener using the data gathered in the experiment.II. METHODOLOGYMaterials and EquipmentThe materials and equipment used in this laboratory experiment were: four (4) ” 1000 mL graduated cylinder (glass), long stirrers, ruler, stopwatch, aluminium pan, weighing scale, one (1) ” 10mL graduated cylinder, flocculant ” Magnafloc, and crushed ore sample.FlocculationA. Sample PreparationThe ore sample used in the experiment was given by the laboratory technician. A weight of 50g of the ore sample was taken through random selection since a ratio of 20:1 ore suspension was required in this experiment. (20mL is to 1g sample: hence, in 1000mL is to 50g sample) The 50g sample was then put in a 1000mL graduated cylinder.B. Main ExperimentIn the 1000mL graduated cylinder with the ore sample, water was added up to the 1000mL mark and then 25mL more water was added in the same 1000mL graduated cylinder. A volume of 5mL of Magnafloc, a type of flocculant, was measured in a 10mL graduated cylinder. The measured flocculant was then poured into the ore suspension and then was stirred using a long stirrer. Stirring was stopped and the timer started when the floccules reached the 1000mL mark. The height of the agglomerated solid at the bottom of the graduated cylinder and the liquid was measured every minute for the first five (5) minutes using a ruler. After the 20th minute, the final heights were taken. All data obtained were noted in the laboratory notebook.SedimentationA. Sample PreparationThe ore sample used in this experiment was from the ore sample used in the previous experiment ” Sampling Methods. Weights of 100g, 200g, and 300g were taken by random sampling from the 5kg -10 mesh ore sample. Each weighed ore samples were placed in their respective 1000mL graduated cylinder, labelled as 10%, 20%, and 30%.B. Main ExperimentA pulp was prepared by mixing the weighed sample with water. A volume of 800mL water was added in the 100g ore sample in the graduated cylinder and was diluted to the 1000mL mark, creating a pulp with 10% solids. A volume of 700mL water was added into the second graduated cylinder and then diluted to the 1000mL mark creating 20% solids. Lastly, 600mL water was added to the third graduated cylinder, diluted up to the 1000mL mark, creating a pulp with 30% solids. The prepared set-ups were then stirred very well using a long stirrers until the mixtures were homogeneous-like. The time was started when the stirring stopped. The height of both clear liquid and the solids were measured using a ruler at varied time intervals ” at 5-minute intervals for the first Ѕ hour, 10-minute intervals for the 1 hour, 15-minute intervals for the next 1-1/2 hour, and lastly, an hour after the last 15-minute interval. Then set-ups were let settled and the final reading was taken after 24 hours. All data obtained were noted in the laboratory notebook.III. RESULTS AND DISCUSSIONFlocculationThe objective of flocculation is to cause growth of size of fine particles, by adding flocculants, to form a stable floc’ such that it will settle. There are two classifications of flocculation, namely, orthokinetic and perikinetic. The first classification is induced by turbulence while the latter is induced by Brownian motion. Brownian motion is the random motion of the colloidal particles in the dispersion medium due to the rapid bombardment by the molecules of the fluid2. It is affected by the size of the particles and temperature; fine particles move more rapidly and vigorously than large particles and it increases with temperature. The data gathered from the experiment is summarized in Table 1.TIME (MINUTES) CLEAR (CM) SOLID (CM) Settling rate(cm/min)1 4.5 28.5 28.52 4 28 143 4 29 9.674 4 29 7.255 4 29 5.820 3.5 29.5 1.48Flocculation occurs immediately after mixing, involving the restructuring of negatively charged clay particles which are surrounded by a positively charged cation shell. The cation shell’s thickness depends on the level of charge; the higher the charge the thicker the shell. Initially, the clay particles are dispersed and occur in a parallel arrangement, due to their negative surface charge and positively charged cation shells repelling each other1. This repulsion may be overcome by van der Waals forces, causing particles to flocculate.Floc formation is achieved through the aid of flocculants. Flocculants are long-chain organic polymers (high molecular weights)3. They carry multiple monomers w/ attached electrolytes that adsorbs to the oppositely-charged particles. For bridging to occur, the electrolyte end must be strongly adsorbed.When the particles are flocculated the van der Waals and other attractive colloidal forces are dominant and the flocculated particles settle together forming a network with a sharp interface between the flocculated sediment and supernatant1. Table 1 shows that as time increases the settling rate decreases as the settled solid tends to be compacted.It is recommended to have more trials with varying amount of flocculants added to show it affects the settling rate. Also, use other flocculating agents to have a legit comparison with the difference of settling rates using different flocculating agent.SEDIMENTATION The process of separating a liquid mixture of suspended particles into clear supernatant liquid and denser slurry having a high concentration of solids is called sedimentation.3 It is usually done using a clarifier or a thickener. Thickeners are used to concentrate solids, while clarifiers are used to purify liquids. A thickener will give you a high density underflow while a clarifier will not. The clarifier overflow is typically cleaner or clearer than the thickener overflow. Thickener can also be used to reduce the size or possibly eliminate settling ponds.3 Both are used to settle solids which results in the separation of liquids and solids. To determine the effect of concentration or height on the settling characteristics of a suspension, batch sedimentation experiments are conducted. The data from these experiments can be used to design continuous thickeners. Continuous thickeners consist of zones similar to a batch sedimentation process except that these zones are of constant height when steady state is achieved.Table B.1 10% solids WEIGHT OF SAMPLE = 100.12 GTIME (MINUTES) SOLID (CM) CLEAR (CM)5 MINUTES 5 4.5 27.4 10 4.0 27.9 15 3.8 28.1 20 3.6 28.3 25 3.55 28.35 30 3.55 28.3510 MINUTES 40 3.5 28.4 50 3.5 28.4 60 3.5 28.4 70 3.5 28.4 80 3.5 28.4 90 3.5 28.415 MINUTES 105 3.5 28.4 120 3.5 28.4 135 3.5 28.4 150 3.5 28.4 165 3.5 28.4 180 3.5 28.460 MINUTES 240 3.5 28.4 300 3.5 28.4OVERNIGHT – 3.5 28.4Table B.2 25% solidsWEIGHT OF SAMPLE = 250.02 GTIME (MINUTES) SOLID (CM) CLEAR (CM)5 MINUTES 5 10.1 21.2 10 8.8 22.5 15 8.1 23.2 20 7.5 23.8 25 7.5 23.8 30 7.3 24.010 MINUTES 40 7.3 24.0 50 7.0 24.3 60 6.8 24.5 70 6.5 24.8 80 6.4 24.9 90 6.4 24.915 MINUTES 105 6.4 24.9 120 6.4 24.9 135 6.4 24.9 150 6.2 25.1 165 6.2 25.1 180 6.2 25.160 MINUTES 240 6.2 25.1 300 6.2 25.1OVERNIGHT – 6.0 25.3Table B.3 30% solidsWEIGHT OF SAMPLE = 300.12 GTIME (MINUTES) SOLID (CM) CLEAR (CM)5 MINUTES 5 20.5 13.1 10 15.0 20.3 15 13.3 20.9 20 12.7 21.4 25 12.2 21.6 30 12.0 22.310 MINUTES 40 11.2 22.6 50 10.9 22.8 60 10.7 23.0 70 10.5 23.1 80 10.4 23.2 90 10.3 23.615 MINUTES 105 10.0 23.6 120 9.7 23.7 135 9.6 23.7 150 9.6 23.7 165 9.5 23.8 180 9.5 23.860 MINUTES 240 9.5 23.8 300 9.5 23.8OVERNIGHT – 9.3 24.0 Fig2. Settling Time vs. Height of interface Figure 2 shows that the settling rate of the ore increases with increasing % solids.The results of the activity show that flocculation has a faster settling rate than sedimentation. IV. REFERENCEWu, Z. H. , Hu, Y. J. , Lee, D. J. , Mujumdar, A. S. and Li, Z. Y.(2010) ‘Dewatering and Drying inMineral Processing Industry: Potential for Innovation’, Drying Technology, 28: 7, 834 ” 842Wu, Z. H. , Hu, Y. J. , Lee, D. J. , Mujumdar, A. S. and Li, Z. Y.(2010) ‘Dewatering and Drying inMineral Processing Industry: Potential for Innovation’, Drying Technology, 28: 7, 834 ” 842Wu, Z. H. , Hu, Y. J. , Lee, D. J. , Mujumdar, A. S. and Li, Z. Y.(2010) ‘Dewatering and Drying inMineral Processing Industry: Potential for Innovation’, Drying Technology, 28: 7, 834 ” 842Wu, Z. H. , Hu, Y. J. , Lee, D. J. , Mujumdar, A. S. and Li, Z. Y.(2010) ‘Dewatering and Drying inMineral Processing Industry: Potential for Innovation’, Drying Technology, 28: 7, 834 ” 8421P. Sargent, in Handbook of Alkali-Activated Cements, Mortars and Concretes, 20152Vanda Broughton, (2006) “Van Nostrand’s Encyclopedia of Chemistry (5th edition)”. Reference Reviews, Vol. 20 Issue: 3, pp.45-46, B. A., & Napier-Munn, T. (2006). Wills’ mineral processing technology: An introduction to the practical aspects of ore treatment and mineral recovery, seventh edition (7th ed.). Amsterdam ; Boston : Oxford, U.K. ; Burlington, Mass.: Elsevier ; Butterworth-Heinemann.4Wu, Z.H., Hu, Y.J., Lee, D.J. Mujumdar, A.S. and Ly, Z.I. (2010) Dewatering and Drying in Mineral Processing Industry: Potential for Innovation, Drying Technology, 28:7, 834-8425Yersel, Erkan (2014). Importance of Dewatering retrieved by