Hydrocyclones and Thickener Notes (METE 254) PDF

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana MATERIALS ENGINEERING DEPARTMENT METE 451 METALLURGICAL PLANT DESIGN AND OPERATIONS Credit Hours: 3 Dr. Mrs. Bennetta Koomson January, 2021 ...

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana MATERIALS ENGINEERING DEPARTMENT METE 451 METALLURGICAL PLANT DESIGN AND OPERATIONS Credit Hours: 3 Dr. Mrs. Bennetta Koomson January, 2021  Kwame Nkrumah University of Science & Technology, Kumasi, Ghana LECTURE 6 DESIGN AND SELECTION OF CYCLONES AND THICKENERS Learning Outcomes Students should be able to 1. Explain classification and relate it to the operation of a typical hydrocyclone 2. State and apply key formulas required in determining the dimensions of a thickener and a hydrocyclone for dewatering and classification purposes Design and Selection of Cyclones www.knust.edu.gh Design and Selection of Cyclones www.knust.edu.gh  Introduction to Hydrocyclones Used in the classification of particles and dewatering in comminution circuits. The practical particle size range for cyclones is 40 to 400 µm. Cyclones are used in both primary and secondary grinding circuits as well as regrind circuits. www.knust.edu.gh  Introduction to Hydrocyclones Classification is achieved by increasing the force acting on the ore particles by replacing the gravitational force with centrifugal forces The feed entry is either tangential to the centre line of entry or forms an involuted entry. The cross-section of the entry pipe is usually circular, oval or rectangular; each of which provide a different velocity profile inside the feed chamber and the cyclone cone. www.knust.edu.gh  Introduction to Hydrocyclones The top of the feed chamber is closed with a plate through which a pipe known as a vortex finder passes The bottom of the vortex finder protrudes below the feed chamber. Below the feed chamber the body of a cyclone is shaped like an inverted cone, which converges to a smaller cone, and serves as the outlet of the coarser size fractions in the feed. www.knust.edu.gh  Introduction to Hydrocyclones The feed chamber and the cones are lined with rubber / synthetic linings. The lining material is hard rubber, neoprene or urethane. In some cases, the protective lining is sprayed to form a hard monolithic bond with the base metal. The apex is sometimes fitted with a concentric, hardwearing synthetic rubber inner sleeve, which can be squeezed hydraulically or pneumatically to alter the diameter of the opening. www.knust.edu.gh  Introduction to Cyclones Some hydrocyclones are occasionally provided with nozzles just above the apex for injecting water to compensate for water loss and loss of fines www.knust.edu.gh  Verity about Hydrocyclones The area of the inlet nozzle at the point of entrance to the feed chamber is normally 6-8% of the area of the cyclone feed chamber The size and configuration of the vortex finder is about 35% of the cyclone diameter The apex should not be smaller than a quarter of the vortex finder and not bigger than a third of the vortex finder diameter. In addition it should not be smaller than 10% of the cyclone diameter When these ratios are distorted, the cyclone malfunctions www.knust.edu.gh Description and Basic Operation www.knust.edu.gh  Basic Parameters (Standard Cyclone) 1. The cyclone diameter. – This is the inside diameter of the cylindrical feed chamber. 2. The area of the inlet nozzle at the point of entry into the feed chamber. – The basic area of the inlet nozzle approximates 0.05 times the cyclone diameter squared. 3. The vortex finder. – The size of the vortex finder equals 0.35 times the cyclone diameter. www.knust.edu.gh  Basic Parameters 4. The cylindrical section. – It is the same diameter as the feed chamber – Its function is to lengthen the cyclone => increase the retention time. – Length should be 100% of the cyclone diameter. 5. The conical section. – The included angle of the cone section is normally between 10o and 20o – Provides retention time. 6. The apex orifice. – The minimum orifice size is 10% of the cyclone diameter and can be as large as 35%. www.knust.edu.gh  Cyclone Performance Two main objectives: 1. Classification or separation required 2. Volume slurry to be handled. Base conditions: 1. Feed liquid – water at 20o C. 2. Feed solids – spherical particles of 2.65 sp gr. 3. Feed concentration – less than 1% solids by volume 4. Pressure drop – 69 kPa. 5. Cyclone geometry – “standard cyclone” www.knust.edu.gh Introduction Hydrocyclone (Dewatering) Hydrocyclone(Classifier) 𝐷𝐶 𝐷𝐶 Inlet diameter 𝐷1 = Inlet diameter 𝐷1 = 4 7 𝐷 𝐷𝐶 Vortex finder diameter, 𝐷𝑂 = 3𝐶 Vortex finder diameter, 𝐷𝑂 = 5 Length or Height. 𝐿𝑐 = 5𝐷𝑐 𝐷 Diameter of underflow= 15𝐶 Length of vortex finder, 𝐿𝑣 = 0.4𝐷𝑐 Length of vortex finder, 𝐿𝑣 = 0.4𝐷𝑐 Length of cyclone= 3𝐷𝑐 www.knust.edu.gh Cyclone Parameter Mular and Jull (11) Arterburn (12) Cross-sectional area of feed 6-8% of the cross-sectional ( 0.015-0.02)𝜋 𝐷𝑐2 Pipe at point of entry Area of the feed chamber 0.35𝐷𝐶 Vortex finder diameter, 𝐷𝑂 35-40% of 𝐷𝐶 12° for 𝐷𝐶 < 250𝑚𝑚 Cone Angle 12° for 𝐷𝐶 < 250𝑚𝑚 20° for 𝐷𝐶 > 250𝑚𝑚 Apex diameter 20° for 𝐷𝐶 > 250𝑚𝑚 > 0.10 𝐷𝑜 > 0.25 𝐷𝑜 www.knust.edu.gh  Classification Classification is the particle size of which 50% reports to the overflow and 50% to the underflow (D50C point). The d50 cut point size is used in the selection of cyclones. Overflow from cyclone has a certain size distribution, normally defined as a given percent passing a specified micron size. www.knust.edu.gh  Classification  Classification is the particle size of which 50% reports to the overflow and 50% to the underflow (D50C point).  As the D50C point changes from one application to another, the recovery curves shift, along the horizontal axis.  In order to determine a single graph which represents the corrected recovery curve, the particle size of each size fraction is divided by the D50C value and a “reduced recovery” curve can be plotted www.knust.edu.gh  Relationship of D50C to Overflow Size Distribution Example: Produce an overflow of 80% passing 149 microns. Multiplier at 80% passing = 1.06. D50C required = 1.06 x 149 = 158 microns. www.knust.edu.gh  Effect of Cyclone Diameter on D50C D50C = 2.84 x D0.66 D = cyclone diameter For example, a 25.4 cm diameter cyclone has a D50C point of 24 microns. www.knust.edu.gh  Flow Rate  The volume of feed slurry that a given cyclone can handle is related to the pressure drop across the cyclone.  The pressure drop across a cyclone is measured by taking the difference between the feed pressure and the overflow pressure. www.knust.edu.gh  Example Problem Select the proper size and number of cyclones for a rod mill/ball mill circuit where new feed to the rod mill is 250 metric tons per hour (MTPH) solids. Both mill discharges join together at cyclone feed sump and are pumped to cyclones. Overflow is to be 60% - 200 mesh (74 microns) at a minimum of 40% solids by weight. Underflow becomes ball mill feed. Specific gravity of solids is 2.9 and estimated circulating load is 225%. www.knust.edu.gh www.knust.edu.gh  Design of Thickeners - Sedimentation It is a process whereby a suspension of solids in a liquid is allowed to settle, until a clear liquid layer tops a mud layer. The clear liquid layer is withdrawn from the top of the thickener while the thickened slurry is collected from the bottom by pumping to the next process stage. www.knust.edu.gh  Design of Thickeners - Sedimentation Purpose: Prepare feed of right pulp density Reduce transportation volume Conserve water Optimize downstream process structures Reduce chemical inventory www.knust.edu.gh Design of Thickeners - Sedimentation Major Equipment Parts www.knust.edu.gh Design of Thickeners – Separation Zones www.knust.edu.gh Design of Thickeners - Separation Zones www.knust.edu.gh  Design of Thickeners Thickener design criterion is to determine the relationship between the settling velocity of ore particles and the dimensions of the Vessel. This begins with sedimentation tests www.knust.edu.gh  Design of Thickeners A graph of mud line against settling time is obtained from the experimental data The slope (zone settling velocity), as determined from a tangent represents the rate of settling (R) www.knust.edu.gh Thickener Design www.knust.edu.gh  Coe and Clevenger Method The cross-sectional area (A) of a thickener (m2) 𝐹−𝐷 𝑊 𝐴= 𝑅𝑆 F: liquid-solid ratio of slurry feed to the thickener D: liquid-solid ratio of slurry discharge from the thickener W: dry solids feed to thickener (t/h) S: specific gravity of liquid. R: particle settling rate, m/h www.knust.edu.gh  Coe and Clevenger Method Sample Question A thickener is to be installed to receive cyclone overflow of density, 30% solids and thicken it to 55% solids. If the plant throughput is 1000 t/h and the sedimentation test gave a settling rate of 750 mm/min, estimate the area of the thickener and the diameter. www.knust.edu.gh Talmage and Fitch Method www.knust.edu.gh  Talmage and Fitch Method Assuming: – H0 is the mudline height at t0 – C0 is the concentration in kg/L of the solids in the original pulp, i.e. at t0 – H is the mud-line height corresponding to a uniform slurry concentration, which is also expressed in kg/L of solids. 𝐶0 𝐻0 CH = CoHo and 𝐶 = 𝐻 www.knust.edu.gh  Talmage and Fitch Method 1 1 𝑊 𝐴= − 𝐶 𝐶𝑢 𝑅𝜌 Where R is the settling velocity, 𝜌 is the density of the liquid W is the dry solid in feed in t/h C and Cu (in kg/L of solids) are slurries of different concentrations www.knust.edu.gh Example Problem www.knust.edu.gh  Example Problem A thickener is to be installed to receive cyclone overflow of solids concentration, 0.43 kg/L and thicken it to 1.22 kg/L. If the plant throughput is 1000 t/h and the sedimentation test gave a settling rate of 750 mm/min, estimate the area of the thickener and the diameter. Assume density of liquid to be 1000 kg/L. www.knust.edu.gh C = 0.43 kg/L 𝝅𝑫𝟐 𝑨= 𝟒 Cu = 1.22 kg/L 𝟒𝑨 𝟎.𝟓 W = 1000 t/h = 106 kg/h 𝑫= 𝝅 R = 750 mm/min = 45 m/h 𝟒 × 𝟑𝟑. 𝟓 𝟎.𝟓 𝑫= = 𝟔. 𝟓𝟑 𝒎 ρ = 1000 kg/m3 𝟑. 𝟏𝟒 𝟏 𝟏 𝑾 𝑨= − 𝑪 𝑪𝒖 𝑹𝝆 𝟏 𝟏 𝟏𝟎𝟎𝟎 𝑨= − = 𝟎. 𝟎𝟑𝟑𝟓 × 𝟏𝟎𝟎𝟎 = 𝟑𝟑. 𝟓 𝒎𝟐 𝟎. 𝟒𝟑 𝟏. 𝟐𝟐 𝟒𝟓 × 𝟏𝟎𝟎𝟎 www.knust.edu.gh