2011 MIME 341 Midterm 1 Notes.docx

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Mining & Materials Engineering
MIME 341
Kristian Waters

MIME 341 Midterm 1 Notes Lecture 1: Introduction  Separation processes o Gravity/density o Froth flotation o Magnetic separation o Electrostatic separation o Optical o Leaching o Dewatering/ flocculation  Learn o Basic mass balance (two components) and economics  Processing economically viable? o Crushing and sizing  Slurry density  Screens  Hydrocyclones o Physico-chemical properties and their roles  What are minerals o Minerals: compound that is generally in crystalline form created through geological processes o Mineral resource: Concentration of naturally-occurring solid, liquid, or gas in Earth’s crust in such form and amount that is currently economically feasible or potentially feasible. Can include effect of meteor impacts o Mineral reserves/ Ores: That part of a resource that is both economically and legally extracted under current circumstances  Mineral Processing o Separate valuable minerals from waste o Concentrate: stream of desired minerals o Must be viable, metal must be able to be recovered from concentrate profitably taking into account mining, processing, smelting and refining costs  Terms o Comminution: breaking rocks o Liberation: having valuable mineral as a distinct particle from waste through comminution o Separation: recover and concentrate valuable mineral from waste o Gangue: waste mineral  Current metal prices o Copper: $9500/tonne o Nickel: $28000/tonne o Aluminum: $2500/tonne o Gold: $1400/oz o Silver: $33/oz o Platinum: $1800/oz  Copper Flotation route o Sulfide ores containing the primary copper mineral of chalcopyrite and bornite are almost always treated by flotation o Comminution liberates minerals and provide optimal size for separation o Final product: 30 % copper o Trace elements such as gold and silver can add value to concentrate o Conversely As, Bi and Sb are common penalty elements  World’s Minerals o Deposits are concentrated in certain areas o Many minerals are regularly associated with other specific ones  Galena (Lead) and Sphalerite (Zinc) o If earth’s crust was homogeneous mix of all minerals, separation would not be economically or technologically viable  Environmental considerations o Mining and mineral processing has large environmental footprint  Mass balances o Mass of feed = mass of concentrate + mass of tailings  Grade vs. Recovery o As recovery of specific metal increases, the grade decrease  This is due to increased recovery of gangue  100 % recovery of metal = give back ore that was given to you  Lecture 2: Mass Balancing  Mass balance calculations o Determine the amount of each mineral, phase and size in all streams o Mass of phases, elements, minerals and chemical changes are conserved o Number of equations = number of unknowns  Multiple product example (Lead/Zinc) o Set up into 3 different equations then set F = 1 o B = AX  o Inverse  To get adjoint: get matrix of minors  Make matrix of minors into matrix of Co-factors  Transpose matrix of co-factors to get adjoint  Multiple stage separation o Feed  Rougher  Rougher tails  Scavenger  Scavenger tails    Slurry Density o Helps in knowing size of vessels required, retention times in conditioning vessels o Slurry/relative density in % = 100%  Thickener o  Mineral process economics o Flotation and smelter financially linked  Increased concentrate recovery at lower grade  Pay for metal (metal recovery vs metal recovered) ; charge for treatment (metal recovery vs metal)  Linear relationship between grade and value in concentrate  Increased recovery results in increased concentrate mass rate o Impossible to get 100% grade/recovery  Net smelter return: o (Grade of concentrate-0.01)*C*Value per ton  Other aspects to take into account o Moisture o Penalty elements may add further charge o Charge for small volumes o Delivery to smelter  Costs of processing o Crushing, grinding, separation (flotation), thickening, filtration, dealing with tailings, reagents, pipeline, water, laboratory, maintenance support, management support, admin, misc.  Other considerations o Most plants do not have just one concentrate stream o Optimal separation is required to maximize returns o Other valuable metals may bring additional revenue such as gold and silver  Economic efficiency o This is the ratio of the actual smelter return to maximum theoretical return  Theoretical return takes an assumption that all the metal is recovered and only the metal bearing mineral reports to the concentrate Lecture 3: Introduction to comminution and sizing  Comminution: Breakage of ore o Liberation of mineral particles o High energy intensive: 1% of world’s energy  Highly inefficient: 3% o Approximately 25-35% of a base metal concentrator’s operating costs  Particle Size o After mining, ore is crushed and milled to good size before separation process  Breaking rocks o Gyratory crusher o Jaw crusher o Roll crusher o Autogenous mill o Semi-Autogenous mill  Crushing, now screening o Screens: meshes with holes at set size o Particles smaller than holes will pass through  Larger ones will be returned to be re-crushed  Results in recycle  The load to screen would contain o Recycle o Feed to crusher  Efficiency o Some “small” enough particles may not reach mesh because it is trapped above larger particles o Long and thin particles may or may not pass through screen, depending upon its orientation o Assume none of the larger particles will pass o while assuming u = 1  Grinding (final stage of preparing feed for separation) o The additional size reduction produces particles fine enough to liberate minerals  Tumbling mill o Tumbling motion: cascading o Contains grinding media or charge  Can be ball, rod or slug usually made of steel o Critical speed  Can’t be too fast  Media would stay at side resulting in no cascade  Critical speed means no milling, or cascading  For rod mill: √  Methods of improving efficiency o High Pressure Grinding Rolls (HPGR) o Microwave radiation  Part of electromagnetic spectrum in frequency range of 300 MHz to 300 GHz  Dielectric heating (dipole rotation)  Different materials behave differently when exposed to microwave radiation  Transparent  Adsorbant  Reflective  Induce intergranular fractures  Lab results have shown:  Decrease in energy requirements  Increased copper recovery  Increased magnetic properties of pyrite  Improved leaching due to creation of porous structure  Effect of breakage: liberation  Size separation after milling: Hydrocyclone o Separate feed stream into two classes  Based on one species system  Overflow: small size fraction  Underflow: large size fraction  Multi-species system: density plays roll o Operation  Large particles pushed to walls by centrifugal force  Drag force brings fine particles toward center  Rotation creates pressure drop towards center, dragging air in  Water, fine particles in center spiral upwards around air core to be recovered as overflow  Larger particles exit at bottom recovered as underflow  Drag force vs centrifugal force o Hydrocyclone partition curve  Commonest way of representing efficiency of cyclone  Relates fraction/percentage of each particle size reporting to apex/underflow fraction  Partition: the probability of particle of going into underflow  Cut point is the particle size for which 50% of particles report to underflow and the other 50% report to overflow  In ideal world, partition curve will have infinite gradient at d50 value  Slope can be expressed by taking d75 and d25 values  Correction of partition curve  where y is actual mass fraction  And R is fraction of feed liquid reporting to underflow  Particle Size  When size range is given, a representative particle size is used  This is often the square root of the product of the upper and lower bounds of size range  Constructing curve  Partition number for particular size range o where U is total mass of underflow, and is mass fraction of particle size range in underflow o F is the total mass of feed and is the mass fraction of particular size range in feed  Mixture of solids  Left partition curve will be denser than right partition curve o Separation bypass  Some of particles report to underflow w/o undergoing size separation  Short circuit: if 30% of feed water reports to underflow then this 30% of feed material will short circuit Lecture 4: Comminution and hydrocyclones part 2  Size reduction: Crushers and mills  Rotational speed o Conventional: rpm o Tumbling mills, a percentage of “critical speed” is used  Speed at which media will be pinned to wall by centrifugal force  No falling media  Critical speed of rod mill: √  Derivation of critical speed  Centrifugal force = m*g*cos(theta) where angle = 0  Material nature o Hardness  Mohr scale  From 1(Talc) to 10 (Diamond) o Structure  Most minerals can be crushed using compressing  Fibrous materials require tearing o Moisture  Material do not flow well with moisture of 5-50% o Friability  Tendency to fracture during normal handling  Work Index o The bond work index expresses the resistance of grinding of a material (kWh/t); will know the specific energy required for target size reduction o Used for selecting mills o To get from an “infinite” feed to a P80 of 100um o Shown as Wi o √ √  Power as a function of time (P=W x T) o Determine 80% passing size after various grinding times o Find linear relationship  Work/energy requirements are linear w/ respect to time  Power draw of tumbling mill depends on two things o Loading  Percent of mill volume occupied by media plus voids  5% in autogeneous mill  40-50% in ball mill  Power increases to a certain load point, then decreases o Rotational speed  Breakage due to two mechanisms: impact and attrition  Particle size-screens o Given mesh number represents number of wires per inch or square apertures per inch  Hydrocyclones o Difficult to model o Plitt model is a famous model to determine d50 value o No model works for all o If we have cut point then we can go backwards to determine size of Hydrocyclone from table o Sometimes we base Hydrocyclone on particular passing size o Selection of Hydrocyclone is based on required size analysis of overflow  There are some conversion tables Lecture 5: Magnetic and Electrostatic Separation  Magnetism o Paramagnetism: Materials only slightly affected by magnetic field. Align in the direction of the magnetic field. Alignment “disrupted” by thermal excitation o Ferromagnetism: Five ferromagnetic elements (Fe, Co, Ni, Gd, and Dy). Capable of achieving high degree of magnetic alignment despite thermal interaction until critical temperature is reached o Diamagnetism: very weakly repelled by pole of strong magnet. When magnetic field applied, the diamagnetic substance will develop a magnetic moment through induction but is in the opposite direction and is therefore repelled  Magnetization: magnetic moment per unit volume of solid o For ferromagnetic materials  Saturation magnetization: increase of magnetic field strength or induction strength while no increase in magnetization  Coercivity: value of Magnetic field strength or induction strength when magnetization = 0  Remanence: value of magnetization at zero magnetic field strength or induction  Measuring magnetism o Magnetic moment can be measured then converted into magnetization (M) o This can then be converted to susceptibility χ  Minerals o FeS₂ pyrite : Paramagnetic/ Diamagnetic o FeS₃O₄ magnetite: Ferromagnetic o PbS galena: Diamagnetic o FeCr₂O₄ chromite: Paramagnetic  Magnetic susceptibility o Ratio of magnetization against magnetic field o Dimensionless o Constant and positive for paramagnetic materials o Constant and negative for diamagnetic materials o Decrease with increasing field strength for ferromagnetic materials  Force acting on a particle o Fm MISSING EQUATION  Rare-earth magnetic roll separator  Induced roll magnetic separator  Wet high intensity magnetic separator  Which method of magnetic separation? o Wet or dry depends on nature of material.  Drum separators can be used wet/dry and will separate highly magnetic material  Pulley magnets are another method of magnetic separation and if used with rare earth magnets, they can remove some paramagnetic material as well as ferromagnetic material  Gauss/Tesla o Gauss (G) is a measure of magnetic flux density or the magnetic induction o Tesla (T) is the SI unit o Field strength often given in kA m^(-1)  Other uses of Magnetic separation o Processing of secondary mineral sources o Plastic recycling: add ferromagnetic particles to polymer chains  Ferromagnetic minerals o Magnetite  Diamagnetic minerals o Galena and pyrite  Paramagnetic minerals o Pyrite, rutile and kaolinite  Electrostatic separation o Utilizes the difference in conductivity of different minerals o Particle size is an issue  Methods of charging particles o Corona charging: ion bombardment o Induction o Frictional charging tribocharging  Corona Electrification o Charges particles as they pass between two electrodes o Conductive particles dissipate charge through grounded surface while non-conducting particles are pinned to the grounded surface  Induction charging o Process where uncharged particles in an electric field assume the field polarity o From conductive surface o From conductive particle  Tribo-electric charging o Electric charges that remain on particle surfaces after solid-solid contact o When distinct particles come into contact, they will gain charges o Due to the nature of this charging and small area of contact, repeated contacts are required to gain sufficient charge o Non-conductors can also be separated this way through the differences in the dielectric properties of the particles o Material with higher dielectric constant becomes positively charged while material with lower dielectric constant becomes negatively charged o Fluidized beds and cyclones are examples of attaining a high degree of contact o Once a particle has attained a charge through tribo-electrification, it can be separated using a free-fall electrostatic separator o Used to remove impurities from talc o Improve whiteness of talc o Must be dry process (humidity changes separation)  Example: magnetic and electrostatic : heavy beach sands o Ilmenite and rutile o Lots of silica o Typical magnetic components of beach sand response to high tension separator  Magnetite: thrown  Ilmenite: thrown  Garnet: pinned  Monazite: pinned o Typical non or weak magnetic components response to high tension separator  Rutile: thrown  Zircon: pinned  Quartz pinned o FLOWSHEET WTF? Lecture 6: Introduction to leaching and thickeners  Hydrometallurgy o Solvent extraction and electrowinning  Copper oxide or low grade sulphide ores  Leaching : CuO + H2SO4 = CuSO4+ H2O  Solvent Extraction: Cu2+aq + 2RHorg = R2Cuorg + 2H+  Electrowinning: 2Cu2+ + 2H2O = 2Cu+ 4H+ + O2  Solvent extraction o Mixer-settlers o Mixing zone o Picket fence  Electrowinning o Inside tankhouse  Copper products: wire, rod, sheets, pipes 60% used in housing and power in China  Sedimentation and Dewatering o Sedimentation is the settling of solid particles in a fluid medium o Often water o Supernatant: clarified liquid at top o Solid particles in a bed at the bottom  Role in mineral processing o Need to dewater both tailings and concentrates o Sedimentation produces pulp of 55-65% solids by mass o This removes 80% of water o Filtration then gives 80-90% solids  Thickening  Particle motion in a fluid o Forces acting on a particle o Buoyancy and drag force vs gravitational force  Terminal velocity o Spherical particle will settle and increase velocity until acceleration force = resistance force o Turbulence of system plays a key role o Two crazy equations  Assumptions made for terminal velocity o Free settling o No wall effects o Fluid is a continuous medium  Regions in sedimentation o Induction o Free settlement o Hindered settlement o Compaction  Increase settling rate o Induce coagulation/flocculation  Makes bigger particles  Increase settling rate o Coagulation: destabilization of colloidal dispersions by adding ions which cause a reduction in the mutually repulsive electrical double layer forces present at the solid-liquid interface o Flocculation: is the aggregation of colloidal suspensions by the action of high molecular weight polymer soluble in the continuous phase by means other than charge reduction  Zeta potential o For any system, there is a balance between attractive and repulsive forces o Repulsive forces prevent particle coagulation and in addition hinders settling o colloidal suspensions are stable with a zeta potential of greater than 30 mv  Other methods of inducing sedimentation o Addition of other compounds to induce flocculation known as flocculants o Common use of polyacrylamides o Bridging flocculation  Polyelectrolytes attach to more than one particle to overcome electrostatic repulsion  Issues o Flocculants induce flocculation of different particles o Resulting floccs are larger than initial particles o Increased sedimentation rate  Thickener area o Volumetric feed rate can be set equal to the feed to a separator o QCPs can be set to the mass flow rate of solids Lecture #7: Filtration, Tailings and Spontaneous Combustion  Filtration o Filtration is the process of removing solids from a liquid using a porous medium o Use vacuum to enhance filtration
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