Presentation on theme: "University of Saskatchewan Geological Engineering GEOE 498.3 Introduction to Mineral Engineering Lecture 10 – Mineral Processing 3."— Presentation transcript:
University of Saskatchewan Geological Engineering GEOE 498.3 Introduction to Mineral Engineering Lecture 10 – Mineral Processing 3
Mineral Processing Overview Mineral Processing Terms, Economics Comminution and Classification Physical processing methods Chemical processing methods Waste products treatment and disposal Process plant flow sheets: uranium and potash
These course notes are a compilation of work conducted by many people. Notes have been taken from the following Edumine courses: The Mill Operating Resource 2 Flotation 1, 2, 3 Mineral Processing Technology, BA Wills
Separation of concentrates often performed prior to chemical processes Solid-liquid separation used everywhere
Typical Beneficiation Steps Beneficiation : the enrichment of ores and separation of unwanted gangue minerals so subsequent treatment to get the metals is more efficient. Uses only mechanical, physical, and physico-chemical methods. Can be divided into two distinct steps. Liberation: rock is broken down by mechanical means so that the individual mineral components become independent of each other, i.e., each is detached or liberated. Separation: valuable minerals are separated from the rest by means of physical and physico-chemical methods making use of differences in specific gravity, magnetic properties, etc.
Mineral Processing Methods = beneficiation + extractive metallurgy Watch for chemical aspects within physical separation methods
Mineral Processing Terminology - Review Concentration: Another word for grade Heads: A term that is used to denote the mineral found in the FEED to a circuit. Head Grade: aka feed concentration Middlings: valuable mineral plus gangue locked together Concentrate: a purified mineral. May require further downstream processing to convert for end uses. Examples: Copper and nickel sulfides Tailings - Material rejected from a mill after the recoverable valuable minerals have been extracted.
Separation Terminology Separation Techniques take advantage of the differences in characteristics between minerals: Flotation: Attachment of minerals to air bubbles - hydrophibicity Flotation: Attachment of minerals to air bubbles - hydrophibicity Magnetic Separation: Apply magnetic field to minerals or heavy media Magnetic Separation: Apply magnetic field to minerals or heavy media Radiometric sorting: Read or apply radiometric field to minerals Radiometric sorting: Read or apply radiometric field to minerals Gravity Separation: differences in specific gravity of materials, Stoke’s law, centrifugal force, sedimentation Gravity Separation: differences in specific gravity of materials, Stoke’s law, centrifugal force, sedimentation Electrostatic Separation: Apply electrostatic polarity Electrostatic Separation: Apply electrostatic polarity Particle size distribution, shape, and density has large influence on results
Liberation spectrum since liberation and separation are not perfect, concentrate contains some gangue, and tailings contains some valuable minerals. One can divide particles by size, then look at liberation for each size fraction. It is possible to make a clean separation of particles of adequate fineness.
Liberation vs. Separation efficiency Grade and recovery are interdependent. If liberation is improved, the amount of middlings is reduced. Improved liberation creates a higher grade recovery curve. However, limits to liberation imposed by: Costs of size reduction Costs of size reduction Subsequent separations become more difficult as particles become finer. Subsequent separations become more difficult as particles become finer.
Mineral Processing Mineral Processing Two basic operating strategies common in separatory circuit operation. Maximize recovery while maintaining concentrate grade. Maximize concentrate grade while maintaining recovery. Strategy ultimately based on sales contract – penalty charges for impurities
Froth Flotation Froth Flotation Froth flotation: Most common method for separating sulfide minerals from each other and from waste minerals or gangue Also used in potash, phosphates A stream of air bubbles is passed through the pulp. Being hydrophobic, the particles attach to the bubbles which, of course, are filled with air. The bubbles float to the surface and collect in a froth layer that either flows over the top
Froth Flotation Froth Flotation Chemical additives: Frother: a long chain alkyl alcohol, is added to stabilize the froth layer. Collector:organic chemical (eg. Xanthates), selectively adsorbs onto the surface of the mineral of interest and renders it hydrophobic (afraid of water) – non-polar head Collector: organic chemical (eg. Xanthates), selectively adsorbs onto the surface of the mineral of interest and renders it hydrophobic (afraid of water) – non-polar head Modifier: adjust pH of water Activator: cause a mineral to float with a collector when it would otherwise not float. Depressant: prevent a mineral from floating. Example: Starch, guar depress flotation of clays in potash
Froth Flotation Froth Flotation Water – mineral surface chemistry: Water is a polar molecule. Hydrogen atoms have slightly positive charge, oxygen slightly negative charge. Thus, pH has a important effect on flotation performance Particles may have an electrical charge on their surface when placed in water. Sign and magnitude of the charge depends on the atoms on the particle surface and the ions in solution. Surface will tend to dominate the flotation properties of the mineral Sulphide minerals can react with oxygen (oxidize) in water used during mining and concentrating processes. These reactions can change the surface charge usually rendering it hydrophilic, thus non-floating.
Flotation Flotation cell froth, carrying wet concentrate Color of froth reflects the mineral particles being recovered. High grade material has a distinct color. More locked particles and gangue will change the color. If froth is completely barren, it will be clear to white and milky.
Flotation Process control parameters: Feed grade - increase in will result in a higher grade final concentrate at approximately the same recovery (mass balance) Feed grade - increase in will result in a higher grade final concentrate at approximately the same recovery (mass balance) Feed size distribution - finer will result in higher recovery and grade in the final concentrate (liberation) Feed size distribution - finer will result in higher recovery and grade in the final concentrate (liberation) Feed % solids - increase will result in a lower grade and higher recovery to the final concentrate (entrainment) Feed % solids - increase will result in a lower grade and higher recovery to the final concentrate (entrainment) Feed tonnage - increase in will result in lower recovery and higher grade in the final concentrate (shorter residence time) Feed tonnage - increase in will result in lower recovery and higher grade in the final concentrate (shorter residence time) Air addition rate - increase will raise recovery and lower the grade of the final concentrate (entrainment) Air addition rate - increase will raise recovery and lower the grade of the final concentrate (entrainment) Froth depth - lowering will increase recovery and lower the grade of the final concentrate (entrainment) Froth depth - lowering will increase recovery and lower the grade of the final concentrate (entrainment)
Flotation Column flotation cells: Do not use mechanical agitation (impellers). Instead, mixing is achieved by the turbulence provided by rising bubbles. Mostly used to produce final grade concentrates because they are capable of great selectivity. Other features: Tall shape – froth much deeper Tall shape – froth much deeper bubble generation system - spargers bubble generation system - spargers use of wash water - high degree of cleaning, entrainment virtually eliminated. use of wash water - high degree of cleaning, entrainment virtually eliminated.
Flotation circuit Flotation process is broadly divided into rougher, cleaner and scavenger stages, each using many (bank of) flotation cells : Concentrate from the rougher stage are further concentrated in the cleaner stage. Tailings from the rougher or cleaner stage are fed to the scavenger stage. With all the internal recycles, operation of a flotation plant is a somewhat delicate balancing act.
Flotation Characteristics: Rougher stage: provide sufficient retention time to achieve target recovery. provide sufficient retention time to achieve target recovery. Eliminates a large portion of unwanted material as tailings, thus greatly reducing size of next stages. Eliminates a large portion of unwanted material as tailings, thus greatly reducing size of next stages. Cleaning stage is to produce the target grade: more than one stage of cleaning. Typically the first cleaners treat rougher concentrate, the second cleaners treat first cleaner concentrate and so on more than one stage of cleaning. Typically the first cleaners treat rougher concentrate, the second cleaners treat first cleaner concentrate and so on Scavenger stage: scavenger concentrates usually contain a high proportion of locked middling particles. scavenger concentrates usually contain a high proportion of locked middling particles. normally sent to regrind. normally sent to regrind.
Flotation Regrind: First grind fine enough to liberate gangue, but too coarse to liberate valuable minerals. Liberated gangue can be discarded in a separation step before regrinding to liberate more of the valuable mineral. Avoids grinding liberated gangue unnecessarily, thus saving power. Also thought that regrinding "cleans" the particle surface, enhancing the effectiveness of the chemicals used in flotation.
Flotation Conditioning - set the correct chemical conditions prior to a flotation stage Use a stirred tank pH adjustment common conditioning step Activator also common, example: copper sulphate for sphalerite flotation Aeration – oxidize selected surfaces, example: pyrite
Flotation circuit design philosophy Keep it simple: Avoid, when possible, unit operation with long time delays, such as thickeners. The best circulating load is no circulating load - circulating loads are inherently unstable. The more complex the ore, the greater the need for a simple circuit arrangement. Deficiencies in liberation or pulp chemistry cannot be corrected by recirculation. In fact the opposite will probably occur. In a plant environment, a simple responsive circuit will almost invariably outperform a complicated circuit. If a human can't understand, balance, or control a circuit, a computer certainly can't either.
Flotation Ancillary equipment: The most important measurement is grade. Usually use X-ray fluorescence (XRF) measurement as an on-stream analyzer (OSA) tool Provides trending, or as an integral part of automatic process control.
Flotation Types Standard flotation: Most common type of circuit separates a single valuable mineral from gangue. Float the valuable mineral Example: Cu
Flotation Types Reverse flotation: gangue is floated practical if small amount of gangue removed from a large stream. example is flotation of pyrite from zinc/lead concentrate
Flotation Types Bulk flotation: Two or more values floated together one set of conditions example is bulk copper / lead flotation
Flotation Types Differential flotation: bulk concentrate is separated into two products Change chemical conditions example is copper / lead concentrate flotation
Flotation Circuit One basic circuit arrangement has been found to be universally applicable to a wide range of ores and minerals. Example: Standard rougher- cleaner-scavenger with regrind Good for low grade, simple ore such as Cu
Flotation Circuit Example: Standard rougher- cleaner- scavenger with regrind, times two Common for Cu/Zn, Cu/Ni, Pb/Zn
Shaft Froth Flotation – Potash - Composition: NaCl: 55% NaCl: 55% KCl:35-40% KCl:35-40% KMgCl 3.6H 2 O :1-5% KMgCl 3.6H 2 O :1-5% Insolubles: 1-8% Insolubles: 1-8% Size Range of Natural Crystals in Ore: 1-10 mm 1-10 mm
Shaft Froth Flotation – Potash - clays scrubbed from ore surface - insolubles separated by cyclone or flotation
Magnetic Separation used on minerals which are affected by a magnetic field Ferromagnetic minerals are magnetic themselves. Example: magnetite paramagnetic or diamagnetic: weakly attracted to or repelled by a magnetic field. Examples: Examples: ilmenite (FeTiO3), rutile (TiO2), wolframite ((Fe, Mn)WO4),
Electrostatic Separation Principle: difference in polarity of the material, or the degree of electrical conductivity of the particles Examples: iron ore, graphite ore, feldspar, fluorspar, rock salt, phosphate, and different plastics pre-charge the particle surface and apply polarity Feed must be dry!
Radiometric Sorting Used in: uranium (gamma from ore) diamonds (X ray source) Reading head Air blast Similar to photometric
Friability Separation Bradford breaker: Used for crushing, sizing, and cleaning of run-of-mine coal and other friable materials. Product is relatively coarse, with minimum fines, and that is 100% to size
Gravity Separation Main principles: Stoke’s and Newton’s laws Gravity and/or centrifugal force Separation based mainly on particle settling rate, which is in turn dependent on: Size Size Shape Shape Density Density Classification is key! A large low density particle = small high density settling rate Size distribution must match equipment capabilities Slimes (-20 micron) detrimental due to viscosity Used for very heavy minerals (W, Sn, Au) or very light (coal)
Gravity Separation Sink/float separation, aka heavy media separation (HMS) uses differences in specific gravity (SG), density and buoyancy forces to separate minerals. By mixing media with a high SG along with ore that has two different minerals each with an SG that is sufficiently different from the other Example is coal and shale. Coal has a SG of 1.0 to 1.5. Shale has an SG of 2.4 to 2.8. Add fine magnetite (s.p. 4.0) to water to make slurry SG between 1.5-2.4 Good for coarse feed, fines pollute the media
Gravity Separation Jig: Handle minerals that have: high density high density Too coarse to be leached or floated Too coarse to be leached or floated Water is pulsed up through the screen. The heavy mineral sinks through the ragging
Gravity Separation Spiral concentrator: Mild centrifugal force Used for fines (~1mm) – but not slimes May use heavy media, can also be water-only
Gravity Separation Shaker table: deck may be tilted to different angles and vibrated riffles (ridges) running lengthways Very efficient Low tonnage
Gravity Separation Centrifugal concentrator: Spins the slurry inside a bowl at a high RPM, generates 200-300 G’s Centrifugal action forces the heavies to the surface of the bowl's interior Commonly used for gold Useful down to ~0.05mm particles
Gravity Separation Crossflow separator: Hindered bed settling Used for fines in many industries Tangential (cross flow) feed Teeter bed – water injection upward through slurry Fluidized bed forms
Gravity Separation Heavy/dense media cyclone: look like classifying hydrocyclones Used for coal and diamonds Typically use fine magnetite or ferrosilicon slurry, can also be water-only or heavy liquid near-horizontal orientation allowing for large apex sizes
Gravity Separation Heavy/dense media: Media recycle system starts to get complex
Gravity is your friend! Most older mills built on hillsides Main flows by gravity down to next circuit = minimal pumps!
Dewatering Terminology Dewatering: To remove water from a substance. Also refers to the circuit where this takes place. Dewatering Techniques: Thickener/Clarifier: Allow gravity settling Thickener/Clarifier: Allow gravity settling Sand filter: Remove entrained solids Sand filter: Remove entrained solids Filter: Apply air pressure/vacuum to draw water out Filter: Apply air pressure/vacuum to draw water out Centrifuge: Apply centrifugal force Centrifuge: Apply centrifugal force Dryer: Apply heat to evaporate Dryer: Apply heat to evaporate Slurry Density: The amount of solids in a slurry, expressed as a percentage by weight. Moisture: The percentage by weight of water in a sample, corollary to density
Dewatering Equipment Thickeners are large diameter conical bottom tanks accelerate settling of solids and dewatering of tailings. As feed enters the thickener in the feed well the solids start to settle immediately. Solids collection by rotating rakes, which force the solids inward and down to the bottom of the cone. Liquids overflow the top of the thickener tank into a launder
Dewatering Equipment Thickener: The thickened solids are pumped from the bottom of the cone as an underflow Further dewatering of underflow may be performed (for example, in a filter press) The overflow is (relatively) clear water which is piped to the next process step Conventional thickener main objective is maximizing tonnage or underflow density
Dewatering Equipment Low power, high gearing, high torque Rakes turn slowly, and lift/lower as required (operational/emergency) Bridge suspends drive and rakes
Dewatering Equipment Flocculation: Accelerates settling Addition of a synthetic polymer, resulting in agglomeration (flocs). Flocculants are typically polyacrylamide or derivatives. Can be cationic, non-ionic, or anionic. Molecular Weight (MW) and degree of ionicity can vary. Often best performance when flocculant charge is opposite of particle surface charge.
Dewatering Equipment Factors Affecting O/F Clarity Settling properties of the solids (material, flocculent vs. discrete, % solids) Settling properties of the solids (material, flocculent vs. discrete, % solids) Area of unit. Area of unit. Feed rate. Feed rate. Factors Affecting U/F Density Depth of compression bed. Depth of compression bed. Residence time (U/F pumping rate, feed rate). Residence time (U/F pumping rate, feed rate). Compressibility properties of the solids. Compressibility properties of the solids. Center well Overflow Underflow Flocculant
Dewatering Equipment Free (particulate) settling near the top. Zone (hindered) settling in the middle, near the water-solids interface. Compression at the bottom.
Dewatering Equipment Counter Current Decantation (CCD) Widely used in mining high capital, low operating costs 2 or more thickeners in series Feed to each stage consists of U/F from preceding stage plus O/F from following stage, O/F and U/F move counter-currently
Dewatering Equipment Counter Current Decantation (CCD) Typically, feed is from leaching, discharge solids to tailings, liquid to recovery Wash ratio = tonnage of wash water / dry tonnage of feed
Dewatering Equipment Counter Current Decantation (CCD) How many stages are required? Assume perfect mixing in each stage of the process. Assume perfect mixing in each stage of the process. R = fraction of dissolved species in feed that is recovered in the first stage O/F O = overflow liquor mass U = underflow liquor mass N = number of CCD stages
Ancillary Equipment Nuclear density gauge: commonly fixed to a pipe. used to measure density of slurry
Clarifiers Clarifiers look like thickeners Main objective is O/F clarity, not U/F density May use fluidized bed – self filter Generally lower density feed Rise rate critical for difficult-to-settle feed
Sand Filter used for removing small amounts of suspended matter. Eg. From thickener overflow solutions. Useful to less than 10 microns (0.010mm) turbid solution passes through bed of granular sand where the suspended matter is retained. When the rate of percolation decreases, the bed is regenerated by back-washing with water to expel the suspended matter out
Sand Filter Typically several in parallel operation One in backwash while remainder online Flowrate of backwash high enough to lift media bed, but not enough to flush it out Backwash to CCD, often via surge tank
Dewatering Equipment Disc filter: Uses vacuum through manifold Rotates through slurry Blowoff dried cake Removable cloth sectors Next generation: ceramic discs - capillary
Dewatering Equipment Other vacuum filters: Drum filter One cloth One cloth Often spray bars Often spray bars Variant: hyperbaric filter Filters often high maintenance
Dewatering Equipment Other vacuum filters: Belt filter Longer drying time Longer drying time Counter current washing possible Counter current washing possible
Dewatering Equipment Ancillary Equipment: Vacuum pump and filtrate pump Receiver tank and/or a barometric leg are required to prevent filtrate from entering and damaging the vacuum pump
Dewatering Equipment Pressure filter: Low moisture product Batch process High maintenance
Dewatering Equipment Filter press: intermittent operation. Has series of shallow compartments whose vertical walls are lined with fabric. suspension is pumped into these compartments under pressure Solids are either retained on the walls as a thin layer, making a cake filtrate runs down the solid walls behind the fabric and escapes through openings at the bottom of the press.
Dewatering Equipment Rotary kiln dryer: heat an air space up and then tumble the wet material through this space until it is dried revolving shell is on a slight incline
Dewatering Equipment Rotary kiln dryer: Direct or indirect fired Chains and hammers prevent concentrate from sticking to the shell.
Assignment / Tutorial #10 Tutorial / Quiz preparation Complete selected EduMine sections: Complete selected EduMine sections: The Mill Operating Resource - 2: Mineral Recovery Part 4 - Thickeners, Filters and Driers – Review #4 The Mill Operating Resource - 2: Mineral Recovery Part 4 - Thickeners, Filters and Driers – Review #4 Flotation 3 - Circuits Part 2: Copper Circuit Example – Review #2 Flotation 3 - Circuits Part 2: Copper Circuit Example – Review #2
Flowsheet examples Metallurgical coal Diamonds Oil Sands – Fort McMurray Heavy minerals – from oil sands, SK research! Gold – Placer, sulphide and oxide
Titanium Corporation – Ft. McMurray / Regina First stage: cleans the sand of rocks and organic using vibrating screens and scrubbing techniques. Attritioning - caustic solution to clean the mineral surface. Second stage: concentrate the valuable heavy minerals using large gravity spirals. concentrate is dried - magnetic, thermal and electrostatic properties used to separate the titanium and zircon minerals.