Mineral Processing: DEWATERING Flashcards
- Solid – Liquid Separation
- Done to produce a relatively dry
concentrate (less shipping volume) - Also for certain processes that requires less or low H2O content
Dewatering
Flocculation and Coagulation
Sedimentation
Porous medium
Filtration
Evaporation of H2O content (<5%)
Thermal Drying
- Rapid settling of solids to produce a decant-able clarified liquid
- Governed by Stokes’ or Newton’s Laws
- Very fine particles are either centrifuged or agglomerated
– Flocculation
– Coagulation
Sedimentation
– altering the stability of the suspended colloidal solids to allow them to adhere to one another
Coagulation
formation of more aggregates than coagulation by using reagent molecules acting as “bridges”
Flocculation
- a.k.a. Thickening
- Cheap, high-capacity
- w/ low shear forces for flocculation
- 2-200m diameter, 1-7m depth shallow tanks
- provided w/ slow moving radial arms for gentle agitation and sediment collection
Gravity Sedimentation
The thickened underflow solids move continually downward to an annular trench at the center
* The clarified liquid moves upward and radially outwards
* The arms “rake” the solids @ 8m/min (perimeter)
– The arm mechanism is the basis for different thickener types!
Thickeners
The degree of thickening is a function of retention time;
Thickener Depth
- S/L separation using porous media
- Production of filter cake depends on:
– Pressure difference from filter to feed
– Area of filtering surface
– Viscosity of filtrate
– Resistance of filter cake
– Resistance of filter medium
Filtration
- Acts as support to filter cake
- The initial layers of cake is the true filter!
- Resistant to blinding, mechanically strong, corrosion resistant, little resistance to flow
- Cotton, wool, linen, jute, nylon, silk, glass fiber, porous carbon, metals, rayon, porous rubber, and other synthetics
Filter Medium
- Consists of alternately arranged plates and frames pressed by screw or hydraulic piston
- Filter cloth is fitted in between plates
- Slurry is fed by a continuous channel at the corners of the plates; clarified liquid is removed the same way the other end
- Cake is washed when the filter is full, prior to discharge
Pressure Filters
- A filter drum rotates horizontally while partially submerged in slurry
- Suction is delivered from the inside of the drum, discharge is at the tail end of rotation
- 0.1 – 3.0 RPM
Rotary Drum Filters
- Similar to drum filters, but discs are used
- The perforated discs rotate through a
through of slurry - Discharge is by pulsating air blow w/ scraper
- Small floor area vs. filtering area
Disc Filters
- Long sloping cylindrical shell
- Hot gases or air is fed either parallel flow or counter-current (more dry product); indirect or direct (drier)
Rotary Thermal Dryers
– Sufficient slopes w/o sharp turns; clean dry ores slide easily w/ 15- 250 steel-faced chutes
– 45-550 working slope is used; control of ore is reduced w/ increased slope
Chutes
- 30 – 60% of total delivered price of raw materials
- Covers the processes of:
– Transportation
– Storage
– Feeding
– Washing
– of the ore en route to, or during its various stages of treatment in the mill
Ore Handling
- Used when space is limited for conveyor installation
- Continuous line of buckets attached to roller chains
- Buckets are pivoted to remain upright until it reaches dumping position
Gravity Bucket Elevators
– Most widely used for loose materials
– 20,000Mt/h; 5000m flights; 10m/s speeds
– Increased capacity by using troughing idlers
– Spacing of idlers along the belt @ maximum w/o excessive sagging
– Tensioning Devices – adjusts the belt for stretch and shrinkage to prevent undue sag
(Standard Rubber) Belt Conveyor
Transport of Wet Ores
– Gently sloping troughs of rectangular, triangular, or semi-circular section
– Pulp advances by gravity; solids by either sliding or rolling
– Larger particle size, greater slope
– Deep launders for suspensions; shallow ones for fast settling solids
Launders
Transport of Wet Ores
– Should be straight as possible to prevent excessive abrasion @ bends
– Oversized pipes – dangerous when encountered due to deceleration of flow; settling of solids and choking may occur
– Design is based on: S/L ratio, pulp density, solids density, particle size and geometry, fluid viscosity
Pipelines
Transport of Wet Ores
– Cheap; small space requirement
– Capable of attaining high slurry velocities
– Single stage pumping may reach 30-100m heights
Centrifugal Pumps
Transport of Wet Ores
Desired for delivery of uniform stream of ore
* Essentially a conveying operation;
– Shorter distance
– Close regulation of rate of ore passage
* Unnecessary for cases when succeeding operations have same rates of flow
* Consists of small bin + suitable conveyor
* Chain Feeders
– Used for smooth control of bin discharge
– Curtain of heavy loops of chain
– Chains lie on the ore @ the outfall of bin @ ≈ angle of repose
– When the loops of the chain move, the ore begins to slide
Ore Feeding
Primary Crusher Feeders
– Remove fines from coarse particles before crushing
– “Scalping” the ore to prevent consolidation of fines
Vibrating Grizzlies
Primary Crusher Feeders
– Long elliptical bars in alternate vertical and horizontal positions all rotating in one direction
– Rocking action sifts the ore; the fines fall through to a conveyor, the coarses tumbles across to crusher
Elliptical Bar Feeder
Primary Crusher Feeders
– Essentially a conveyor w/ abrasion-resistant steel pans attached in series
– Most widely used feeder for coarse materials
Apron Feeders
Removal of Harmful Materials
used when in abundance of cheap manual labor, or additional equipment installation is not economically justified
Hand Sorting
Removal of Harmful Materials
situated above conveyors to pick up large pieces of tramp iron and steel; used w/ metal detectors
Magnetic Separators
Removal of Harmful Materials
removal of wood from pulp (wood causes choking)
Vibrating Scalping Screens
Removal of Harmful Materials
removal of slimes
Washing
- Necessity arises from successive processing units performing @ different rates
– Intermittent vs. continuous, for repairs, and for successive batch and continuous processes - Depends on:
– The equipment in the mill / method of operation
– Frequency of maintenance work required - Accomplished by Stock piles, Bins, or Tanks
Ore Storage
- Can be reclaimed by front-end loaders or bucket-wheel reclaimers
- Most economical is tunnel reclaim system
– Conical ____ can be reclaimed by running a tunnel through the center
– Live storage – amount of reclaimable material, is 20- 25% of total; 30-35% for elongated ______
Stock Piles
- Used as intermediate between crushing and grinding circuits
- They allow the steady discharge of ore w/o segregation or choking
- Flat-bottom ____ always retain some ore @ the bottom for cushioning
- Sloping-bottom ________ for easily oxidized ore
Ore Bins
- For storing suspensions of fine solids
- w/ provisions for agitation for suspension of solids or prerequisite chemical reactions
- Surge tanks are also provided to control small variations in feed rate
Slurry Tanks / Conditioning Tanks
- Disposal of mill tailings causes environmental problems such as visual effect on landscape and ecological effects of water run-off
- Depends on nature of tailings:
– Slurries w/ high water content
– Dewatered sludges
– Dry coarse materials from DMS
Tailing Disposals
Methods of Tailings Disposal
– application of techniques to recover additional values
Recycling
Methods of Tailings Disposal
filling of mined-out areas or stopes using mill tailings
Back Filling
- Impoundment of the volume of mill tailings
- The dam construction in itself is made of the tailings or the overburden from the mine
- Must be constructed as cheap as possible
- Must be constructed as close to the mine / mill as possible
Tailings Dam
- Essentially the reverse of the upstream method
- The dam wall is raised at the opposite side of the starter dike
- The only design acceptable by engineering standards
- Limited by the amount of sand needed to raise the wall
Downstream Method
- A starter dam is placed @ the extreme downstream point
- Dam wall is progressively raised on the upstream side; the tailings discharge by spigoting off the top of the starting dike
- When the pond is filled, the dike is raised, and the cycle is repeated
- Dam height is limited by stability of unconsolidated slimes retained by the dam walls
Upstream Method
- The crest remains in the same position as the dam wall is raised
- Uses less sand to raise the crest to any height
- The dam may be constructed ahead of the tailings pond
Center-line-method
to neutralize acids and precipitate
heavy metals as insoluble hydroxides
* Removal of decanted surplus water from dam
– may be recycled to plant upon treatment
Lime
- Impervious foundations and interception ditches
– to prevent seepage to groundwater
Mitigating Techniques
waste water treatments
– neutralization /
precipitation
Chemical stabilization
waste water treatments
ctive carbon and
clays
physical absorption
waste water treatments
– micro-algae species
Biological Oxidation
