MINERAL PROCESSING

MIN211 LECTURE28/09/2015

ESSENTIAL STAGES

• PROSPECTING • EXPLORATION

• DEVELOPMENT• ROCK BREAKING• HAULING

• CRUSHING • GRINDING• PROCESSING • METALLURGY

REFINING

FINAL PRODUCT

TYPES OF FEEDERS

PRIMARY• Vibrating Grizzly • Stepdeck Grizzly • Apron• Reciprocating Plate

SECONDARY• Vibrating Pan • Reciprocating Plate • Conveyor Belt

Primary Feeders: Vibrating Grizzly Feeder

• Mechanically actuated, low in cost, self-cleaning• Design Considerations– Typically horizontal, however an increase of slope of 5°

results in in approximately 20% increase in capacity– Bottom of feed hopper should be parallel to the grizzly – Feeder skirt assemblies should be bolted to the hopper

floor and allow the removal for ease of service– Feeder should be long enough to provide sufficient room to

accept a truck load without overflowing into the crusher– Should be long enough to provide uniform feed to the

crusher

Primary Feeders: Apron Feeder

• Continuous steel belt unit made of overlapping flights supported on steel chains or bars

• Suited toward handling heavy, coarse, or lumpy materials

• Can provide close control of feed rate• Design Considerations– Usually inclined 10-15°– Slope has little effect on feeder capacity, but may

increase horsepower required

Secondary Feeders: Vibrating Pan Feeders

• Sloped trough with a flat or slightly rounded bottom having shallow sides and an open end

• Usually self-cleaning• Handle a wide range of particle sizes• Design Considerations

– Capacity• Influenced by pan width• Material Depth• Inclination of the pan• Characteristics and density of the material • Amplitude and frequency of vibration• For any pan width, capacity is directly related to material weight and bed depth

– Vibrating motion• Electromagnetic feeders use high frequency, low amplitude motion to impart

velocity to the feed

Secondary Feeders: Conveyor Belt Feeders

• Provide positive, easily regulated, continuous method of feed control

• Constructed with standard conveyor components, but can be made with troughed carrying idlers

• Best served with smaller sized materials• Design considerations:

– Moisture higher than 5 to 6% typically requires a return belt cleaner– Large particles or particles with sharp edges can damage belts– Capacity varies with gate opening and belt speed– Belt speed is typically limited to 150 fpm– Skirts should be divergent from feed end to discharge end to provide

less friction

Conveyors• Factors determining Selection

1. Capacity2. Length and lift3. Material weight and size– Normal range of belt speeds is between 300 and 600 fpm– Belt width is a compromise between speed selected and capacity to be

handled– Conveyor Idlers are used for support

• Design Considerations– Belt selection is influenced by drive horsepower and nature of material being

handled– Typical inclinations up to 18°– Special belts can handle 25° or more– Transfer points should be at about 5 to 10°, sloping upward in a radial curve

Elevators and Screw Conveyors• Elevators

– Bucket elevators can convey material vertically in a minimal horizontal space for small, free flowing materials

• Screw Conveyors– Fine, free flowing materials are often handled by screw

conveyors– Have pitched auger flights on a rotating shaft that is

enclosed by a tube or U shaped housing– Performance characteristics depend on selection of the unit– Typically used for materials finer than minux No. 8 sieve

Chutes and Hoppers

• Openings should be at least four times as great as the largest particle handled

• Chutes should be at least 0.60m wide to provide maintenance access

• Chutes should have easily removable covers for replacing linings or clearing plug-ups

• Flowing material should always be turned by a shelf that retains the material, and placed onto the center of a belt through a divergent opening

Measuring• Weight– Trucks

• Weight of trucks are tared before loading and weight of aggregate is determined after second weight is taken

– Rail• On-site rail scales are certified by the railroad• Off site rail scales are typically agreed upon an “average weight

agreement”– Scales

• Primary Scales– Most accurate and certified

• Secondary Scales– Used for internal control of loading process– Typically accurate ±1% of total weight

Other Processing Equipment: Rock Breaker

• Pneumatically (pressurized air) or hydraulically controlled

• Piston actuated reciprocating hammers fractures oversized particles to prevent bridging of near size pieces in the feed to the primary crusher

Removal of Metal

• Magnets– Protect crushers from damage by tramp metal (unwanted metal)– Strong electromagnets remove ferrous metal from the flow without flow

disruption– Magnets should be placed as close to the conveyor as possible without

impeding flow for maximum efficiency• Metal Detectors

– Detect metallic particles– If metal is detected belt stops and metal is removed

• Metal Detectors and Magnets are often both used to minimize flow disruption

• An automated divert or following a metal detector can bypass a segment of contaminated material with minimal flow disruption

Plant Design Example

• Processing plant flow sheet– Model that attempts to predict plant performance– Objective of following example is to produce 500

tph of minus 1 in. sized product from a biotite granite quarry

– Quantitative values are from manufacturers, design experience, or experimentation

Primary Crusher Selection

• First step in flow sheet preparation• Compressive strength and abrasiveness of granite suggests

that compression type machines throughout the plant• 500 tph is within the range of a jaw primary crusher fed

with a vibrating grizzly feeder• Approximately 35% of the stone passes a 150mm Opening

and is small enough to bypass the primary crusher– Feeder Grizzlies are typically inefficient, so assume that only

20% (100 tph) bypasses the primary jaw crusher– Remaining 400 tph is processed through the jaw crushers

Secondary Crusher Selection

• Jaw crusher product and bypass product are combined on a conveyor and sent to secondary screening and crushing process

• Aggregates are then blended• Table 8.7 shows the capacities of various cone

crushers used as secondary crushers

Tertiary Crushing

Tertiary crusher flow path is a closed circuit systemStone not passing the 1 in. top deck is fed into the

tertiary crusherStone is repeatedly fed into the tertiary crusher until

it passes the 1 in. top deck screen

Circulating Load

SurgesSurge piles limit issues from erratic production rate from the primary crusher station

Primary Crusher Surge Protection A hopper capable of holding several truck loads provides surge

protection between time periods of reloading Secondary Crusher Surge Protection

Primary crushers often have consistent flow problems Surge bin in the Secondary Crushing Station increases plant productivity

Surge Pile for Tertiary Crusher Tertiary crusher must be sensitive to market demands which determine

amount of each size product required Primary surge piles segregate aggregate during reclamation

Fine material is fed first followed by coarse material Despite this surge piles after primary crushers are necessary

Fractioning

Process of Screening finished product into closely sized fractions Advantages and Disadvantages

Flexibility in responding to market demands Quality control improvements

1. Tightly sized fractions minimize segregation2. blending tightly sized fractions produces more consistent

product gradation3. Final rinse stations can be implemented into blending

systems loadout, assuring a clean product• Automated loadout systems can be centralized for many

products• Surplus coarse aggregate can be reprocessed to control

inventory