Gravity concentration introduction
Gravity concentration is a process to concentrate the gold mineral of interest using the difference of specific gravity of gold and gangue minerals. The specific gravity of gold is 19.5 and the specific gravity of quartz (the common gangue mineral associated with gold) is 2.65 (i.e., gravity concentration works because gold is heavy, and quartz is light). Often gravity separation methods are confused with size classification because large particles of light minerals can behave like a small particle of a heavy mineral. The most effective gravity separation processes occur when applied to ore particles of about the same size. The most important factor for a successful gravity separation is liberation of the gold particles from the gangue minerals. It is not easy to establish the degree of liberation of low-grade minerals such as gold. Classical microscopy of screened fractions to establish mineral liberation is unreliable with gold ores. The most recommended method to establish the optimum gold liberation size is grinding for different times (or grain size distributions) and applying gravity concentration to the ground products. This is a classical and important procedure to recommend any type of gravity concentration process. Because most artisanal miners do not classify (screen) the crushed/ground material (i.e. they work in open circuit), their chances to improve gold recovery are very limited.
The main advantages of gravity concentrators over gold cyanidation are:
- relatively simple pieces of equipment (low capital and operating costs)
- little or no reagent required
- works equally well with relatively coarse particles and fine grained materials
Gravity by Sluices
Sluices are inclined, flat-bottomed troughs that are lined on the bottom with a trapping mechanism that can capture particles of gold and other heavy minerals. They can be used either for alluvial or for primary ore (sluices are sometimes called “strakes” or “blanket tables”). Ore is mixed with water and the pulp poured down the trough.
Sluices work on the principal that heavy particles tend to sink to the bottom of a stream of flowing water while the lighter particles tend to be carried downstream and discharged off the end of the sluice. Sluices are used in various sizes, from small hand-fed sluices to large sluices found on dredges or fed by trucks, front-end loaders or bulldozers, which can process as much as 150m3 of alluvial ore per hour. Much like in the past, today’s hand-fed sluices are usually 1 to 2 meters long, 30 to 50 cm wide, with walls 10 to 30 cm high. Sluices are usually inclined at a 5 to 15 degree angle. Many miners working alluvial deposits today use large sluices when sufficient water and operating capital is available. In monitor-gravel pump systems, slurry is pumped through 7.5-15 cm hoses onto 1-1.5 m wide by roughly 5 meter long sluices, such as those used in Guyana, Indonesia and Brazil.
Used correctly, sluices are efficient devices to separate gold from gangue. While sluices are not necessarily more efficient than panning, they do allow miners to increase the amount of ore they process, thus boosting their income considerably. Unfortunately, the resulting increase in the volume of ore processed can put large amounts of silt into streams, damaging regional water supplies and thus harming people, animals and aquatic life. Sluices can cause other environmental problems as well–those lined with mercury coated copper plates are especially destructive because the slurry solids scratch the mercury from the copper plate and carry it downstream, poisoning fish and people.
Good sluice design
Even though sluices have been used throughout the world for thousands of years, they are often not designed or operated correctly. Limited knowledge of the basic operating principles, lack of capital and access to more efficient modern lining carpets greatly reduces gold recovery, especially recovery of fine gold.
Particles suspended in a slurry stream settle when the intensity of the turbulence cannot support them. A well-designed sluice insures that the maximum amount gold can settle near the bottom of the slurry stream where it can be caught by trapping mechanisms such as carpets or riffles. Trapping mechanisms shelter gold particles from being lifted back into the current by turbulent forces, holding the gold from being washed off the end of the sluice.
Gravity causes gold to settle in water faster than silica and other gangue minerals. The rate of settling depends on particle density, size and shape: Large, dense, spherical grains settle quickly, whereas small, less dense and flatter particles settle much more slowly. Coarser grained low-density particles can settle at the same rates as finer high-density particles. In sluices where turbulence is low, the difference in settling rate between heavy and light particles tends to separate the slurry into loosely stratified zones. As the slurry stream flows down a sluice, the densest and largest particles accumulate in a zone close to the bottom where they can become trapped within the lining carpet’s pile or weave and sheltered from the current, while the smaller, lighter particles tend to stay in suspension near the top of the stream and be carried off the end of the sluice.
The rate of flow influences how gold and gangue particles in the feed stream settle to the bottom of the sluice, and how they become re-suspended. Flow velocities are controlled by the amount of feed pulp, and by the sluice box’s inclination, width, and length. At low flow velocities, the densest and largest particles settle to the bottom, while the less dense and smaller particles remain suspended in the feed stream. On the bottom of the sluice, sediments in the surface layer move slowly down the sluice by rolling and sliding. Increased flow velocities can cause these sediments to be lifted and suspended or bounced downstream: High flow velocities cause turbulent currents that, if strong enough, can fully re-suspend the bottom bed load and carry it all downstream.
For efficient operation, the slurry flow velocity must be adjusted fit both the range of gold particle sizes in the feed, as well as the trapping mechanism used. Flow should be fast enough to insure that the trapping spaces created by the riffles or carpet liner are not filled and blocked with gangue (i.e., the carpet must be kept from “sanding up”), yet slow enough to allow as much fine gold as possible to settle to the bottom where it can be trapped.
Increasing the angle of the sluice causes the flow velocity to increase; increasing the slurry depth by narrowing the width (or by increasing the input) also causes the flow velocity to increase; lengthening the sluice also increases the flow velocity as the slurry moves down the sluice because the fluid accelerates with distance. For a given feed rate and sluice width, the optimum flow velocity is empirically determined by incrementally increasing the angle of inclination until the trapping mechanism is clear of silica and other light gangue minerals (or the other way round, reducing the angle of inclination until the sluice starts sanding out, and increasing it again slightly).
The flow rate should be constant. The highly variable, discontinuous feed rates in hand-fed sluices are not efficient because the bottom carpet quickly becomes clogged with gangue, blocking the trapping spaces. In hand-fed sluices, water is poured onto the sluice one bucket at a time, but even at the peakflow of each bucket pour, the velocity is usually too low to lift much of the gangue and keep the trapping mechanism open. Even though some gold particles can become entrained within the surface sediments as they roll and slide down the sluice, the trapping efficiency of these surface sediments is much lower than that of a carpet with exposed fibers. Continuous gravity flow from a diesel barrel filled with water is better than pouring one bucket of water at a time into hand-fed sluices.
Gold ores typically contain a mixture of coarse and fine-grained gold particles. Because fine gold settles much more slowly than course gold, it is often best to use multiple stage sluices—capture the coarse gold using riffles, coarse expanded metal or/and vinyl loop carpets in a relatively steeply inclined first stage (faster flow velocity); then screen the coarse material off by using an inclined grizzly screen at the end of the first sluice, and feed the passing fine material (plus the water) onto a more shallow angled, perhaps wider sluice, where the remaining fine gold is recovered on a more tightly woven or pile carpet. This second sluice can be oriented either perpendicular to, or underneath the first sluice in a zigzag configuration. Differently angled zigzag sluices allow variable flow velocities, while reducing acceleration of the slurry stream by shortening the length of the bed. The feed box for zigzag sluices usually has to be higher than straight sluices, so they work best when the feed can be pumped to the sluice, and when the discharge can be in the opposite direction as the feed. Zigzag sluices are used often in large alluvial mining operations.
The ore should first be screened so that the particle size is as uniform as possible and the coarse barren material is eliminated. Under ideal conditions, the feed should not be coarser than the largest possible gold particle. Large rocks on the sluice create eddies and turbulence that keeps the fine gold in suspension; the high flow velocities required to move rocks off the sluice also leads to loss of gold. In alluvial sluices, a “grizzly” (inclined parallel bars spaced about 1-2.5 cm apart) can be used to screen the feed and make sure
larger rocks are kept out of the sluice. Grizzlies also remove clumps of clay that can roll down the sluice bed, sticking to gold particles and carrying them into the tailings.
Do not over-grind primary ore. Grinding too much can make smaller and flatter gold grains that tend to stay in suspension and ultimately be washed off the end of the sluice. Gold particles are very difficult to be concentrated when the material is a slurry of fine-grinding minerals. The same happens when the ore is rich in clayminerals. This forms a muddy-viscous pulp that must be adequately dispersed with caustic soda or dispersants to create conditions for gold to be concentrated by gravity processes. The more fine particles in the ore, the less % of solids must be used in the concentration process.
The ideal feed contains between 5 to 15% solids. A high percentage of solids makes the slurry too viscous – dense particles are buoyed upwards by less dense particles, limiting the ability to the slurry to stratify according to density. If very little water is available, and the gold is not too fine, coarse gangue particles can carefully be raked out of the sluice.
Sluice design and construction
Miners should design sluices to accommodate the anticipated feed rate by adjusting the width (increasing width decreases depth and flow velocity). Note that adjusting the width strongly influences the flow velocity – width is considered by some researchers to be the best control of flow velocity. Flow rates can be fine-tuned by adjusting the slope. When possible, miners should design sluice features (e.g., angle, width, etc.) so that they can be changed to insure optimum recovery. Wide sluices need to have carefully designed feed boxes to insure even slurry distribution over the whole width of the sluice.
Flow accelerates with distance, making it harder for the trapping mechanism to capture small gold particles. Research has shown that 90% of gold is recovered in the first 1/3rd of the sluice, 9% in the 2nd 1/3rd, and only 1% in the last 1/3rd.
Most gold is caught in the first 0.5 meter of the sluice, so keep the sluice length short (less than 2 m for hand-fed sluices). Zigzag configurations break flow velocity and help to increase recovery; three 2m zigzag sluices are usually better than one single 6m sluice.
The optimal slope is usually between 10 and 15 degrees, but can be as low as 5% for fine grained primary feed.
Use multiple stage sluices to capture coarse and fine gold in different passes. Capture the coarse gold first, then the fine gold. The turbulence from faster flows needed to capture coarse gold in riffles can be calmed by placing a short smooth section (a “slick plate”) before the next stage.
Clean-up time is a critical activity. Trapping mechanisms should be easily removable and cleaned. Complex assemblies reduce the likelihood of cleaning. Trapping efficiency can be monitored by checking sluice tailings constantly by panning. Clean-up time can be as often as once an hour to prevent blocking of the carpet, especially for primary ore with high sulfide content. To enable continuous operation, parallel sluices should be installed (one sluice in operation, one in cleanup and preparation). To improve recovery and prevent theft, the top sections of alluvial sluices should be washed at least once per day.
Secondary sluices can be used to re-concentrate the concentrate recovered by the primary sluice, therefore reducing the mass of material to be amalgamated. Secondary sluice tailings should be recycled to the primary sluice.
Bed linings should be firmly fixed to the bottom of the sluice, especially when not backed, to prevent captured gold from migrating down the sluice underneath the lining and being lost off the end of the sluice.
a) Cross riffles made from railroad rails, angle iron, wood or split bamboo are often used to trap gold particles >1mm. The simplest riffles are stones, but these can cause turbulence likely to cause gold loss. Carpet and/or expanded metal should be used underneath the riffles.
b) Rudimentary riffles do not necessarily improve recovery–turbulence can break up stratification, and cause the loss of fine gold. While catching some of the coarse gold, riffles often only leave the impression that recoveries have improved.
c) Riffles protect the carpet lining from wear and keep it firmly on the bed of the sluice.
d) Ore with a range of coarse particle sizes may need to utilize several kinds of riffles (e.g., large and small expanded metal riffles).
e) Select riffle size and spacing, then select the flow rate that keeps the sheltering spaces behind the riffles clear of sand.
f) 25 mm angle iron riffles are commonly used with 4.0-6.5 cm gaps, canted uphill at about 15 degrees. There should be very little sand between the riffles. If there is too much sand, the flow is either too slow, or the riffles are too high.
g) Expanded metal grating (see picture, below) forms shallow riffles which cause a local
turbulence that keeps the sand moving downstream while providing effective shelter for gold grains less than 0.1 mm. Wider sluices need a heaver gauge metal to hold the liner flat.
a) Type of carpet lining is usually determined by what is available.
b) Fibrous or hairy fabrics like sacking, sisal, blankets, or old carpets have hairs that can trap fine gold particles and prevent them from being lifted back up into the current by turbulence. Animal hides are usually not a good option, because they tend to fowl.
c) In general, the best carpets have open fibrous structures that let gold particles settle deeply in the lining.
d) If rubber backed carpets are not available, use a tighter weave cloth backing underneath the carpet to prevent loss of gold.
e) Wash carpets in a series of buckets, in barrels cut lengthwise into troughs,
or in tubs.
Optimal slurry flow velocity
Different trapping mechanisms require different flow velocities. Adjust the width and/or slope to control flow velocity to optimize the performance of the various riffles and carpets used. Coarse gold recovery needs faster flow velocity (narrower and/or steeper sections); finer gold recovery requires slower speeds (broader and/or less steep sections).
Keep feed rate and pulp density constant. Increasing the flow can increase turbulence and make it more difficult for gold particles, especially the fine gold grains which tend to stay in suspension, to contact and be trapped at the bottom of the sluice. Slowing or stopping the flow fills the trapping mechanism with gangue. Avoid turbulent flow, especially when trying to capture fine gold. Higher flow velocities can be necessary to keep the gangue from clogging riffles and carpets, but high speed tends to push fine gold off the end of the sluice. Lower flow velocities can yield higher recovery (fine gold is recovered in addition to the coarse gold), but if too slow, can lead to clogging of the trapping mechanism.
Distribute the flow evenly over the sluice bottom by making sure the sluice bottom is flat—avoid twisting and sagging.
When water supply is short, use narrower sluices to insure adequate flow velocity to keep trapping mechanism clear.
Assess the efficiency of recovery by panning the tailings, or by passing them over a short test-sluice.