Canadian Mining Journal


Innovative device offers solution to waterless mineral processing

A Dry Gravity Concentrator has been developed for mineral deposits where water is not available, where there are competing interests for a limited water supply or, for jurisdictions where the use of cyanide is prohibited.

A Dry Gravity Concentrator has been developed for mineral deposits where water is not available, where there are competing interests for a limited water supply or, for jurisdictions where the use of cyanide is prohibited.

In the current global environment, precious metal mining companies are also struggling with high capital and operating costs and because of this, new and innovative ways to help make mining more profitable are always welcome.

Because of this, we feel that it’s a good time to take a look at dry gravity concentration.

Of all of the unit operations in mineral processing, gravity concentration has the lowest capital and operating costs and in the words of Pennsylvania State University Mineral Processing Professor Frank Aplan; “Gravity Concentration, for economic reasons, is applicable where the situation dictates the least expenditure of money.”

A Hydraulics Systems Approach

To come up with a novel dry concentrator, a different approach was to consider the dry gravity process in terms of a hydraulic system, with two flow regions.

Flow regions are described as follows:

  • Turbulent (10,000 to 1,000 mircon particle size range)
  • Intermediate (1,000 to 100 micron particle size range)

With a hydraulics approach and employing “Air Sizing and Dust Collection” formulas, the Terminal Velocity of particles for these three flow regions can be defined by formulas that provide the main variables required for dry gravity separations.

Flow regions are described as follows:

  • Turbulent (10,000 to 1,000 micron particle size range)
  • Intermediate (1,000 to 100 micron particle size range)

In the Intermediate Flow Region, most fine dry solids can be aerated with low pressure air. Once aerated they will flow like a fluid.

In pneumatic conveying, fines aeration is the basis upon which commercially available air-activated gravity conveyors, (commonly referred to as “air-slides”) operate. They are employed in dry industrial mineral plants to empty rail cars, storage bins, and to convey materials between dry process operations. They are available in sizes that vary between four and 24 inches wide, require 2 to 6 degree slopes, if the elevation is available, lengths can be up to one mile, and they can convey a variety of low and high bulk density materials.

Generally, the higher bulk density materials can only be conveyed in the finer particle sizes. Since most precious metal ores are in the 1,400 to 1900 kg/cubic meter range with liberation sizes finer than 100 mesh; air-slide conveyors can be readily adapted as dry gravity concentrators. 


In addition to low capital and operating costs, a dry gravity concentration process has a number of inherent advantages, such as:

  • WATER – Only requires 70 to 90 kg of water per ton of ore for dust suppression. Wet processes generally require two to three tons of water per ton of ore.
  • AIR is a compressible fluid that can replace water.
  • POWER – A dry system will take 833 times less energy to move an equivalent volume of air than it would take for water. (1.2 kg/cubic metres for air compared to 1,000 kg/ cubic metres for water).
  • VISCOSITY OF AIR – Depending on the temperature, the viscosity of air is 28 to 90 times lower than it is for water. That effectively eliminates surface tension and viscosity agglomeration effects in the dry concentration of minerals.
  • REAGENTS are not required.
  • VIBRATION is not required when air is the fluid medium.
  • All of the process support equipment is CURRENT TECHNOLOGY.

Air-Slide Conveyors

Since the 1880s, many variations of the air-slide, dry gravity concentrator concept have been patented in the United States. Very few (two) of these patents has made it to a commercial production stage. This may be because the variables required to achieve commercialization were not properly defined.

Those patent descriptions show material fed to conveyed by and discharged to a separation device. What was missing were the front-end material preparation and the separation steps.

Gravity systems require constant-flow streams to produce optimum results.

In a dry gravity system, this is best achieved with steady head feed bins.  Then, within the separation device, first the material must be segregated, then concentrated and finally separated.

To accomplish these tasks the design variables for concentrator geometry, slopes, lengths, air pressures, porous medium types and an effective mineral recovery device must be determined.  

The Terminal Velocity formulas show that dry gravity separations are based primarily on particle size and specific gravity. Perfect separations would be achieved when both the gangue and heavy particles have the same particle size. Dry gravity separations can be made more efficient by separately processing groups of smaller particle size ranges.  

When the price of gold increased from $300 to $1,500 per ounce, the average grades of gold ores mined and processed dropped from 5 grams per ton down to the 1 gram per ton range.

To process these lower grade ores, a dry processing system will be required to treat several particle size intervals separately. This will be necessary to achieve high separation recoveries and low cost unit operations.

Particle Size Interval – Treatment Examples

In the 1970s, a dry process pilot plant was designed to develop a gold deposit in a desert, where water was a scarce commodity.

The ground feed was separated into five different plus 200 mesh particle size groups and achieved 90% gold recovery, from a 5 gram per tonne feed. Air tables (aerated deck Wilfley-type tables) were the concentration device used.

A feasibility study determined that air table capacity limitations with their attendant vibrations would not provide a viable process.  However, the use of several particle size ranges and the air table concentrators demonstrated there were possibilities for developing viable dry gravity separation methods. 

An additional example with the efficiency of processing multiple particle size intervals was experienced in April, 1997.  Process development work was being conducted on metallurgical samples from a large lode gold property, which provided higher than normal gravity gold recoveries.

However, no gold values were found in the bulk autogenous grinding samples. This raised questions. A variation of the multiple particle size intervals was used to concentrate and recover the gold particles in metallurgical sub-samples. They were determined to be salted placer gold particles, which allowed that gold mining company to exit the bidding process. 

For the past four years, a modified air-slide conveyor has been developed to function as a dry gravity concentrator. It was selected to operate on materials where the 80% liberation size is between minus 300 microns (50 mesh) and 45 microns (325 mesh).

The particle size range was limited in the coarse size by the pneumatic carrying capacity of solids, and in the fine particle size, to eliminate dust problems generally associated with dry systems. Eliminating the minus 45 micron fraction also increases the separation efficiency because it reduces the particle size range being processed.

The development program was conducted on Barite, Magnetite and with Tungsten-Carbide. The design variables were determined or calculated and incrementally improved through four prototype generations.

In the final stages, it was decided to focus on the precious metal industry. 

For the precious metal development work, an artificial binary standard was made up of silica sand (SG=2.6) and tungsten-carbide (SG=14.6), which is magnetic.

For each test run, a magnetic separator was used to recover tungsten-carbide in the individual time-weight samples. In this way, the binary standard facilitated the development work required to establish the design variables.

After 110 test runs, carbide metal recoveries in the 80 to 90% range have been obtained from a 0.5% tungsten-carbide grade standard.

The basic concept is simple; but difficult to copy. This novel concept now requires real-world samples containing middlings to prove the concept and determine what can be realistically accomplished.

*George Rodger, Inventor, Rodger Mineral Services, Coquitlam, BC  and Tony Mariutti, President of TM Engineering Ltd., Burnaby, BC.

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