Introduction to Dry Mineral Processing

A major issue in the world, and directly effecting the mining industry is water or rather lack of water.  Rivers are running dry causing economic impacts and humanitarian issues in many parts of the world.  Many, say most, mining and mineral processing operations require water.  Even with best practices water scarcity is becoming a problem.

An alternative is dry mineral processing.  This falls into two general categories: sorting and pneumatic or air separation. 

SORTING

One of the earliest, if not the earliest, methods of mineral processing was sorting.  This made use of the difference in color, luster, or similar property of one mineral from another.  This (along with gravity) remained the main method of concentration until the later parts of the 19th century (and for some minerals well into the 20th), primarily by doing hand sorting.  Even today hand sorting is often the final stage for small artisanal mining and prospecting (selecting diamonds by eye and picking gold from the pan).

Today sorting has also grown beyond this and expanded to become commercial operations using how minerals react to electromagnetism.  Electromagnetism has three properties all of which are used in mineral processing: electric fields, magnetic fields, and electromagnetic radiation (light).

Electrical Fields

Some minerals are relatively good conductors, while others are relatively poor conductors of electricity.  This can be used to separate them. Most of the sulphide minerals and most metals are, to some extent, conductors of electricity, while the gangue minerals are generally poor conductors.  If ore particles are brought into contact with an electrode containing a charge, the better conductors become similarly charged and are repelled, in the same way that pith balls would be under like conditions, while the poorer conductors are not repelled or at least not as far.

Magnetic Fields

Attraction to a magnet is quite strong in some minerals and metals; particularly magnetite, some forms of pyrrhotite, cast iron, wrought iron, steel, nickel, and cobalt. Other minerals, such as franklinite, chromite, or iron-bearing sphalerite, garnet, etc., have very weak magnetism. Still others, such as quartz, calcite, gypsum, feldspar, etc., exhibit no attraction at all.  This property may be used to separate the magnetic from the non-magnetic.

Electromagnetic Radiation

Under the stimulation of electrical magnetic radiation, some minerals have distinctive optical properties.  The easiest to observe are color and luster which was (and is) used in hand picking and optical sorting.  Slight differences in color or in luster - for instance, the brass yellow of chalcopyrite, the pale yellow of pyrite, the vitreous luster of quartz, the resinous of sphalerite, the adamantine of diamond, the dull of chalk, and the pearly of talc can furnish aids in hand picking and can be used for optical sorting.  Further on in the spectrum particular under ultra-violet or even on to x-rays, mineral fluorescence is used to sort.

Sorting as a Process

Sorting, particularly mechanized sorting, requires the material to be between 1 mm and 20 microns in particle size, and in feed rates of 150 tons per hour or less.  For sorting to be effective, and its major limitation, is that the ore and the waste material must be well liberated in the feed to the sorter (that is into practically discrete particles of ore and waste). The larger the liberation size the more likely sorting is to be cost effective. 

Picking the correct ore sorting equipment for an operation is important and often special machine features are developed for an individual application.  As with any process, and more so than some, good sorter performance requires extensive test work.  Often using prototype or preferably on production model sorting machines at close to design tonnages and conditions as possible.

If set-up and used properly, a high grade concentrate can be produced, but recovery may suffer due to blocked or locked particles.

PNEUMATIC CONCENTRATORS

Pneumatic concentrators are those in which a gas, invariably air, is used to effect differential movement of particles of different specific gravities. They parallel the water-gravity concentrators In that beds, quicksands, and the direct impulse of air on feed particles are all utilized.

 Bed-Type Pneumatic Concentrators

 This group is made up of those machines in which separation is effected by differences

In settling rates of particles in a pulsated bed of which they themselves are parts. The group includes all of the pneumatic machines best known and most used. The essential elements in all are a porous supporting surface for a mass of grains; an air supply flowing upward through the supporting surface and thence through the interstices of the mass of grains, means to produce flow of the mass of grains and to constrain layers at different vertical depths therein to move in different directions to different discharge points. These are the essential elements of a water jig, except for the difference in interstitial medium.  The kind of stratification effected, with large, heavy material of a given specific gravity at the bottom of the layer of that material, is the same in both machines; the responses to controls are, of course, also the same.  In other words, these apparatus, though having the general form and appearance of shaking tables used for water-gravity concentration, and called pneumatic tables, are actually pneumatic jigs.

 Pneumatic Quicksand

 A pneumatic quicksand may be formed by blowing air through a finely porous diaphragm into a relatively deep box of fine sand (e.g., <20-m. siliceous river sand).  With such material an air pressure of 1 1/2 to 3 in. Hg, sufficient to cause dilation without boiling, produces a spindle-hydrometer density of 1.45. This corresponds to an interstitial volume of 44.%, or 15 to 20% dilation, Such a quicksand floats bituminous coal, and permits slate and pyrite to sink. Flotation of the coal is due to the fact that at this dilation, dry, the effective density of the quicksand is well above the composite density.

 Blowing

 Blowing (sometimes called dry panning) involves direct impulse of a stream of air across a stream of free-falling particles and across a 1-grain-deep layer of particles supported on a smooth plane surface. Effectiveness is greater the shorter the size range treated and the greater the differences in specific gravity.  Blowing is also a method of sorting and can also be included in that area.

 Pneumatic Gravity Equipment

 The most common type in use today is the Bed-Type, of which there are three main styles. The simplest type and used mainly for artisanal gold operations is the dry washer.

 

Dry Washer (after West)

 Basically a screen that feeds an air pulsed set of riffles.  Small versions can be carried by a couple people.  The riffle bed is of a porous media, with air blown upwards through it.  Some even have off set weights on the blower to provide a shaking motion to transport the material.

 In general these handle a cubic yard or cubic meter an hour or less.  They are used primarily for a preliminary concentration, usually followed up with other processes later.

 Next in size and complexity are air tables and air jigs.  They work similar to regular tables and jigs, but using air instead of water.  Because of this their generally efficiency is less and more stages may be needed. 

Air Table (after Forsberg’s Inc.)

Air Jig (after Weitkaemper & Wotruba)

 In most cases, the feed needs to be dry and free flowing.  Also the energy consumption is much higher per unit of volume processed.  This is true for both sorting and pneumatic bed processes.

 Unit capacity is also lower, with the largest generally used being around 100 cubic yards or cubic meters per hour.

 

o   40+ years’ experience in the mining industry with strong mineral processing experience in precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience performing studies and determining feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).

o    E-mail:  info@smartdogmining.com