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An Introduction to Mineral Processing

Mineral Processing: converting a mineral to an ore

There are some basic principles in mining and mineral processing that often get overlooked or at least not paid enough attention to. This is especially true when people start dealing with minerals that have a special hold on them (gold being a good, but not the only, example).

 A good place to begin is with an ore.   An ore is a natural occurrence of materials (rocks) from which a mineral of some value can be recovered with profit on a large scale.  More goes into a having a mine than just the minerals in the ground.  You can have a high grade mineral deposit, that is easily mineable, but due to socio, political, and environmental issue you do not have an ore body and do not have a mine.  On the other hand with the proper conditions a low grade, difficult to mine deposit can be economical making it an ore body and a mine.

 I looked at a deposit that averages around 1 oz/ton (31 g/tonne) of gold and can be recovered by dredging from a depth of 10 to 20 feet.  But due to those conditions, it is not economic to mine and thus should not be considered an ore.

 More commonly the issue is if the mineral can be extracted economically from the rock. When the percent of the value mineral is too low for profitable extraction the rock ceases to be an ore.

 Mineral processing, mineral beneficiation, or ore dressing is the process of separating and saving valuable minerals from the valueless material of an ore, where the valuable minerals are concentrated into smaller bulk and weight by discarding some of the waste.  It is also the process of separating two or more minerals, which combined have little value, into two or more products, each of increased value.

 The advantages gained by concentrating the valuable minerals into a smaller bulk are: first, that the cheaper mechanical method of rejecting the waste material is substituted for the more expensive chemical method of the smelting furnace; and second, the rejected waste material is not shipped, and this saves freight. In some cases, particularly with non-metalliferous ores, such as coal and precious stones, smelting may not be an option.

 The advantage gained by separating two valuable minerals from each other lies in the fact that the mineral of less prominence is advanced from being of no value or even a positive detriment, to being a standard ore, salable to smelting works; while the mineral of more prominence has advanced in selling value from being a poorer grade of ore to being a better one, and commands a higher price in consequence.

Rock Properties for Mineral Processing: what makes it work (or not)

Above I discussed that it takes more than a high grade to make a mine.   It also takes physical properties of the rocks and ore.  Mineral processing is often divided into two parts, severing and separating.  It is the properties of the minerals and rocks that explain how these parts work (or do not work).  Mineral processing makes use of the physical properties of minerals and rocks; and the difference in behavior between the valuable and waste minerals affords methods for the severing and separation.

 Severing (or Detaching). – Also called comminution.  The valuable minerals as they occur in the rock are attached to waste minerals, and to sever the one from the other, the various steps of breaking, crushing, and grinding are used.  The method depends on the type of rock. With solid rocks, which are to be separated without essentially change the nature of the rock, the almost universal method is some form of breaking or comminution.  Chemical severance (often related to in-situ mining or leaching) usually involves either dissolution in a solution or decompositions of rock at high temperatures, with or without a selective phase change.

 The main physical properties of interest are

Hardness or softness

Tenacity, brittleness or friability

Structure and fracture

Aggregation

 Separating - After the crushing has severed the valuable minerals from the waste, the two are still mixed together; and the true separation, which puts the good ore into the store bin and sends the waste to the dump, must then take place. Separation methods are much more diversified than those of severance. In mineral processing they are based upon utilization of some property in which the intermingled severed minerals differ either in kind or degree, to effect a differential response to some impulsive force.

 The main physical properties of interest are:

Specific gravity and settling power

Hydrophilic/hydrophobic

Electro-conductivity

Magnetic susceptibility

Dissolution

Optical Characteristics (Color and luster, etc.)

Adhesion

Change in properties on heating

non-magnetic to magnetic.

dense to porous.

Breaking of chemical bonds.

 

Breaking up can be hard to do

Yes breaking up can be difficult, but understanding what allows it, can make it easier to do.

Properties for Comminution: severing the good from the bad

This is a continuation of a previous post on the physical properties that make mineral processing work (or not) (1). Previously I discussed the overall groupings, in this post I will discuss the properties that effect severing.  A following post will discuss the properties that effect separating.

 Hardness or Softness.

 Minerals differ greatly in their hardness, ranging from the hardness of the diamond to the softness of talc, their ability to scratch one another being considered the measure of hardness. The table of hardness adopted by Dana in his "Mineralogy" is as follows

10        Diamond

9          Sapphire

8          Topaz

7          Quartz

6          Feldspar

5          Apatite

4          Fluorite

3          Calcite

2          Gypsum

1          Talc

 Each mineral in the list can scratch all those below it. Hardness affects the wear of crushing machines - the harder the mineral the greater the wear. It does not necessarily affect the tendency of the minerals to produce fines or slimes in crushing.

 Tenacity, Brittleness or Friability

 Some minerals, such as native copper, mica, talc, and gypsum, are very tough though they may at the same time be soft, and this makes them difficult to break. Some forms of hornblende and feldspar exhibit extraordinary toughness, although they are not very hard; other minerals are brittle and break up with comparative ease, as, for example, some varieties of quartz. A hard, brittle mineral will, when in agitation, make more fines or slime much more than one which is soft and tough.

 Structure and Fracture.

 The structure of a mineral tends to modify the shape of the particles resulting from crushing. Cleavable minerals may break into cubical blocks, as galena; into elongated fragments, as galena, feldspar, calcite, and sphalerite; into needle-like or thread-like shapes, as asbestos; or into flat scales, as galena, mica, graphite, and talc. Granular minerals will drop naturally into separate rounded grains when broken up, as magnetite, garnet, and some varieties of galena. Minerals with massive structure, free from any special tendency to break in one more than in another direction, may have earthy fracture, as hematite; or conchoidal (oyster shell like), as pyrite crystal, quartz crystal, and obsidian. The shapes of these grains have an important bearing upon their power to settle in water or in air.

 Mineral Aggregation

 The valuable minerals may occur in pure masses, as in the banded vein structure and in pockets or vugs. They may also be in large crystals mixed with the waste minerals. Both these conditions are favorable for complete separation. On the other hand they may occur much intermingled with the waste minerals: either in granular structure (such as a porphyry), rounded grains or small, compact crystals; or a radiating mass of slender, needle-like crystals (acicular structure, the valuable and waste minerals penetrating each other in all directions, to the eye a hopeless tangle; or, finally, of laminated structure, in thin layers alternately of good and worthless mineral. All of the latter structures add difficulty to the problem of mineral processing.

Properties for Beneficiation: separating the good from the bad (or the better from the not quite as good)

You have an ore body, mined the ore, and broken it up into smaller pieces.  It is now time to separate the good from the bad. To do this you use the physical properties described above.

 Specific Gravity

 The difference in densities of equal volumes of materials (often referred to as specific gravity) allows for their separation, and this property may be employed in two different ways, namely, as affecting settling power, or as affecting momentum.

 Settling Power - In general for two particles of the same size and shape, the heavier will settle faster than the lighter, and for two particles of different specific gravities and the same settling velocity, the higher specific gravity particle will be of smaller diameter. The ratio between these two diameters will have an approximately constant value under similar conditions, and called the settling ratio.

 Momentum - When a particle is given a high velocity in a horizontal direction, the path it follows is called its trajectory. Of two particles of the same size and shape, the heavier will have the longer trajectory, and of two particles with different specific gravities but the same trajectory, that with the higher specific gravity will be of smaller diameter than the other.

 Hydrophobic/hydrophilic

 The basis of froth flotation is the difference in wettability of minerals. Particles range from those that are easily wettable by water (hydrophilic) to those that are water-repellent (hydrophobic). If a mixture of hydrophobic and hydrophilic particles are suspended in water, and air is bubbled through the suspension, then the hydrophobic particles will tend to attach to the air bubbles and float to the surface. The froth layer that forms on the surface will then be heavily loaded with they hydrophobic mineral, and can be removed as a separated product. The hydrophilic particles will have much less tendency to attach to air bubbles, and so it will remain in suspension and be flushed away. 

 Particles can either be naturally hydrophobic, or the hydrophobicity can be induced by addition of chemicals (reagents). Naturally hydrophobic materials include hydrocarbons, and non-polar solids such as elemental sulfur. Coal is a good example of a material that is typically naturally hydrophobic. Chemical treatments (addition of reagents) to render a surface hydrophobic are essentially methods for selectively coating a particle surface with a monolayer of non-polar oil.

Electro-Conductivity

The fact that some minerals are relatively good conductors, while others are relatively poor conductors, of electricity makes it possible to separate them. It has been found that the greater part of the sulphide minerals and the metals themselves are, in varying degrees, conductors of electricity, while the gangue minerals are, in general, very poor conductors. If neutral ore particles are brought into contact with an electrode containing a static 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 so far.

Magnetic Susceptibility

The attraction to the magnet is quite strong in some minerals and metals, notably 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. By using properly constructed magnets this property may be made of great value, not only separating the magnetic from the non-magnetic, but those that are more magnetic from those that are less so.

Dissolution

Various chemical solutions (lixiviants) can selectively dissolve minerals and elements allowing their separation from the rock. Lixiviants may work by altering the reduction-oxidation reaction state of an ore, or by altering the pH.  Acidic lixiviants, such as sulfuric acid, are commonly used to leach base metals such as copper, whereas basic lixiviants such as a solution of sodium cyanide are used to leach precious metals. A key component is to have this reaction be reversible.

Optical Properties

Under the stimulation of electrical magnetic radiation, some minerals have distinctive optical properties.  Thea 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, the fluorescence is used to sort.

Other properties

Adhesion - As with hydrophobic/hydrophilic characteristics, some minerals will preferentially adhere to other substances.  The most common are gold adhering to mercury in plate amalgamation and the use of grease tables in diamond recovery.

Change of Property by Heating - Heating of some minerals will change their basic characteristics so that separation that was not feasible becomes possible.  Examples include autoclaving to convert from sulphide to oxide.  Some minerals, when heated, decrepitate or fly to pieces through the unequal expansion which overcomes the cohesion of the molecules. Calcite, fluorite, and barite are examples of this. A mineral which decrepitates may be separated from one which does not, by decrepitating and sifting; the latter mineral will be found on the sieve, while that which was finely decrepitated will have gone through.

The Use of Supplementary Principles

A process usually consists of two or more successive steps, in which the later is supplementary to the earlier. Thus, sorting in classifiers is followed by sizing on slime tables; and sizing by screens is followed by sorting on jigs. In each case the first step prepares the ore for the second, and the second supplements and completes the work which the first step was incapable of performing alone. Neither step is complete without the other.

The use of graded crushing and graded separation to diminish the amount of fines or slimes produced is quite frequently resorted to with brittle minerals.

 

 

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