Concentration of Ore

Ores found on the earth contains many impurities like sand, rocks, and grit in them, which are collectively known as “Gangue”. The concentration of ore is a method of removing gangue particles or other impurities in order to purify the ore. This process is also called Ore-dressing and benefaction. It is a very crucial step to obtain a pure metal from ore and to extract the metal.

Definition of Concentration of Ore

Ore concentration is the chemical process of eliminating impurities like sand, rocks, silt, grit etc. from the ore to extract the metals in their pure form. In other words, it is the method of separating ore from the gangue, as the gangue or matrix particles are the valueless substances that are of no use. Both physical and chemical means can be used for the separation of ores. The purified ore obtained after the completion of the ore concentration is called concentrate.

What is Ore?

Ore is defined as a solid substance (like a rock) that contains minerals or a mixture of minerals, from which the metal can be extracted by a series of methods like the concentration, isolation and refining.

Types of Ore

Ores can be mainly classified into four types:

1. Native ore: They are the type of ores which are non-reactive and contains metal in a state. Eg: Silver, gold etc.
2. Oxidized ore: They contain oxides or oxysalts such as carbonates, phosphate and silicate of metal. Examples: Oxide ores (Fe₂O₃, Al₂O₃ etc.), Carbonate ores (CaCO₃, ZnCO₃ etc.).
3. Sulphurized ore: They contain sulphides of metal like iron, and lead. Eg: FeS₂ (Iron pyrites), PbS (Galena), HgS (Cinnabar) etc.
4. Halide ore: They contain halide of various metals. Eg: Chloride ore: AgCl (Horn silver), CaF₂ (Fluorspar) etc.

Methods of the Concentration of Ore

Ores found in the earth’s ground surface contains unwanted earthy materials like rocks, sand, silt, and other impurities colloquially termed as gangue. Concentration is basically the separation of something useful out of the impure substances. Thus, we can actually extract and refine metals through concentration of ore from such impurities,. We can use various physical and chemical processes to concentrate or separate ore from the gangue matrix.

Physical Methods

The physical methods for the ore concentration include traditional handpicking, hydraulic washing, froth floatation, and magnetic separation.

• Hydraulic Washing: This method concentrates the ore by passing it through an upward stream of water whereby all the lighter particles of gangue are separated from the heavier metal ore. This is a type of gravity separation.
• Magnetic Separation:This involves the use of magnetic properties of either the ore or the gangue to separate them. The ore is first ground to fine pieces and then passed on a conveyor belt passing over a magnetic roller. The magnetic ore remains on the belt and the gangue falls off the belt.

• Froth Flotation Method:This method is mainly used to remove gangue from sulphide ores. The ore is powdered and a suspension is created in water. To this are added, Collectors and Froth Stabilizers. Collectors (pine oils, fatty acids etc) increase the non-wettability of the metal part of the ore and allows it to form a froth. Froth Stabilizers (cresols, aniline etc) sustain the froth. The oil wets the metal and the water wets the gangue. Paddles and air constantly stir up the suspension to create the froth. This frothy metal is skimmed off the top and dried, to recover the metal.
• Leaching:Leaching is used when the ore is soluble in a solvent. The powdered ore is dissolved in a chemical, usually a strong solution of NaOH. The chemical solution dissolves the metal in the ore and it can be extracted and separated from the gangue by extracting the chemical solution. Extraction of the Aluminium metal from Bauxite ore is done using this process.
• Breaking and Crushing: Big masses of rocks containing the ore must be reduced to small lump so that they may be completely exposed to the action of the subsequent operations. Machines like crushers, etc., usually do it.
• Grinding and Pulverizing: Sometimes, some ore’s concentration and further treatment demand their reduction into a very fine state. Machines like grinding and pulverizing mills are used for this work.
• Hydraulic classifier: In this method, the finely powdered ore is dropped through a hopper into a conical reservoir called a hydraulic classifier from the top. A powerful stream of water is introduced from the bottom of the reservoir. The lighter gangue particles are carried up by the current water and pass out along with water through the outlet provided near the top. The heavier ore particles get collected at the base of the cone. The conical shape of the reservoir helps reduce the velocity of water and thus prevents the ore particles from being carried away along with the stream of water.

(ii) Wilfley table: The other method of gravity separation is the Wilfley table. It is a wooden table having a slanting floor with riffles or grooves or cleats fixed on it.
The pulverized ore is placed on the trays provided at one end of the table, and a stream of water under pressure is passed over it. Thus, a rocking motion is given to the table. The lighter gangue particles are carried away by the stream of water while the heavy ore particles get deposited in the grooves. The ore particles deposited in the groove move towards one side of the table due to the rocking motion given to the table and are finally collected.
This method is used when the ore particles are heavier than the earthy or rocky gangue particles. This method is usually concentrated by the oxide ores iron (haematite), tin (Tin stone) and native ores of \({\rm{Au,}}\,{\rm{Ag,}}\) etc.

f. Electromagnetic separation: This method of concentration is employed when either the ore or the impurities associated with it are magnetic in nature. For example, chromite, \(\left( {{\text{FeO}}{\text{.C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{=FeC}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{4}}}} \right)\) an ore of chromium, magnetite \(\left({{\text{F}}{{\text{e}}_{\text{3}}}{{\text{O}}_{\text{4}}}} \right){\text{-}}\) an ore of iron and pyrolusite \(\left({{\text{Mn}}{{\text{O}}_{\text{2}}}} \right){\text{-}}\) an ore of manganese being magnetic in nature, are separated from non-magnetic siliceous gangue by this method. Similarly, tinstone or cassiterite, an ore of tin being non-magnetic, can be separated from magnetic impurities like those of tungstates of iron and manganese, which are generally associated with this method.
In this method, the powdered ore is dropped over a conveyor belt moving around two rollers – one of which has an electromagnet in it. As the ore particles roll over the belt, the magnetic particles are attracted by the magnetic roller. As a result, two heaps are formed separately. The heap collected below the magnetic roller contains the magnetic particles, while the heap formed away from the magnetic roller contains the non-magnetic impurities. In the case of tinstone, the tungstate being magnetic fall in a heap under the magnetic roller while the ore particles that is \({\text{Sn}}{{\text{O}}_{\text{2}}}\) fall in a separate heap away from the magnetic roller.

g. Electrostatic separation: This method is used for the concentration/ separation of ores, which are good conductors of electricity, while impurities are poor conductors of electricity. The method is based upon the principle that when an electrostatic field is applied, the ore particles, which are good conductors of electricity, get electrically charged and hence are repelled by the electrodes having the same charge and thus are thrown away. This method is used for the concentration /separation of \({\text{PbS}}\) and \({\text{ZnS}}\) ores occurring together in nature. The mixture of ores is powdered and dropped over a roller subjected to an electrostatic field. \({\text{PbS}}\) being a good conductor, immediately gets charged and thrown away from the roller, whereas \({\text{ZnS}}\) is a poor conductor, does not get charged and falls vertically from the roller.
h. Froth floatation process: This process is commonly used for sulphide ores and is based upon different wetting characteristics of ore and gangue particles. The finely powdered ore is mixed with water, pine oil (frother) and ethyl xanthate or potassium ethyl xanthate(collector) in a big tank. Then, the whole mixture is agitated with air. The ore particles wetted with oil come in the froth and are taken off, while impurities wetted with water settle at the bottom.

1. Here, the foaming agent is pine oil, and the froth stabilizer is cresol and anisole.
2. Collectors used are ethyl xanthate and potassium ethyl xanthate.
3. The activator used in froth floatation is \({\text{CuS}}{{\text{O}}_4}\) while the depressant used in this method is \({\text{KCN}}\) or \({\text{NaCN}}{\text{.}}\)

Chemical Methods

The chemical methods include calcination, roasting and leaching for the concentration of ore.

Calcination

It is the chemical method of separating carbonate or hydrated oxide ores.

• A carbonate ore produces carbon dioxide under heat exposure.
  
• The hydrated oxide ore releases water under heat exposure.
  

Roasting

It is a process generally applicable for sulphide ores under the influence of heat and air. In roasting, the sulphide ore is heated at a temperature below the melting point. For example:

Separation of Two Sulphide Ores

If the mineral to be concentrated consists of sulphides of two metals, then adjusting the proportion of oil to water is often possible to separate one sulphide from the other. Additional reagents called froth depressants to prevent one type of sulphide Ore particles from forming the froth with air bubbles. For example, sodium cyanide is used as a depressant to separate lead sulphide ore from zinc sulphide ore. It is because \({\text{NaCN}}\) forms a soluble zinc complex \({\text{N}}{{\text{a}}_{\text{2}}}\left[{{\text{Zn}}{{\left({{\text{CN}}} \right)}_{\text{4}}}} \right]\) which goes into the solution thereby preventing it from forming the froth. Under these conditions, only \({\text{PbS}}\) forms a froth and hence can be separated from \({\text{ZnS}}\) ore.

\({\text{4NaCN + ZnS}} \to {\text{N}}{{\text{a}}_{\text{2}}}\left[{{\text{Zn}}{{\left({{\text{CN}}} \right)}_{\text{4}}}} \right]{\text{ + N}}{{\text{a}}_{\text{2}}}{\text{S}}\)

Leaching

The process of treating the powdered ore with a suitable reagent that can selectively dissolve the ore but not the impurities is known as leaching.

(a) Leaching of Aluminium from Bauxite

The bauxite ore contains aluminium in the form of \({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\) along with \({\text{Si}}{{\text{O}}_2},\) iron oxides and titanium oxide as impurities. The powdered ore is heated with a concentrated solution of \({\text{NaOH}}\) at \({\text{473-523}}\,{\text{K}}\) and \(35-36\) bar pressure. As a result, alumina is leached out as Sodium aluminate and \({\text{Si}}{{\text{O}}_2}\) as sodium silicate leaving behind the impurities.

\({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 2NaOH + 3}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{2Na}}\left[{{\text{Al}}{{\left({{\text{OH}}} \right)}_{\text{4}}}} \right]\)
\({\text{Si}}{{\text{O}}_{\text{2}}}{\text{ + 2NaOH}} \to {\text{N}}{{\text{a}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{3}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}\)

The resulting solution is filtered to remove the undissolved impurities. Then it is cooled, and its \({\text{pH}}\) is adjusted either by dilution or by neutralization with \({\text{C}}{{\text{O}}_2}\) when aluminium hydroxide gets precipitated. In this stage, the solution is seeded with freshly prepared samples of hydrated alumina to get it precipitated, leaving behind sodium silicate in the solution.

\({\text{2Na}}\left[{{\text{Al}}{{\left({{\text{OH}}} \right)}_{\text{4}}}} \right]{\text{ + C}}{{\text{O}}_{\text{2}}} \to {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.x}}{{\text{H}}_{\text{2}}}{\text{O + 2NaHC}}{{\text{O}}_{\text{3}}}\)
The hydrated alumina is filtered, dried and heated to 1400 K temperature to give back pure \({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.}}\)

\({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.x}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + x}}{{\text{H}}_{\text{2}}}{\text{O}}\)

(b) Leaching of Silver and Gold

Some metals like gold and silver are extracted from their concentrated ores by leaching. They are dissolved in suitable reagents like acids or bases, leaving behind insoluble impurities. Then the metal is recovered from the solution by precipitation or crystallization. For example, Silver ore is leached with a dilute solution of sodium cyanide. As a result, silver dissolves, forming a complex sodium dicyanoargentate(I). This solution is further treated with scrap zinc to get the precipitate of silver.
\({\text{A}}{{\text{g}}_{\text{2}}}{\text{S + 4NaCN}} \to {\text{2Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_{\text{2}}}} \right]{\text{ + N}}{{\text{a}}_{\text{2}}}{\text{S}}\)
\({\text{2Na}}\left[{{\text{Ag}}{{\left({{\text{CN}}} \right)}_{\text{2}}}} \right]{\text{ + Zn}} \to {\text{N}}{{\text{a}}_{\text{2}}}\left[{{\text{Zn}}{{\left({{\text{CN}}} \right)}_{\text{4}}}} \right]{\text{ + 2Ag}}\)

Similarly, native gold is leached with a potassium cyanide solution, and it is recovered from the solution by the addition of scrap zinc.
\({\text{4Au + 8KCN + }}{{\text{O}}_{\text{2}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{4K}}\left[{{\text{Au}}{{\left({{\text{CN}}} \right)}_{\text{2}}}} \right]{\text{ + 4KOH}}\)
\({\text{2K}}\left[{{\text{Ag}}{{\left( {{\text{CN}}} \right)}_{\text{2}}}} \right]{\text{ + Zn}} \to {{\text{K}}_{\text{2}}}\left[ {{\text{Zn}}{{\left( {{\text{CN}}} \right)}_{\text{4}}}} \right]{\text{ + Au}} \downarrow \)

Summary

All materials found on Earth are made up of chemical elements. Most elements found on Earth are metals and nonmetals. There are numerous ways to carry out the concentration of ores. The correct method is chosen based on the physical and chemical properties of the metal. This article discussed important procedures like hand-picking, hydraulic washing, magnetic separation, froth floatation method, and leaching processes for the concentration of the ores.

Frequently Asked Questions (FAQs) on Concentration of Ore

The commonly asked queries on the concentration of ores are answered here: