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Concentration or Dressing of Ores

  • The ores obtained from the earth contain large quantities of foreign matter.

  • These unwanted impurities, e.g. earth particles, rocky matter, sand limestone etc. present in an ore are called gangue or matrix.

  • Prior to the extraction of the metal from the ore, it is necessary to separate, the ore from the gangue. This separation can often be achieved by physical means since mineral and gangue generally occur as separate solid phases.

  • The process of removal of gangue from the ore is technically known as concentration or ore dressing and the purified ore is known as concentrate.

  • These are various physical and chemical process involved in this step.

Hand Picking

  • The ore is separated from the main stock in a sufficient degree of purity by simply picking it by hand and then breaking away the adhering rocky materials with a hammer.

  • This is done on the basis of differences in their colour, luster and lump shape.

  • It may be accomplished on an ore-sorting conveyer.

Gravity Separation or Hydraulic Washing (Levigation)

  • This method of concentration of the ore is based on the difference in specific gravities of the metallic ore and gangue particles.

  • Generally metal ores are heavier than the gangue associated with them.

  • By flowing the powdered ore in a current of water, the lighter rocky impurities can be washed away and the ore particles are left behind.

  • For this, either wilfley table or Hydraulic classifier is used. Generally oxide and carbonate ores are concentrated by this method. i.e. haematite and cerussite.

  • There are two gravity concentration methods.

    • Jigging : It is removal of the lighter portions of an ore by means of a stream of water or air which rises through a bed of coarse ore particles.

    • Tabling : It is similar removal by passing a mixture of ore and water over an inclined vibrating table.
      Gravity Separation or Hydraulic Washing (Levigation)

Magnetic Separation

  • This method of separation is used when either the ore particles or the gangue associated with it possess magnetic properties. For example, chromite Fe(CrO2)2 being magnetic can be separated from the non-magnetic silicious gangue by magnetic separation.

  • This method is widely used for the separation of two minerals, when one of them happens to be magnetic.

  • The magnetic mineral can be separated from the non-magnetic one by this method. For example, mixture of FeWO4 (magnetic) and cassiterite SnO(non-magnetic) are separated by this method.
     

  • Magnetic Separation

  • Similarly, rutile TiO2 (magnetic) can be separated from chlorapatite, 3Ca3(PO4)2. CaCI2 by magnetic separation.

Froth Floatation Process

  • This method is extensively employed for the preliminary treatment of the minerals especially sulphides.

  • The process is based on the difference in wetting characteristics of the gangue and the ore with water and oil.

  • The former surface being preferentially wetted by water and of the latter by oil.

  • The crushed ore along with water (slurry) is taken in a floatation cell. Various substances (additive) are added depending on the nature of the ore and a current of air is blown under pressure.

  • The air is broken into tiny bubbles which are distributed throughout the volume of the pulp, attaching themselves to those solid particles which have become water repellent after reagent treatment.

  • The bubble-particle aggregates float to the surface of the suspension where a mineral-laden froth forms. The additives added are usually of three types.

    • Frothers: Frothers increase froth stability within desired limit. For example, pine oil, soaps, resins etc.

    • Collectors: These attach themselves by polar groups to grains of some mineral and form water repelling films on those minerals. Hence, these minerals attach with bubbles and go to froth. Collectors will attach with themselves only to minerals with definite chemical composition and lattice structure. They are high molecular weight organic compounds. The most common among them are xanthates, carboxylic acids and their salts.

    • Activators and Depressants: Minerals similar in chemical composition, such as sulphides of copper, lead and zinc exhibit an almost equal ability to absorb collectors; for this reason, when present in the same suspension, they will tend to froth together. For the purpose of selective floatation, this tendency may be controlled by supplementary reagents, known as depressors. Depressors are inorganic compounds, which form films on solid particles, thereby preventing the absorption by collectors. The film is produced through a chemical reaction between the depressor and the surface layer of the mineral.

  • The collector effect may be enhanced by activators. They are inorganic compounds soluble in water. Added to the suspension, an activator can destroy or modify the depressor film on the solid particles so that they are now able to absorb the collector ions or molecules and becomes floatable. For example, galena (PbS) is usually associated with zinc sulphide (ZnS), pyrites (FeS2) and quartz (SiO2).

  • Floatation is carried out by using potassium ethyl xanthate (Collector) along with sodium cyanide and zinc vitriol (depressants). They depresses the floatation property of ZnS grains by forming a complex, so mainly PbS passes into the froth when air is blown in. The froth spills over and is collected. After galena as been removed with the froth, the process is repeated by adding CuSO(activator). This break the depressor film on ZnS grains hence, now these grains are available for collector, which are removed with the froth. The acidification of remaining slurry leads to the floatation of pyrites.

    Froth Floatation Process

 

Refer to the following video for Concentration of Ores


Illustration

Question : How does NaCN act as a depressant in preventing ZnS from forming the froth?
Solution: NaCN forms a layer of zinc complex, Na2[Zn(CN)4] on the surface of ZnS and thus prevents it from the formation of froth.

Leaching

  • This a chemical method of concentration.

  • Here the powdered ore is treated with certain reagents, which dissolves the ore leaving behind impurities.

  • The impurities left undissolved are removed by filtration.

  • Leaching method is used for concentrating ores of aluminium, silver, gold etc. For example, bauxite (AI2O3.2H2O), is concentrated by this method. Crude bauxite contains ferric, oxide, titanium oxide and silica. These impurities are removed by making use of the amphoteric nature of alumina. Finely powdered bauxite is treated with an aqueous solution of caustic soda at 420-440 K under pressure for several hours. Alumina present in bauxite dissolves forming soluble sodium aluminate.

    AI2O3 + 6NaOH → 2Na3AIO3 + 3H2O

    The impuritie remain unaffected and separate as insoluble red mud, which is filtered off. The filtrate is diluted and a little freshly precipitated aluminium hydroxide is added which causes the precipitation of aluminum hydroxide. This is filtered and calcinated to get highly pure alumina.Na3.AIO3 + 3H2O → AI(OH3) + 3NaOH
    2AI(OH)3 AI2O3 + 3H2O

  • Leaching is also used to concentrate silver and gold ores and is known as Mac Arthur Forrest cyanide process. 

4Ag + 8NaCN + 2H2O + O2 → 4Na[Ag(CN)2] + 4NaOH
                                                 (Sodium argentocyanide)

4Au + 8NaCN + 2H2O + O2 → 4Na[Au(CN)2] + 4NaOH
                                           (Sodium aurocyanide)

Now, Ag and Au can be recovered easily from the solution by the addition of electropositive metal like zinc.

2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag↓

2Na[Au(CN)2] + Zn → Na2[Zn(CN)4] + 2Au↓
                                        Soluble complexes

  • Thickening: Prior to precipitation, it is sometimes advantageous to concentrate the solution. This is especially true to learn materials, the leached solution of which are usually diluted or contain large amount of impurities. This concentration is called thickening. It is accomplished by means of ion-exchange method.

  • Precipitation The metal sought or its compounds obtained by leaching are precipitated from the solution after it has been separated from the undissolved residue by means of filtering or settling. In elemental form, a metal can be precipitated from a solution either electrochemically, as in copper, zinc or nickel or by cementation according to reaction.

Question 1: The ores that are concentrated by floatation method are

a) Carbonates

b) Sulphides

c) Oxides

d) Phosphates

Question 2: Magnetic separation is used to increase concentration of

a)  Horn Silver

b) Calcite

c) Haematite

d) Magnesite

Question 3: Which of the following substances is not used as a frother?

a) Pine oil

b) Soaps

c) Resins

d) Ethanol

Question 4: Which of the following is a chemical method of concentration of ores?

a) Froth floatation process

b) Magnetic separation

c) Hydraulic washing

d) Leaching

Q.1

Q.2

Q.3

Q.4

b

d

d

d

Related Resources

To read more, Buy study materials of General Principles & Isolation of Elements comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.

 

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