Concentration or Dressing of Ores

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 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 SnO₂ (non-magnetic) are separated by this method.
   

• 

• 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 CuSO₄ (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.
  
  

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

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