1. Crushing and grinding of the ore
2. Concentration or benefaction of the ore
3. Hand Picking
4. Hydraulic washing or levigation
5. Electromagnetic separation
6. Froth flotation
This is to be verified that this project to study the"Extraction of Metals" has been completed under my supervision and this student namely Ravi Rathore has completed this report.
Signature of Lecturer
As a student of Class XI-A, I did this project as a part of my studies entitled :"Extraction of Metals".
I owe a deep sense of gratitude to my Chemistry Teacher whose valuable advice, guidance and lovingly nurturing this project from conception to completion.
Finally, I am very thankful to my parents for helping economically and my friends for giving me a helping hand at every step of this project.
Signature of Student
Crushing and Grinding of the Ore
The ores usually occur in nature in form of big lumps. These lumps are broken into small pieces with help of crushers or grinders. This process is called crushing. These small pieces are then reduced to a fine powder with the help of ball mill or stamp mill. This process is called Pulverization.
Concentration or Benefaction of the Ore
Ores as they are obtained from earth's crust are never pure. They are usually associated with earthy and siliceous impurities (in addition to the impurities of other minerals) called gangue or matrix. These impurities have to be removed from the ore before the extraction of the metal is actually done.
The finely ground ore is concentrated by the following methods :
1. Hand Picking :
In case the impurities are quite distinct from the ore so that these may be differentiated by naked eye, these may be separated by hand picking.
2. Hydraulic Washing or Levigation :
The process by which lighter earthy particles are freed from the heavier ore particles by washing with water is called levigation. For this purpose, the ore is either agitated with water or washed with an upward stream of running water. The lighter impurities are washed away while heavier ore particles sellle down.
This method is used when the ore particles are heavier than the earthy or rocky gangue particles. The oxide ores such as those of iron (haematite), tin (tin stone) and native ores of Au, Ag etc. are usually concentrated by this method. The process is carried out in specially designed tables called Wilfley tables.
3. Electromagnetic Separation :
This method of concentration is employed when either the ore of the impurities associated with it are magnetic in na- true. For example, chromites, FeO.Cr2O3 = FeCr2O4) - an ore of chromium, magnetite (Fe3O4) - an ore of iron and pyrolusite (MnO2) an ore of manganese being magnetic are separated from non-magnetic siliceous gangue by this method. Similarly, tinstone or cassiterite (SnO2), 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 it, by this method.
In this method, the powdered ore is dropped over a travelling 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 heat collected below the magnetic roller contains the magnetic particles while the heap formed away from the magnetic roller contains the non-magnetic impurities. In case of tinstone, the tungstates being magnetic fall in a heap under the magnetic roller while the ore particles i.e. SnO2 fall in a separate heap away from the magnetic roller.
4. Froth Flotation :
This method is widely used for the concentration of sulphide ores such as zinc blende (ZnS), copper pyrites (CuFeS2), galena (PbS) etc. This method is based upon the fact that the surface of sulphite ores is preferentially wetted by oils while that of gangue is preferentially wetted by water.
The ore is crushed into a fine powder and mixed with water to form a suspension in a tank. To this suspension are added collectors (e.g. pine oil, xanthates and fatty acids) which enhance the non-wettability of the ore particles and froth-stabilizers (e.g., cresols and aniline) which stabilize the froth.
The suspension is violently agitated by the rotating paddle which draws in air causing frothing.
During this process, the ore particles which are preferentially wetted by the oil become lighter and thus rise to the surface along with the froth while the gangue particles which are preferentially wetted by water become heavier and thus settle down at the bottom of the tank. The froth is skimmed off. It is allowed to collapse and finally dried to get the concentrated oe.
If the mineral to be concentrated consists of sulphides of two metals, then by adjusting the proportion of oil to water, it is often possible to separate one sulphide from the other. Sometimes additional reagents called depressants are used 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 (PbS) ore from zinc sulphide (ZnS) ore. This is due to the reason that NaCN forms a zinc complex, Na2 [Zn(CN)4] on the surface of ZnS thereby preventing it from the formation of froth. Under these conditions only PbS forms froth and hence can be separated from ZnS ore.
4 NaCN + ZnS -------> Na2 [Zn(CN)4] + Na2S
Sod. tetracyanozincate (II)
5. Leaching :
This process consists in treating the powdered ore with a suitable reagent such as acids, bases or other chemicals) which can selectively dissolve the ore but not the impurities.
1. Modern ABC of Chemistry.
2. NCERT Class X.
3. Comprehensive Practical for XI