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Concentration of Ore - Magnetic Separation, Hydraulic Washing, Froth Floatation, Leaching- Baeyer's Process, Practice Problem, FAQs
One can do wonders if one has a scientific temperament and is attentive to observations. A washerwoman had an innovative mind too. While washing a miner’s overalls, she noticed that sand and similar dirt fell to the bottom of the washtub. What was strange was that the copper-bearing compounds from the mines that had made their way to the garments were caught in the soapsuds and rose to the top. Mrs. Carrie Everson, one of her clients, worked as a pharmacist. Mrs. Everson was informed of the washerwoman's incident. The latter believed that the concept may be applied to large-scale separation of copper compounds from rocky and earth components. This way an invention came up. At that time only those ores were used for the extraction of copper, which contained large amounts of the metal. The invention of the Froth Floatation Method made copper mining profitable even from the low grade ores. World production of copper soared and the metal became cheaper.
Let's study more about such methods!
Table of contents:
- Introduction
- Hydraulic washing
- Magnetic separation
- Froth floatation process
- Leaching
- Practice problems
- Frequently Asked Questions-FAQs
Introduction:
Concentration of ore refers to the removal of undesirable particles from ore. This also goes by the names dressing and benefaction.
The choice of method of concentration of ore depends on the difference in the physical properties of the component of metal and the gangue.
Various methods involved in the concentration of ore are as follows:
1. Physical Methods:
1.1. Hydraulic washing
This technique is based on variations in the gravities of the gangue particles and the ore. Another name for it is the gravity separation technique. In this process, the powdered ore is washed with an upward stream of running water so that the lighter gangue particles are washed away and the heavier ores are left behind, or vice versa.
1.2. Magnetic separation
This method is based on the differences in the magnetic properties present in the ore particles. It is generally used to separate magnetic impurities from non-magnetic ores or vice-versa. Example -Tin stone, SnO2 (non-magnetic) is separated from its magnetic impurity FeWO4 + MnWO4(Wolframite). Powdered ore is allowed to move over the magnetic roller and falls down. Magnetic material makes a new heap as it is held to a roller for a longer time.
1.3. Froth floatation process
This technique works by differentially wetting the mineral with oil and the impurities with water. Typically, it is employed to extract gangue from sulfide ores. There are several processes in the froth flotation process, including
1. Preparation of suspension of powdered ore: Finely powdered ore is mixed with water to make slurry.
2. Addition of collectors and froth stabilizers: There are also collectors added, such as tar, crude coal, and pine oil. These substances make mineral particles more resistant to wetting. Froth stabilizers are substances that create stable froth, hence lowering the surface tension of water. Aniline and cresol are examples. Oils make mineral particles wet, whereas water makes gangue particles wet.
3. Agitation of mixture: A rotating paddle agitates the mixture and draws air in it such that it forms froth that causes ore particles. Froth is light and can be skimmed off. Dried ore particles are recovered.
If two sulfide ores are present in a particular ore can be obtained by using depressants. Depressants are substances that are added to suppress the floating characteristic of the ore particles. Example-NaCN/KCN.
Example- To froth containing ZnS and PbS, depressant NaCN/KCN is added which results in the formation of soluble complex of ZnS as PbS doesn’t react with the depressants added.
2. Chemical Method:
2.1. Leaching
It is generally used if the ore is soluble in a suitable solvent.
Leaching of alumina
Alumina is extracted from its ore using this technique. The primary source of aluminium is bauxite. Impurities of SiO2, TiO2 and Fe oxides are present in bauxite.
One of nature's most captivating sights is the Cava di Bauxite near Otranto, a little lake with deep emerald green hues encircled by rocky cliffs painted a stunning imperial red. It is a deposit of water from the residues of bauxite released in the crater of an old disused quarry abandoned in 1976.
Various steps involved in the leaching process are:
1. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This way, Al2O3 is leached out as sodium aluminate (and SiO2 too as sodium silicate) leaving the impurities behind.
2. The aluminate in the solution is neutralized by passing CO2 gas and hydrated Al2O3 is precipitated.
3. The hydrated alumina is filtered, dried, and heated to produce pure Al2O3, leaving the sodium silicate in the solution.
Leaching of Silver and gold
1. Metal is leached with a dilute solution of NaCN or KCN in the presence of air ( O2).
2. Pure metal is obtained by replacement with zinc.
Practice Problems:
Q1. Oxidation states of the metal in the minerals magnetite and haematite, respectively are
A). III in magnetite and II, III in haematite
B). II in magnetite and II, III in haematite
C).II, III in magnetite and II in haematite
D). II, III in magnetite and III in haematite
Answer: D
Solution: Magnetite is Fe3O4. Haematite is Fe2O3, where the oxidation number of iron is III. And in magnetite iron is a mixed oxide (FeO.Fe2O3). Hence, iron is present in both III and II oxidation states.
Q2. Which of the following ores is best concentrated by the froth floatation method?
A). Siderite
B). Galena
C). Malachite
D). Magnetite
Answer: B
Solution: Froth floatation is a process which is used to concentrate sulfide ores. Only galena (PbS) is a sulfide ore. Siderite is FeCO3, magnetite is Fe3O4, and malachite is Cu(OH)2.CuCO3.
Q3. The idea of the froth floatation method came from person X and this method is related to the process Y of ores. X and Y, respectively are:
A). Fisher woman and concentration
B). Washer woman and concentration
C). Fisherman and reduction
D). Washer man and reduction
Answer: B
Solution: The idea of froth floatation method came from washerwomen, and this process is related to the concentration of sulfide ores.
Q4. Pure silver metal forms a water-soluble complex with an aqueous solution of NaCN in the presence of
A). Nitrogen
B). Oxygen
C). Carbon dioxide
D). Argon
Answer: B
Solution: Native (impure) metal is leached with NaCN in the presence of O2 to form a [Ag(CN)2]- complex. From this complex, silver is precipitated by using metallic zinc dust.
Frequently Asked Questions-FAQs:
Q1. Why is zinc used to replace pure metal in the leaching process of silver?
Answer: Zinc is used to replace silver in its complex because of the higher reactivity of zinc over silver. The following reaction occurs in the leaching of silver to obtain pure silver.
Q2. What is the principal used in hydraulic washing?
Answer: Ore and gangue particles are separated based on the difference in between the densities of the two and this washing is based on gravity. It is typically used to concentrate oxide and native ore.
Q3. Give examples of ore which can be concentrated using froth floatation?
Answer: Froth floatation is generally used for the concentration of low-grade sulfide ores like PbS, ZnS, Cu2S, CuFeS2.
Q4. What is the role of stabilizers in the froth floatation process?
Answer: Froth can last for a longer period in presence of a stabilizer. Ultimately stabilizers stabilize the froth formed in the froth floatation process.
Related topics:
|
Extraction of Crude Metal from concentrated Ore |
Extraction of iron |
|
Occurrence of Metals |
Thermodynamical principle of metallurgy |
|
Vapor phase refining |
Refining of metals from concentrated ores |
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