Concentration Of Ores

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Concentration Of Ores

Generally, the ores are associated with nonmetallic impurities, rocky materials and siliceous matter which are collectively known as gangue. The preliminary step in metallurgical process is removal of these impurities. This removal process is known as concentration of ore.

It increases the concentration of the metal of interest or its compound in the ore. Several methods are available for this process and the choice of method will depend on the nature of the ore, type of impurity and environmental factors. Some of the common methods of ore concentration are discussed below.

Gravity Separation or Hydraulic Wash

In this method, the ore having high specific gravity is separated from the gangue that has low specific gravity by simply washing with running water. Ore is crushed to a finely powdered form and treated with rapidly flowing current of water.

During this process the lighter gangue particles are washed away by the running water. This method is generally applied to concentrate the native ore such as gold and oxide ores such as haematite (Fe2O3), tin stone (SnO2) etc.

Froth Flotation

This method is commonly used to concentrate sulphide ores such as galena (PbS), zincblende (ZnS) etc. In this method, the metallic ore particles which are preferentially wetted by oil can be separated from gangue. In this method, the crushed ore is suspended in water and mixed with frothing agent such as pine oil, eucalyptus oil etc. A small quantity of sodium ethyl xanthate which acts as a collector is also added.

A froth is generated by blowing air through this mixture. The collector molecules attach to the ore particle and make them water repellent. As a result, ore particles, wetted by the oil, rise to the surface along with the froth. The froth is skimmed off and dried to recover the concentrated ore. The gangue particles that are preferentially wetted by water settle at the bottom.

When a sulphide ore of a metal of interest contains other metal sulphides as impurities, depressing agents such as sodium cyanide, sodium carbonate etc are used to selectively prevent other metal sulphides from coming to the froth. For example, when impurities such as ZnS is present in galena (PbS), sodium cyanide (NaCN) is added to depresses the flotation property of ZnS by forming a layer of zinc complex Na2[Zn(CN)4] on the surface of zinc sulphide.
Occurrence of Metals img 3

Leaching

This method is based on the solubility of the ore in a suitable solvent and the reactions in aqueous solution. In this method, the crushed ore is allowed to dissolve in a suitable solvent, the metal present in the ore is converted to its soluble salt or complex while the gangue remains insoluble. The following examples illustrate the leaching processes.

Cyanide Leaching

Let us consider the concentration of gold ore as an example. The crushed ore of gold is leached with aerated dilute solution of sodium cyanide. Gold is converted into a soluble cyanide complex. The gangue, aluminosilicate remains insoluble.

4Au (s) + 8CN– (aq) + O2 (g) + 2H2O (l) → 4[Au(CN)2]– (aq) + 4OH–(aq)

Recovery of Metal of Interest from the Complex by Reduction:

Gold can be recovered by reacting the deoxygenated leached solution with zinc. In this process the gold is reduced to its elemental state (zero oxidation sate) and the process is called cementation.

Zn (s) + 2[Au(CN)2]– (aq) → [Zn(CN)4]-2 (aq) + 2Au (s)

Ammonia Leaching

When a crushed ore containing nickel, copper and cobalt is treated with aqueous ammonia under suitable pressure, ammonia selectively leaches these metals by forming their soluble complexes viz. [Ni(NH3)6]2+, [Cu(NH3)4]2+, and [Co(NH3)5H2O]3+ respectively from the ore leaving behind the gangue, iron(III) oxides/hydroxides and aluminosilicate.

Alkali Leaching

In this method, the ore is treated with aqueous alkali to form a soluble complex. For example, bauxite, an important ore of aluminum is heated with a solution of sodium hydroxde or sodium carbonate in the temperature range 470 – 520 K at 35 atm to form soluble sodium meta-aluminate leaving behind the impurities, iron oxide and titanium oxide.

Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4] (aq)

The hot solution is decanted, cooled, and diluted. This solution is neutralised by passing CO2 gas, to the form hydrated Al2O3 precipitate.

2Na[Al(OH)4] (aq) + 2CO2(g) → Al2O3.3H2O(s) + 2NaHCO3(aq)
The precipitate is fitered of and heated around 1670 K to get pure alumina Al2O3.

Acid Leaching

Leaching of sulphide ores such as ZnS, PbS etc., can be done by treating them with hot aqueous sulphuric acid.

2ZnS (s) + 2H2SO4 (aq) + O2(g) → 2ZnSO4(aq) + 2S(s) + 2H2O

In this process the insoluble sulphide is converted into soluble sulphate and elemental sulphur.

Magnetic Separation

This method is applicable to ferromagnetic ores and it is based on the difference in the magnetic properties of the ore and the impurities. For example tin stone can be separated from the wolframite impurities which is magnetic. Similarly, ores such as chromite, pyrolusite having magnetic property can be removed from the non magnetic siliceous impurities.

The crushed ore is poured on to an electromagnetic separator consisting of a belt moving over two rollers of which one is magnetic. The magnetic part of the ore is attracted towards the magnet and falls as a heap close to the magnetic region while the nonmagnetic part falls away from it as shown in the figure 1.2.
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General Trends in Properties of P-Block Elements

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General Trends In Properties Of P -Block Elements

We already learnt that the properties of elements largely depends on their electronic configuration, size,  ionisation enthalpy, electronegativity etc. Let us discuss the general trend in such properties of various p-block elements.

Electronic Configuration and Oxidation State:

The p-block elements have a general electronic configuration of ns2, np1-6. The elements of each group have similar outer shell electronic configuration and differ only in the value of n (principal quantum number). The elements of group 18 (inert gases) have completely filled p orbitals, hence they are more stable and have least reactivity.

The elements of this block show variable oxidation state and their highest oxidation state (group oxidation state) is equal to the total number of valance electrons present in them. Unlike s-block elements which show only positive oxidation state, some of the p-block elements show negative oxidation states also.

The halogens have a strong tendency to gain an electron to give a stable halide ion with completely filled electronic configuration and hence – 1 oxidation state is more common in halogens. Similarly, the other elements belonging to pnictogen and chalcogen groups also show negative oxidation states.

General Electronic Configurations and Oxidation States of P-Block Elements

General Trends in Properties of P-Block Elements img 1

The tendency of an element to form a cation by loosing electrons is known as electropositive or metallic character. This character depends on the ionisation energy. Generally on descending a group the ionisation energy decreases and hence the metallic character increases.

Figure 2.1 P-Block Elements With Their Ionisation Enthalpies, Electronegativity and Metallic Nature.

General Trends in Properties of P-Block Elements img 2

In p-block, the elements present in lower left part are metals while the elements in the upper right part are non metals. Elements of group 13 have metallic character except the first element boron which is a metalloid, having properties intermediate between the metal and nonmetals. The atomic radius of boron is very small and it has relatively high nuclear charge and these properties are responsible for its nonmetallic character.

In the subsequent groups the non-metallic character increases. In group 14 elements, carbon is a nonmetal while silicon and germanium are metalloids. In group 15, nitrogen and phosphorus are non metals and arsenic & antimony are metalloids. In group 16, oxygen, sulphur and selenium are non metals and tellurium is a metalloid. All the elements of group 17 and 18 are non metals.

Ionisation Enthalpy:

We have already learnt that as we move down a group, generally there is a steady decrease in ionisation enthalpy of elements due to increase in their atomic radius. In p-block elements, there are some minor deviations to this general trend. In group 13, from boron to aluminium the ionisation enthalpy decreases as expected. But from aluminium to thallium there is only a marginal difference.

This is due to the presence of inner d and f-electrons which has poor shielding effect compared to s and p-electrons. As a result, the effective nuclear charge on the valance electrons increases. A similar trend is also observed in group 14.

The remaining groups (15 to 18) follow the general trend. In these groups, the ionisation enthalpy decreases, as we move down the group. Here, poor shielding effect of d- and f-electrons are overcome by the increased shielding effect of the additional p-electrons. The ionisation enthalpy of elements in successive groups is higher than the corresponding elements of the previous group as expected.

Electronegativity

As we move down the 13th group, the electronegativity first decreases from boron to aluminium and then marginally increases for Gallium, there after there is no appreciable change. Similar trend is also observed in 14th group as well. In other groups, as we move down the group, the electro negativity decreases. This observed trend can be correlated with their atomic radius.

Anomalous Properties of the First Elements:

In p-block elements, the first member of each group differs from the other elements of the corresponding group. The following factors are responsible for this anomalous behaviour.

  1. Small size of the first member
  2. High ionisation enthalpy and high electronegativity
  3. Absence of d orbitals in their valance shell

The first member of the group 13, boron is a metalloid while others are reactive metals. Moreover, boron shows diagonal relationship with silicon of group 14. The oxides of boron and silicon are similar in their acidic nature. Both boron and silicon form covalent hydrides that can be easily hydrolysed. Similarly, except boron trifluoride, halides of both elements are readily hydrolysed.

In group 14, the first element carbon is strictly a nonmetal while other elements are metalloids (silicon & germanium) or metals (tin & lead). Unlike other elements of the group carbon can form multiple bonds such as C=C, C=O etc. Carbon has a greater tendency to form a chain of bonds with itself or with other atoms which is known as catenation. There is considerable decrease in catenation property down the group (C>>Si>Ge≈Sn>Pb).

In group 15 also the first element nitrogen differs from the rest of the elements of the group. Like carbon, the nitrogen can from multiple bonds (N=N, C=N, N=O etc…). Nitrogen is a diatomic gas unlike the other members of the group. Similarly in group 16, the first element, oxygen also exists as a diatomic gas in that group. Due to its high electronegativity it forms hydrogen bonds.

The first element of group 17, flourine the most electronegative element, also behaves quiet differently compared to the rest of the members of group. Like oxygen it also forms hydrogen bonds. It shows only -1 oxidation state while the other halogens have +1, +3, +5 and +7 oxidation states in addition to -1 state. The flourine is the strongest oxidising agent and the most reactive element among the halogens.

Inert Pair Effect:

We have already learnt that the alkali and alkaline earth metals have an oxidation state of +1 and +2 respectively, corresponding to the total number of electrons present in them. Similarly, the elements of p block also show the oxidation states corresponding to the maximum number of valence electrons (group oxidation state).

In addition they also show variable oxidation state. In case of the heavier post-transition elements belonging to the groups (13 to 16), the most stable oxidation state is two less than the group oxidation state and there is a reluctance to exhibit the group oxidation state.

Let us consider group 13 elements. As we move from boron to heavier elements, there is an increasing tendency to have +1 oxidation state, rather than the group oxidation state, +3. For example Al+3 is more stable than Al+1 while Tl+1 is more stable than Tl+3. Aluminium (III) chloride is stable whereas thallium (III) chloride is highly unstable and disproportionates to thallium (I) chloride and chlorine gas.

This shows that in thallium the stable lower oxidation state corresponds to the loss of np electrons only and not ns electrons. Thus in heavier posttransition metals, the outer s electrons (ns) have a tendency to remain inert and show reluctance to take part in the bonding, which is known as inert pair effect. This effect is also observed in groups 14, 15 and 16.

Allotropism in P-Block Elements:

Some elements exist in more than one crystalline or molecular forms in the same physical state. For example, carbon exists as diamond and graphite. This phenomenon is called allotropism (in greek ‘allos’ means another and ‘trope’ means change) and the different forms of an element are called allotropes. Many p-block elements show allotropism and some of the common allotropes are listed in the table.

Table 2.2 : Some of common allotropes of p-block elements

Element

Most Common Allotropes
Boron Amorphous boron, α-rhombohedral boron, β-rhombohedral boron, γ-orthorhombic boron, α-tetragonal boron, β-tetragonal boron
Carbon Diamond, Graphite, Graphene, Fullerenes, Carbon nanotubes
Silicon Amorphous Silicon, Crystalline Silicon
Germanium α-germanium, β-germanium
Tin Grey tin, white tin, rhombic tin, sigma tin
Phosphorus White phosphorus, Red phosphorus, Scarlet phosphorus, Violet phosphorus, Black phosphorus.
Arsenic Yellow arsenic, gray arsenic & Black arsenic
Anitimony Blue-white antimony, Yellow, Black
Oxygen Dioxygen, ozone
Sulphur Rhombus sulphur, monoclinic sulphur
Selenium Red selenium, Gray selenium, Black selenium, Monoclinic selenium
Tellurium Amorphous & Crystalline

Occurrence of Metals

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Occurrence of Metals

In general, pure metals are shiny and malleable, however, most of them are found in nature as compounds with different properties. Metals having least chemical reactivity such as copper, silver, gold and platinum occur in significant amounts as native elements. Reactive metals such as alkali metals usually occurs in their combined state and are extracted using suitable metallurgical process.

Mineral and Ore

A naturally occurring substance obtained by mining which contains the metal in free state or in the form of compounds like oxides, sulphides etc is called a mineral. In most of the minerals, the metal of interest is present only in small amounts and some of them contains a reasonable percentage of metal. For example iron is present in around 800 minerals.

However, some of them such as hematite magnetite etc., containing high percentage of iron are commonly used for the extraction of iron. Such minerals that contains a high percentage of metal, from which it can be extracted conveniently and economically are called ores.

Hence all ores are minerals but all minerals are not ores. Let us consider another example, bauxite and china clay (Al2O3.2SiO2.2H2O). Both are minerals of aluminium. However, aluminium can be commercially extracted from bauxite while extraction from china clay is not a profitable one.

Hence the mineral, bauxite is an ore of aluminium while china clay is not. The extraction of a metal of interest from its ore consists of the following metallurgical processes.

  1. Concentration of the Ore
  2. Extraction of Crude Metal
  3. Ref Ning of Crude Metal

Occurrence of Metals img 1

Table 1.1 List of some metals and their common ores with their chemical formula
Occurrence of Metals img 2

Chemistry | Definition, Topics & History Concepts, Free Resources

Chemistry | Definition, Topics & History Concepts, Free Resources

Metallurgy

P-Block Elements – I

P-Block Elements – II

Transition And Inner Transition Elements

Coordination Chemistry

Solid State

Chemical Kinetics

Basic Concepts of Chemistry and Chemical Calculations 

Quantum Mechanical Model of Atom 

Periodic Classification of Elements 

Hydrogen 

Alkali and Alkaline Earth Metals 

  • s-Block Elements
  • Alkali Metals
  • General Characteristics of the compounds of Alkali Metals
  • Biological Importance of Sodium and Potassium
  • Alkali Earth Metals
  • General Characteristics of the Compounds of the Alkali Earth Metals
  • Biological Importance of Magnesium and Calcium

Gaseous State 

  • Intorduction of Gaseous State
  • The Gas Laws
  • Ideal Gas Equation
  • Mixture of Gases – Dalton’s Law of Partial Pressure
  • Deviation form Ideal Gas Behaviour
  • Pressure-Volume Isotherms of Carbon Dioxide
  • Liquefaction of Gases

Thermo Dynamics 

  • Introduction of Thermodynamics
  • System and Surrounding
  • Zeroth Law of Thermodynamics
  • First Law of Thermodynamics
  • Enthalpy (H)
  • Thermochemical Equations:
  • Measurement of ΔU and ΔH Using Calorimetry
  • Hess’s Law of Constant Heat Summation
  • Latice Energy (ΔHlattice)
  • Second Law of Thermodynamics
  • Gibbs Free Energy
  • Third Law of Thermodynamics

Ionic Equilibrium 

Electro Chemistry 

Surface Chemistry

Hydroxy Compounds and Ethers

Carbonyl Compounds and Carboxylic Acids

Organic Nitrogen Compounds

Biomolecules

Chemistry In Everday Life