(a) Give an account of external changes in flora due to the presence of anomalous concentration of base metal in a terrain. (20…

(a) External changes in flora over anomalous base-metal concentrations

Geobotany is the use of plants in exploration. Soil over or near a base-metal deposit holds an abnormal amount of Cu, Zn, Pb, Ni or Co, and this changes the vegetation in ways that can be seen without analysis.

Causes. Plants take up metals from the soil solution through their roots. In excess, the metals inhibit enzymes, upset the balance of Fe, Mn and Mg, damage membranes and lower chlorophyll production and root growth. Tolerant species survive and sensitive ones disappear.

External changes.

(i) Chlorosis, yellowing of leaves from loss of chlorophyll, and necrotic patches.

(ii) Stunting and dwarfing, with small leaves, short stems and stunted roots with few root hairs.

(iii) Abnormal colour and shape of leaves and flowers, and altered flowering time.

(iv) Change of plant community. Normal vegetation is replaced by a sparse, stressed cover, or by bare patches, or by metal-tolerant species that occur only on mineralised ground (metallophytes).

(v) Indicator plants. Indicator species grow only on mineralised ground. Universal indicators are found anywhere; local ones in one region. Examples: the copper flower Becium homblei (Ocimum homblei) and Haumaniastrum of the Zambia–Katanga copper belt, Viola calaminaria and Thlaspi caerulescens for zinc, and Alyssum for nickel, where it is a hyperaccumulator.

Use and limits. Zones of vegetation are mapped along traverses and from aerial photographs, and remote sensing shows stress as a blue shift of the red edge of the reflectance curve. The results are checked by soil and plant geochemistry (biogeochemistry). Soil pH, rainfall, season, drought, salinity and fungal disease also produce similar symptoms, and deep cover or leached soil can hide anomalies.

In India, geobotanical and biogeochemical surveys have been used in the Khetri copper belt of Rajasthan and in Singhbhum.

(b) Flotation as a beneficiation method

Principle. Froth flotation separates minerals by the difference in wettability of their surfaces. The ore is ground to liberate the minerals, and mixed with water into a pulp. Air is blown through the pulp. Valuable minerals, made water-repellent (hydrophobic) by reagents, attach to air bubbles and float as froth, while hydrophilic gangue stays in the pulp and sinks.

Reagents.

(i) Collectors make the mineral surface hydrophobic, such as xanthates for sulphides and fatty acids for oxides.

(ii) Frothers make a stable froth by lowering the surface tension of water: pine oil, cresylic acid and methyl isobutyl carbinol (MIBC).

(iii) Modifiers: activators such as copper sulphate for sphalerite; depressants such as cyanide, lime and sodium silicate for pyrite and gangue; pH regulators such as lime and soda ash.

Parameters. Particle size (fine enough to liberate, but not sludge), pulp density (about 25–40% solids), pH, type and dose of reagents, conditioning time, aeration rate and bubble size, agitation, and temperature.

Frothing. The frother is added at the end of conditioning. The froth is skimmed off as concentrate, in rougher, cleaner and scavenger stages. In differential flotation of complex Pb–Zn ore, as at Rampura Agucha, lead is floated first, with zinc depressed, and zinc is then activated by copper sulphate and floated. Copper sulphide at Malanjkhand is concentrated by flotation.

(c) Industrial minerals and five examples

Industrial minerals are non-metallic, non-fuel minerals and rocks used in industry for their physical or chemical properties, such as hardness, softness, inertness, heat resistance or chemical content, and not for extraction of a metal.

(i) Limestone, CaCO₃, from Madhya Pradesh, Rajasthan and Andhra Pradesh. It is used in cement, flux in steel and in chemicals.

(ii) Gypsum, CaSO₄·2H₂O, mainly from Rajasthan. It is used in cement as a retarder, plaster of Paris and fertiliser.

(iii) Mica (muscovite), KAl₂(AlSi₃O₁₀)(OH)₂, from Jharkhand, Bihar and Andhra Pradesh. It is used in electrical insulation and electronics.

(iv) Barytes, BaSO₄, from Mangampet in Andhra Pradesh. It is used in drilling mud and paints, for its high density.

(v) Magnesite, MgCO₃, from Almora and Salem. It is used in refractories for furnaces, and in chemicals.