BALCO has two captive bauxite mines, namely, the Mainpat bauxite mine and the Bodai-Daldali bauxite mine, in the state of Chhattisgarh in central India.

Bodai-Daldali.
Regional geology.
The area constitutes central part of the extensive laterite-capped plateau of the Maikal Range hills. In plains to the north-western part of the area around Mawai and Kathaitola, gneissose and schistose rocks form the core of the range, whereas around Kukrapani and Taregaon in the south-east, phyllite, slates and quartzites belonging to Chilphi Group are exposed. In the lower part of the plateau, narrow patches of Lametas, represented by calcareous gritty sandstones and pink & white clays are seen in the Kanai river section at Kukri. These are overlain by horizontally disposed basaltic lawa flows of Deccan Trap. Good exposures of Deccan Trap are noticed along the Kukrapani Daldali ghat section. The traps are capped by about 25 m thick laterite. Bauxite is normally found within upper part of the laterite capping.

The Bodai-Daldali deposits are located approximately 260 km from Korba in the Kawardha district of the state of Chhattisgarh.

The Chhattisgarh bauxite deposits are situated over a plateau with steep scarps on both sides, at an elevation of approximately approximately 940 metres above the surrounding land, for Bodai-Daldali. Bauxite is generally one metre to three metres thick and lies within a laterite sequence overlying thick tertiary basalts of the Deccan Traps. The cover of laterite and thin top soil is up to five metres thick but is generally less than two metres. Bauxite outcrops around much of the plateau rims.

A typical profile of the Chhattisgarh deposits comprises topsoil and soft overburden above the laterite. The upper laterite consists of hard, lose or indurated bauxite pebbles and boulders with a clear contact with the underlying hard bauxites. The bauxite occurs in discontinuous lenses up to four metres in thickness with laterite infilling joints and fractures with the bauxite. The contact with the softer lower laterite is usually gradational and irregular.

The bauxite is hard with a natural moisture content of 5.0% to 10.0%, with an in-situ density of 2.4 tonnes per metre to 2.4 tonnes per metre3. It comprises primarily gibbsite with boehmite and minor diaspore. The reactive silica content is low and iron is present in the form of hematite and aluminous goethite.

Mainpat.
Regional geology.
Areas are mostly covered by soil or laterite / bauxite capping. The low plateau consists of rocks of Achaean age. These include mica schist and quartzites, calc silicate rocks, amphibolites, granite and granite gneiss, pegmatite’s and quartz veins. Immediately overlying the Achaeans with an unconformity occur over rocks of Gondwana age consisting of coarse ferruginous sandstone and conglomerate. The Gondwana’s are overlain by Lameta with an unconformity. The Lameta starts with conglomerate at the base and other rock units include ferruginous grits, sometimes with shaly & clayey partings and calcareous grits. Overlying the Lameta occur almost horizontal Deccan Trap basalts which are massive, dark grey, hard and compact, finally overlying this trap rock occurs laterite with lithomarge clays and weathered basalts in between.

Local Geology.
The bauxite occurs in lensoid form within the laterite horizon which comprises various recognizable lithologic units.

The zone of lithomarge is quite distinctive and well defined and forms the lowest horizon of the laterite profile. It occurs just above the trap and thus separates it at few places in deep nalla beds and also along the scarps. The lithomarge is quite soft and plastic in nature. The average thickness of this zone is varies from 1.5 to 3.5 m.

The bauxite occurs in lensoid forms in laterite profile, generally below soil and pisolitic laterite. The bauxite can be put into two categories, i.e. pisolitic bauxite and massive bauxite. The pisolitic bauxite consists of closely packed pisolites ranging from few mm to 5 cm. It occurs mostly in upper horizon of the laterite above bauxite zone. The main mineral constituent is Gibbsite. These ore bodies persist up to maximum depth of 10 m. from the surface. The average thickness of bauxite deposit varies from 0.4 m. to 4.0 m. in the form of lenses. The intensity of bauxite is more in central part of the plateau and decreases towards the scraps. The upper contact of bauxite with overlying pisolitic laterite is quite sharp where as its lower contact is gradational.

BALCO has two captive bauxite mines, namely, the Mainpat bauxite mine and the Bodai-Daldali bauxite mine.

All mining and transportation at both mines are undertaken by contractors. One thin top soil layer is removed by an excavator and is either transported to an adjacent storage point or an area that is being backfilled. The laterite layer is drilled and blasted. The overburden is then removed by backhoe excavators and 15 tonne dumpers. Broken ore is hand-sorted, leaving waste material behind. Ore productivity is around 2 to 3 tonnes per person per day in the dry season which decreases to 1.25 to 1.75 tonnes per person per day in the wet season.

The bauxite extraction limit for Mainpat mine granted by MoEF is 750,000 tpa. The bauxite extraction limit for Bodai-Daldali Mines granted by MoEF is 1,250,000 tpa.

The bauxite deposits of Mainpat and Bodai- Daldali mines are situated over the plateau and the surface terrain is generally flat.

As the Bauxite deposits are found in shallow depth ranging from 0.40-10.0 meter, the method of mining of Bauxite is mechanized opencast mining, crushing, sizing, and sorting, by using excavator of 1- 5 Cum bucket size, dumper of 15-25 tons capacity, water sprinkler, blast hole drill machine, 100 mm dia, dozer, grader, crusher, screens and belt conveyor etc.

The over burden is top soil & soft laterite of 0 to 3.00 meters thickness after which there is a hard overburden of thickness varying from 0 to 5.70 meters. The thickness of ore zone is 0.40 m to 4 m. In general working is of shallow depth of avg. 4 to 5 m (maximum 10 m).

The general practice is to scrap the top soil separately by dozer or excavator dumper combination and use it for directly spreading over already backfilled compacted area. Drilling and blasting is practiced in hard OB benches and ore benches. Excavation is done by excavator dumper combination.

Since the deposits are strata-bound irregular and bouldery in nature, bauxite needs to be separated from laterite and murrum. It is proposed to mechanically crush and screen the ore so as to obtain desired grade bauxite with acceptable silica content.

The backfilling is done in same sequence as it is found in the nature, i.e. hard OB in bottom, then soft OB above the hard OB, then top soil. The height of OB benches is kept maximum of 6 meters and the ore benches are kept generally of 1 to 4 meters of height as per thickness of ore body.

The blast boreholes of 2 to 4 m depth are being drilled by DTH drills, which are required for blasting. Controlled blast technique is used for keeping the ground vibration, blast through and noise within permissible limit. The blast design and charge per delay are kept strictly as per the requirement of DGMS.

For getting the bauxite of desired grade and size, from blasted ore muck, it is subjected to mineral dressing, and this will be carried out by two ways.

* Where the deposits is homogeneous in quality, then the ROM will be processed, through mechanized way of mineral dressing by crusher and screening plant sets
* And, where deposit is of heterogeneous w.r.t. Quality, then the ROM will be subjected to manual method of sorting and sizing.

The sized and sorted bauxite found after crushing /screening or manual sorting is stored and analyzed for its quality before dispatch to plant by truck.