Indian miner relies on bucketwheel excavators to mine lignite

By Simon Walker, European Editor

At the beginning of February, India’s biggest lignite miner, Neyveli Lignite Corp. (NLC), took delivery of two new Tenova Takraf bucketwheel excavators (BWEs). Ordered in 2012, the two machines were erected and supplied by Takraf India Ltd., based in Chennai (formerly Madras) in collaboration with its parent company in Germany.

Not only was it the first time in NLC’s nearly 60-year history that two excavators of this size were delivered simultaneously, but the work was completed five months ahead of schedule, Takraf reported. Built with state-of-the-art technology, they replace two BWEs that were supplied by another German manufacturer more than 30 years ago.

Traces of lignite were discovered by chance in the Neyveli district of the state of Tamil Nadu in the 1930s. The deposits were confirmed in the late 1940s, with exploration during the 1950s revealing their extent. India’s lignite resources are now estimated at some 42 billion metric tons (mt), of which around 80% lie within Tamil Nadu.

Today, the company, in which the state still has a 90% holding, operates three mines at Neyveli, with a combined capacity of 28.5 million mtpy. It also has a 2.1-million-mtpy lignite-mining operation at Barsingsar in the northwest Indian state of Rajasthan; both mining centers are linked to power-generation stations that have a total capacity of 2,740 megawatts (MW).

The Neyveli operations consist of Mine I, with a capacity of 10.5 million mtpy of lignite, Mine 1A (3.5 million mt) and Mine II, which has recently been expanded from 10.5 million to 15 million mtpy. Production in the financial year to the end of March 2014 — the last for which figures are publicly available — totaled a little more than 25 million mt (27.6 million tons), involving the stripping of 161.6 million m3 (211 million yd3) of overburden.

 
India’s Lignite Center

NLC was established as a state-owned company in 1956 with the specific aim of working these deposits, which lie around 200 km (125 miles) south of Chennai. Mining began in 1957 using conventional equipment, with continuous mining systems — BWEs, conveyors and spreaders — being introduced in stages between 1958 and 1961. Early BWEs had bucket capacities of 350 and 700 liters (12 and 25 ft3). The initial mine, based on an area holding some 200 million mt (220 million tons) of lignite, was designed for a production rate of 3.5 million mtpy (3.9 million tons), with the main seam running between 10 and 25 m (33-82 ft) thick beneath 50-80 m (165-260 ft) of overburden.

The operation was soon in trouble. After six years, the 3.5-million-mt production target had still not been achieved, the principal cause being the hardness and abrasiveness of the sandstone overburden. According to a paper presented by two authors from NLC at a meeting in the late 2000s, typical strata cutting resistance at Neyveli was 15-20 MPa (2,100-2,800 psi), compared to conditions of 7-10 MPa that were then found in Germany’s lignite mines, and on which the machine designs had been based. Downtime for changing excavator bucket teeth was excessive.

And not only were teeth wearing out in as little as 3.5 hours, but the shock loading from digging this material was causing extensive structural damage to other machine components. NLC was forced to undertake major modifications to the teeth, buckets, booms, slewing mechanisms and drives, among other things. As the authors noted in their paper, the modifications resulted in substantial performance improvements, while “the experience gained helped in arriving at the present set of equipment in which most of the problems have been eliminated.”

 
Expansion From the 1980s

With Mine I established, in 1978 the Indian government gave the go-ahead for NLC to develop Mine II, initially with a capacity of 4.7 million mtpy, but soon increased to 10.5 million mtpy. Although located only 5 km from Mine I, overburden removal for the new operation threw up a different set of challenges, as well as those involving the hard sandstone. Here, the surface alluvium overlying one area of Mine II consists of a sticky clay that expands when it gets wet, becoming soft and plastic. It stuck in the excavator buckets, clogging them and causing extensive soil spillage.

Again, NCL found remedies through lining the buckets and material chutes of machines working in this material with HDPE teflon sheets, and using open-backed buckets with chains to knock the clay free. Mine II began producing lignite in early 1985.

The next stage of expansion came in 1998, with the development of Mine IA at a capacity of 3 million mtpy. Output from this mine feeds a local private-sector company’s power plant, and also helps balance NCL’s feed to its own generating stations. Mine IA came into production in 2003, with a capacity increase to the current 15 million mtpy at Mine II coming on stream in 2010.

As for the future, NCL is currently in the process of updating both its lignite-fueled power plants, with civils works for a new 2×500-MW station to replace the original Thermal Power Station (TPS) I — commissioned in the 1960s — having started last year. Meanwhile, expansion of the second existing station has involved the construction of two 250-MW circulating fluidized-bed units, which came on stream in March — six years behind schedule as a result of boiler technology problems. Completion of these projects will increase its total generating capacity at Neyveli to 3,390 MW, to which the company is adding 10 MW through the construction of a solar power plant.

In terms of fuel, the previously completed Mine II expansion was designed to supply the additional requirements for TPS II, while a restructuring of operations in Mines I and IA will provide an additional 1.5 million mtpy for the new TPS I. Capacity at Mine I is being cut to 8 million mtpy, while that of Mine IA is being increased to 7 million mtpy, at a cost of some $250 million.

Neyveli Lignite operates 29 bucketwheel excavators in India.

 
The Mining System

According to a presentation made by NLC’s chairman and managing director, Shri B. Surender Mohan, at a workshop on technology development and the mechanization of mines, held in New Delhi in January, the company currently operates 29 BWEs. Of these, 13 are fitted with 1,400-liter (50 ft3) buckets, with the remainder at 700 liters. At the other end of the overburden-removal system, there are four 20,000-mtph (22,000 tph) spreaders, seven capable of handling 11,000 mtph and four rated at 5,000 mtph.

Mohan described the working system used at Mine II, where five overburden stripping benches have a total capacity of 78 million m3py (102 million yd3). The top four benches are each 25 m (82 ft) high, with the lowest overburden bench at 18 m (59 ft). In sequence from the top, the first three benches are each equipped with two 1,400-liter bridge-type BWEs jointly feeding a 20,000-mtph spreader. The bench below that has a single 1,400-liter machine that feeds an 11,000-mth spreader, while the bottom bench uses two 700-liter BWEs feeding another 11,000-mtph spreader.

Lignite seam mining, using an 18 m-high bench, is carried out using four, 700-liter excavators. In his presentation, Mohan explained that a 1,400-liter BWE working 4,000 hours a year has the capacity to move 9 million m3 of overburden — a volume that would require a fleet of 26, 10-12 m3 mining shovels and 230, 100-mt-capacity haul trucks to accomplish.

Not only does NLC face challenges with the hard, abrasive overburden, but also with materials handling and water control. The stripping ratio is high, at around 6 m3 of waste per metric ton of lignite, while the presence of a pressurized artesian aquifer beneath the lignite seam means that the operation has to pump 8-10 m3 (2,100-2,650 gallons) of water per ton of fuel mined. With the aquifer exerting an upward thrust of 5-8 kg/cm2 (70-110 psi), depressurization through permanent pumping is the only way of preventing the seam from heaving once overburden has been stripped, Mohan told the meeting.

And in terms of overcoming the hard sandstone, NCL now drills and blasts each bench before stripping, using around 7,300 mtpy of explosives — mainly site-mixed emulsion — to fragment the rock and reduce the potential for damage to the bucketwheels. Overburden blasting has led to a remarkable improvement in production, Mohan stated.

 
Updating the Excavator Fleet

According to Tenova Takraf, a total of some 32 BWEs have been supplied to NLC since operations began at Neyveli, with almost all of them having come from various German companies. Some have worked for an impressively long time, an example being a machine that NLC recently renovated at a cost of some $7 million. Originally supplied by Buckau Wolf and commissioned in 1983, the 700-liter machine had completed more than 124,000 hours in Mine II. Including a major update of its operating systems, the renovation work was designed to give the machine a further 15 years of life.

Meanwhile, Tenova Takraf received its order for the two new 1,400-liter BWEs in July 2012. The erection site was officially inaugurated, including a traditional blessing, a year later, and Surender Mohan performed the handover ceremony on February 5.

Standing 37 m tall, each machine weighs some 2,600 mt, and is equipped with 34-m-long (110 ft) bucketwheel and discharge booms. The wheel itself, 8.2 m (27 ft) in diameter, carries 14 buckets, giving a maximum digging capacity of up to 6,800 m3ph.

Using an 11,000-volt power supply, the machines are carried on six crawler units that give it a tramming speed of 8 m per minute. Two 750-kW motors power the bucket-wheel drive, while the hoist winch has 2×220-kW drive motors. The entire superstructure can rotate through 180° in either direction, Tenova Takraf said, while the machine has a maximum digging height capability of 30 m (98 ft).

 
NLC’s Future

While NLC maintains it main operations at Neyveli, the company is looking to expand in other directions, in other parts of India and overseas. Following the opening of its Barsingar mine and 250-MW captive power plant in Rajasthan in 2009, it has plans for a further three mines in the state, adding 4.75 million mtpy in capacity. In Tamil Nadu, it is working on two hard coal-based power projects, based on both local and imported coal. Elsewhere in India, it is looking at developing hard coal mines in the states of Odisha and Uttar Pradesh in joint venture with other organizations — projects that could also involve the construction of new thermal power plants — and it is investigating opportunities for mining coal outside of India.

In a national media interview earlier this year, Mohan noted that the Neyveli mines achieved their highest output on record in 2014-2015, at 26.5 million mt (29.2 million tons), with a target of 27 million mt for this financial year. However, he warned that actual output will be dependent on the power market in the five southern Indian states that the operation’s power stations supply, with recent off-take cutbacks having led to nighttime output reductions.

Whatever the outcome, India’s highest-capacity surface mines will continue to produce fuel for this part of the country, with the new machines from Tenova Takraf playing an important role in keeping the rock on the move.