Longwall mining at La Esmeralda, located in Northern Mexico began in September 1999. The mine is owned by Minerales Monclova S.A. de C.V. (MIMOSA) which operates three underground mines, in the gassy Olmos Coals of the Sabinas Basin.

The three underground longwall mines operated by MIMOSA exploit coals of the Upper Cretaceous Los Olmos Formation in the state of Coahuila in northern Mexico. This region contains Mexico’s largest coal reserve, 311.391 million tons, of which an estimated 273.808 million tones is recoverable.

The coals are medium to high volatile in rank, and supply related steel making operations in the city of Monclova, and in the last five years, steam coal for the power plant has generated 8% of Mexico’s total electricity.

The resource is located at a depth of 180m to 350m. The Los Olmos formation contains two distinct coal seams (locally known as the Double seam) that are mined commercially where the rock parting between them is between 0.1m and 0.2m, for a combined thickness of approximately 2.5m.

High levels of gas have been a major technical challenge at the operations and MIMOSA operates horizontal in-seam boreholes in three of its mines, has gob wells in Esmeralda mine, and plans on drilling in two mines in this year.

The La Esmeralda mine started its first longwall panel in September 1999 and almost immediately ran into problems with unanticipated high gas levels. MIMOSA’s previous four mines had gas problems in the development sections but a gob well program had to be introduced in the first panel when increased air volumes could not dilute the methane sufficiently.

The MIMOSA degas engineer Mario Santillan, had to implement a new way to minimize the methane gas concentration underground to keep the methane gas concentration below 1.5 % in the return entry and bring safety conditions to the equipment and workers.

“The solution was a gob well drilled from the surface,” said Rogelio Hernandez Cuevas - operation general manager of MIMOSA mines.

Four gob well were drilled at the mine during the studies at an average depth of around 262.25 meters generating 252 litres per second (lps) of high quality gas. Operational delays due to methane gas concentration in the ventilation circuit were reduced by 48%. With help from the Coalbed Methane Program of US Environment Protection Agency, the mine developed a project to use the methane to heat water.

The current status of the degasification project is the following. Horizontal bore holes averaging 300 meters in depth are being drilled at Esmeralda and at mine 3 also with an average depth of 200 meters. A program of horizontal drilling began in February at mine 6, and mine 7 in the Saltillito Basin.

A planned vertical borehole to collect the gas from the horizontal boreholes will be implemented by June and as soon as the surface facilities (vacuum pumps) are completed drilling of in-seam boreholes will start.

Santillan said Esmeralda plans to purchase a drill machine and survey equipment so that drilling of long holes using a directional drilling system, can be done in-house to reduce costs.

“Our staff will be able to drill long holes to protect at least four panels, three year in advance,” he said.

A gob well program is planned for 2004 at Mines III, VI and at Esmeralda, with three gob wells per longwall panel. A pilot project to produce 1 MW of electricity is planned for one mine, and if successful will be introduced to the rest of the mines.

Santillan said the operational benefits of the horizontal drilling program are very clear. Gas emission in development sections measure 230 lps with 0.6 % CH4 in the return air, very low compared with levels before the degasification program.

Monitoring of the methane gas levels by the safety department is being undertaken in each development section with close analysis by Armando Diaz - MIMOSA safety manager. With the proper application of the horizonal and gob well programs at every mine, Santillan said adequate procedures were in place to prevent a methane related accident.

Esmeralda is noteworthy for having been the first coal mine in Mexico that was recovered and put back in operation after a mine fire started in the development section in May, 1998.

Mine workers were involved in the mine recovery which entailed sealing the vertical ventilation shaft and pumping water into the mine to reduce any probability of fire during the recovery process.

During May to August 1998, the command center took gas samples from the mine and with the help of a chromatograph analyzed the gas content of the environment underground. During this period management developed a training program for rescue teams who had no previous experience of mine recovery. Santillan said the most important thing was the recovery of the mine without any injuries.

Turning to the longwall face itself. (See related Table

for equipment details.) The seam height is roughly 2.5m and coal is cut at a height of 2.2m average. Meters retreat on longwall per month averages 120m for average daily tonnage of 5000 tons.

The roof supports are 500 ton DBT shields with a working range of 1.4m- 2.7m. The 4LS10 Joy shearer has a drum diameter of 1,400 mm and installed power of 4160 volts.

The face conveyor is an MTA PF4-932-AI1505 with a chain size of 34 mm, powered by 375 kW Morley motors. The BSL is also MTA TR-015 and the crusher is DBT supplied.

In the 2002/03 year the mine produced 2.329 million tons and the mine is budgeted to produce 1.737 million tons in 2003/04.

Development roadways at the mine are driven in two sections using 3 IBS and 4 Alpine Miners 4 Joy shuttle cars. Roadways are 4 m wide, with 70 x 70m meters wide pillars. Roadways are supported with 4 x 3m arches and 5.1 x 3.575m arches in main entries and bleeders. Wooden posts and I-beams are also used in longwall development entries. Metres achieved per month averages 850 – 900m.

Esmeralda employs 584 workers and 472 of them work underground. Twenty workers man the longwall face each shift and the mine works 3 x 8 hour shifts per day. One 8 hour maintenance shift is worked per week, serviced by up to 12 people.

Recently Jose Guadalupe Duron Candelas, operation GAN director approved the introduction of ten hour shifts in mine seven. If successful, ten hour shifts will be introduced in the other mines.