Published in the December 2004 American Longwall Magazine

 

Solvay is one of only four such mines in the world. The state of WY contains the world’s largest known resource of natural sodium carbonate in 22 beds of natural trona in the Green River Basin. The state generates about 90% of all soda ash produced in the US.

 

Solvay mine owner, Solvay Chemicals, is a major producer of soda ash from trona. The project consists of an underground operation and the Green River plant which is among the most technologically advanced in the world. Other products manufactured at the site include sodium sulfite, sodium metabisulfite and sodium bisulfite.

 

Other than the Solvay operation, neighboring FMC runs a longwall to mine trona while the other two major trona producers, General Chemical and OCI, use continuous miners and borers. Collectively, these four companies produced 18 million tons of trona in 2002.

 

The Solvay minesite controls 62 square miles of sodium leases with mining activities focused on Trona Bed No 17. This is the best quality seam and is 12ft thick under a depth of 1500-1700ft.

 

The relatively small demand and low value of trona (compared with coal) made it difficult to justify the capital investment to buy a new longwall system. However, Solvay was able to add a longwall to its existing room and pillar operations in 2003 after it acquired a second-hand system, vintage 1996, from Canadian operation Smoky River.

 

Prior to being installed at the mine the two-leg Glinik roof supports were rebuilt by DBT and fitted with a PM4 control system. These supports are serviced by Kamat emulsion pumps. The DBT panline is a 950mm wide, PF4 model.

 

The DBT EL3000 is a dual ranging arm machine with 1200hp cutter motors, incorporating some special feature to overcome the hard cutting conditions associated with mining trona.

 

The compressive strength of trona deposits is low but the tensile strength is high, making it tough to cut. Slabbing can occur placing heavy demands on the shearer. To accommodate this, the DBT EL3000 shearer has some added features - high installed haulage and cutter power, and high mass to reduce vibration and increase stability.

 

The rams are mounted on top of the shearer, avoiding the potential danger which can arise when they are located on the armored face conveyor. The shearer mainframe is made from 100mm thick plate (versus the standard 60mm thick) which increases the shearer mass to 100 tons to maximize stability.

 

To facilitate delivery into the mine, the mainframe was manufactured in two parts and connected by a bolted and dowelled joint. Finally, in order to reduce cutting/loading power and maximize reliability, the shearer was supplied to operate without cowls.

 

Trona has one big advantage - being a natural lubricant, it creates less wear on major components, significantly increasing the life of components such as panlines and gearboxes.

 

The shearer can achieve speeds of up 70ft per minute, cutting uni-di, half web. Typically up to 18in of trona is left in the roof.

 

Despite the fact the longwall equipment wasn’t specified for the mine it has performed well to date.

 

“We’ve had a few mechanical issues but the biggest issue has been training people who were not conversant with longwall mining,” mine manager Hilton Tregilgas said. “There were only about six people on site who had any experience with a longwall, though it’s been a good learning experience for all of us.”

 

The mine’s first longwall move had been a highlight, Tregilgas said. Solvay owns about three quarters of another face conveyor and three quarters of a stageloader and crusher, as well as a spare set of cables and hoses. These were pre-installed ahead of the main move.

 

“The move was really good – we fully expected to do it in 21 days but we achieved it in 14 days. For a bunch of guys who were not experienced it went very well.

 

“Mike Montoya, our technical superintendent did an excellent job. He made little wooden models and showed how to pull the shields, where you pick them up, where the parts of the machinery goes. I’d advise anyone who hasn’t done a longwall move before or wants to refresh staff on the process, to look to do that.”

 

Logistically the mine tries to plan longwall moves to coincide with plant shutdowns, but it only has a 90,000t stockpile available so getting the timing right is crucial.

 

The mine works on two 12-hour shifts and a six-hour maintenance shift worked seven days a week.

 

Solvay produces around 12,000t per day of ore with around 2.65Mt of product produced every year. For every 1.65t of trona mined one ton of soda ash is produced. Roughly half the mine’s output comes from its room and pillar operations.

 

An interesting feature of mining operations at Solvay is the reclamation of about 300,000t per annum of trona suspended in solution. This is achieved by drilling water into the lowest part of the mine which is later pumped out and dried so the trona can be harvested.

 

The mine is serviced by two production shafts. The 26ft south shaft has a compartment for trona skips and one for the men and material cage. This also provides intake air on the men/material side and air is heated in winter by steam from the processing plant.

 

The two production hoists are each rated at 720t per hour and have two 20t skips delivering ore to the surface. The entire system has an annual capacity of about 8Mt though the mine produces about 4.5Mtpa.

 

The shaft access has posed special challenges for equipment maintenance – Tregilgas said once equipment went underground it rarely came back up again. Eight full-time mechanics service and maintain the mine’s fleet of equipment in a well-equipped underground workshop.

 

The mine uses eight Marietta borers for development and one 12CM15 continuous miner for construction, supported by 14 10SC22 shuttle cars.

 

During September one of the Marietta borers on the workshop floor, originally brought underground in 1979, was undergoing its sixth rebuild. Amazingly, this piece of machinery had produced over 10Mt.

 

The mine is also using an old Klockner-Becorit continuous haulage machine, delivered to Solvay in 1988. Tregilgas had this rebuilt after it was scrapped and every 2Mt it is subject to a full overhaul.

 

Tregilgas said the mine’s strategic development formula was to mine 17.5% development and 2% mainline development annually. Gateroads are typically 9ft high by 15ft wide with minimal bolting (5ft roof bolts on 4ft centers) required to support the strata. Longwall gateroads are supported by 14ft cable bolts.

 

The ore itself contains no gas but oil shale above and below the seam seeps gas into the workings. Gas liberated when the floor breaks is kept captive in the gob.