By Lee Buchsbaum
Nationwide, North American Coal Corp., a subsidiary of NACCO Industries, Inc., mines and markets lignite primarily as fuel for power generation and provides selected value-added mining services for other natural resources companies. Corporate headquarters are located in Dallas, Texas. North American Coal operates six lignite mines in North Dakota, Louisiana, Mississippi, and Texas. All together, NACoal produces more than 33 million tons of lignite per year.
NACoal’s Sabine Mining Co. operates the South Hallsville surface mine approximately 150 miles east of Dallas. Sabine generally produces more than 4 million tons annually, depending on the demands of its primary customer, American Electric Power’s 650 megawatt Henry W. Pirkey Power Plant. Initially built by SWEPCO and located adjacent to the mine, the vast majority of Sabine’s coal is shipped to this pulverized coal power plant in a classic mine mouth operation. Sabine also produces some tonnage annually which SWEPCO sells to a second customer, Norit Americas.
One of the common characteristics of all of NACoal’s mines is their expertise with dragline operations. Sabine is no different. Operating four draglines of varying sizes to remove varying amounts of over and interburden presents tremendous scheduling challenges, particularly with multiple, splitting seams and in the wet weather conditions typical of the Gulf region.
Over the years, the mine has migrated from producing in areas of relatively shallow cover to various deeper cover regions. As it has done so, it has been forced to move ever increasing volumes of overburden and develop new site specific mining techniques. Several years into a new contract with AEP and with Norit requesting additional tonnage, ensuring that each piece of mobile equipment is in place and each operator understands how to safely perform their vital role has become the key to Sabine’s continued success.
History of the Sabine Mine
Development of the mine began in 1981 after North American and SWEPCO agreed to terms. Mine construction commenced the following year and in 1983, Sabine began erecting its first dragline. Initial lignite shipments began in 1984. Since then, Sabine has gone through a pair of contract extensions, the most recent in 2008. All together, through the end of 2009, Sabine has delivered more than 87.8 million tons of lignite to its customers and has moved more than 1 billion cubic yards (cu yd) of overburden, most of that by its four draglines.
In east Texas where the Sabine mine is located, the terrain is fairly rolling. However, the coal has its own topography and does not necessarily follow what’s above it. As the miners chase those undulating and splitting seams, they revise their plans and techniques accordingly.
Throughout the 25-year life of the mine, Sabine has produced from several mine areas. Though South Hallsville was the initial mine area, Sabine’s current active area is South Marshall. Sabine terms its active areas M and Q, with reclamation activities on-going in area S.
The mine employs roughly 256 workers, 50 on the staff, 141 in operations, and 65 in maintenance. It also has a variety of contractors doing reclamation work and often employ several temporary employees depending on the mine’s needs.
Though Sabine generally produces more than 4 million tons per year, it is captive to the needs of its primary customer, and its annual production reflects Pirkey’s major outage schedule. Though 2009’s production was less than average, Sabine and AEP forecast that 2010-2011 should be back to normal with production well above 4 million tons.
On average then, the annualized needs of Sabine’s customer translates to roughly 400,000 delivered tons per month. It’s up to the mine’s management and staff to be prepared to meet those requirements.
Reserves and Coal Quality
AEP controls the reserves that Sabine mines and a total of 175 million tons remain with an average effective strip ratio of 14.6:1. Sabine produces from multiple seams, generally three to four per pit, but has and will mine over seven seams at once. When blended, the coal has a calorific value that averages 6,400 Btu/lb with 16.5% ash, depending on the seam. More recently Sabine has been mining a slightly higher quality coal, more in a 6,550 to 6,600 Btu/lb range with roughly 13%-14% ash.
Similar to most lignite deposits, Sabine’s product is roughly 33% moisture but remains low in sulfur. Higher moisture levels generally prevent lignite from traveling long distances. As a leading producer of lignite, NACoal historically has been involved in several pilot projects to enhance lignite’s viability and market share. NACoal remains a driving force behind efforts to dry lignite coals and increase its use.
Going by Colors Not Numbers: Individual Coal Seams
Currently Sabine is mining seven seams from four pits. Moving through the seams, the first one they encounter is the Brown Seam that typically is about 1.5 feet in Area M and roughly 4 feet in area Q. The Red Seam is next and it is the primary seam in Area M. It will generally split into the Red Upper and the Red Lower, with both seams being approximately 3 feet in thickness. In the Q area, the Red remains one 3.5 foot seam.
Below the Red Seams, Sabine also mines the AY seam, which stands for “Almost Yellow.” AY is generally above the Yellow Seam. In places, Sabine also mines the Tan Seam which is above the Brown where it exists.
Commonly, one or more of these seams will split further. “We know when they’re going to split and when they’re not. We have a pretty good model. Most of our pits are about 5,000 to 6,000 ft long, and currently each dragline is operating in its own pit,” said Andy Hawbaker, maintenance manager, Sabine Mining.
With multiple splitting seams, the challenge is managing the amount of overburden and interburden needed to be moved to reach the targeted seams. Sometimes the interbuden is down to inches.” To maintain a proper blend, we have to be careful of what we load and when. The difficulty is cycling the draglines properly as they are supposed to handle nearly all of the over and interburden,” said Hawbaker.
The Mining Process
Long before it can begin moving the overburden to mine the coal, Sabine has to clear the land of brush and trees and preserve and protect the top 4.5 feet of oxidized soil. “We work hard every day to ensure that we are following the rules regarding the environment. We pride ourselves on being respectful to this land, and we make sure that it is put it back in as good, if not better, when we are finished mining,” said Hawbaker. Sabine works to comply with both OSM and the Texas Railroad Commision standards. They stockpile or direct respread this reddish soil that will eventually formulate the top layer of a reclaimed pit. Once the oxidized material is removed, Sabine prepares a bench and one of their draglines will remove the rest of the overburden.
Sabine uses several different mining techniques. “Our favorite is to simply dig and side cast with a dragline. I wish we had conditions that allowed for more of that,” said Hawbaker.
In areas where Sabine has multiple seams or a deeper overburden it generally employ a two-pass method where the dragline on the highwall side will dig a wedge of dirt out of the key cut, cast it in the spoil, and build a new spoil bench. The dragline on the spoil side will then remove the remaining overburden, uncovering the first coal seam. As that coal is removed, the dragline will make another pass along that spoil bench and remove the interburden. “We’ll do that for however many seams are required,” said Hawbaker. The dragline on the highwall basically digs a trench to build the spoil bench and the dragline on the spoil side removes the remaining overburden.
Sabine employs two Bucyrus 1570 draglines, each equipped with a 93-cu-yd bucket. It also has a Marion 8200 dragline and a smaller 736 Page. The 8200 is equipped with a 75-cu-yd bucket and the Page has a 21-cu-yd bucket.
All four draglines run on a 24/7 schedule. The maintenance and dragline shop also run 24/7. Coal loading is also a round the clock operation. Each dragline is worked by three people, an operator, oiler and a groundsman.
It’s All About Working the Angles
As Hawbaker explains, the functionality of a dragline is determined by the angles it is able to work. “You have the swing angles—which are horizontal and you have vertical angles. The vertical angles affect how far out you can reach. This in turn determines how far from the dragline your pile of dirt can be. The further out you can reach, the more dirt you can pile on. The further out you can put your spoil, the more you can utilize the dragline,” said Hawbaker.
Using these principles, Sabine has engineered a scenario where the draglines can handle up to 120 ft of overburden instead of the usual 90 ft. “We added 30 ft of depth to what the draglines are able to move at significantly less cost,” said Hawbaker.
To move 120-ft of overburden with a dragline, without pre-stripping, the dragline would be forced into some sort of an “extended bench.” This is a commonly used method where the bench material can be placed by a cast blast and is quite economical. Sabine’s overburden does not cast. As a sandy clay, the dirt simply does not move, and the draglines are able to free-dig the overburden, so blasting is not required anyway. To build an extended bench with the dragline requires a significant amount of dragline rehandle. “If I fill the pit up with dirt that’s level across, I have to move so much of it twice. I have to pick it up from the face, dump it along the highwall, then pick it up again to actually get rid of it,” said Hawbaker.
The system Sabine’s engineering team developed virtually eliminates that amount of re-handling. Compared to other methods that Sabine investigated, “we’ve probably reduced the built-in rehandle from 50% to less than 10%,” said Hawbaker.
Most overburden removal falls to the three big draglines. But the Page generally operates in its own little area. In the near term future, as production shifts to a a newer mine area, it will be used to assist with the interburden removal between the coal seams.
Coal Loading and Haulage
Once the coal is uncovered, Sabine uses a unique loading system. Using one of several Huron Manufacturing Easi-Miner 1224s and paced by a Kress haul truck, Sabine loads coal on the fly. The HMC Easi-Miner looks like a modified highway milling machine but operates as a continuous surface miner. A massive 7-ft, 4-in. diameter cutting head sizes coal so effectively that the need for a primary crusher is eliminated. “As the cutterhead moves downward, it shatters the coal and places it on a belt. This initial belt feeds to a second that hauls and dumps it into the trailing Kress truck,” Hawbaker said. In optimal conditions, the Easi-Miner is able to load at a rate of nearly 3,000 tons per hour.
When loading, a Kress truck will trail slowly behind the Easi-Miner, speeding up to make several passes as indicated to allow for more even loading. Once full, the Kress operator will drive between 7 and 13 miles to the coal hopper feeding the Pirkey power Plant.
In total, Sabine has three Kress trucks and is running three smaller tractor-trailer combinations. The latter of these are being phased-out as the mine is moving toward a total reliance on Kress trucks by 2012. “They really suit our needs. They’re very fast, mobile, and operator friendly. And with our long hauls over relatively level ground, and without the steep ramps typical of other coal producing regions, they simply haul the most tons at the fastest speed. Out here where the cover is in the 100- to 140-ft range, you can build ramps that are within their tolerance allowing them to thrive,” said Hawbaker.
In addition to the Kress haul trucks, the mine has several Komatsu end dump trucks used primarily for prestripping and interburden removal.
Draglines do the Heavy Lifting
Sabine’s draglines are vital to the mine’s successful operations. Interestingly enough in 2009, out of a production run of 4 million tons, each dragline was roughly responsible for uncovering an estimated 1 million tons. “That’s purely a definition of ratio,” said Hawbaker.
The “newest” of the three large draglines, the Marion 8200 was brought on-line as stripping ratios went up and extra stripping capacity was needed. Prior to purchasing the dragline, management considered using a large electric shovel/trucks combination, an option that was later dismissed. “Conditions here simply don’t favor that sort of operation. We get upward of 50 inches of rain a year which makes our roads muddy and difficult to traverse. Using draglines, we don’t have to muck the mud off so trucks can drive on it. The draglines are able to safely operate round the clock in wet conditions and are nearly impervious to the mud which would hobble a fleet of trucks,” said Hawbaker.
While determining which overburden removal method to implement, NACoal located a never assembled dragline which they had shipped and assembled on site. It’s a brand new 30-year-old machine. Sabine assembled and rebuilt its never-been-used 8200 in 13 to 14 months, upgrading its components and ensuring it will be viable for decades to come.
Excelling at Communication
Like many mines, Sabine is a self-contained entity, doing its own engineering on property. “The engineer that’s doing the 10-year plan is also doing the five year plan and next year’s plan. The production planner is within steps of him so he can go up there and ask him ‘What is the thinking behind a certain step.’ That kind of immediacy is vital to the success of a complex operation like this,” said Chris Taylor, area manager for overburden removal, Sabine Mining.
To keep the draglines properly coordinated, Sabine breaks down the mine and looks at each pit individually to figure out what’s working and what needs to be tweaked. Then it puts all four pits together with the resources available while asking themselves ‘what is the most important task at hand and what sequencing of equipment and personnel do we need to accomplish that task?’
“That’s going to change week to week. From a scheduling perspective, our job is to ensure that the various moving pieces don’t get in each other’s ways or impede their own progress. What we want is to get the required amount of coal out of the ground and ready to be shipped just in time for when it’s needed by our customer,” said Taylor.
Several years ago, Sabine’s engineering and planning group began using a different method of scheduling. Using Microsoft Project, Taylor and his team have basically broken down all of their overburden and loading task, segmented each pit and assigned tasks and durations to each component. “By doing this we’re able to determine when to begin moving dirt in one location as opposed another. We are able to go out in the field, look at a situation and say ‘this coal will be in the way sooner, even though the dragline is not too close to this coal yet.’ We can prepare by having a truck/shovel combination ready. But we have a schedule and a timeframe on when certain things have to be complete so we can keep the draglines at optimal performance,” said Taylor.
For longer term, bigger picture mine planning, Sabine uses Maptek’s Vulcan system and Autocad primarily. All the pits are designed in Vulcan and in the Autocad system. They also use an Excel spreadsheet for each pit. “Each has a sequence file and each one of those is a block model, a 140- x 500-ft piece of property that the dragline will mine through. We demonstrate in each sequence file how many yards of dirt are in each block, how many tons of coal, in which seams, and the expected coal quality. We then combine the MS Project and MS Excel files and come up with one overall plan,” said Taylor.
At the end of the day, “you are just moving dirt around. Though its not rocket science, there is a lot of science behind the scenes. There is a science between each step the operator takes. Bottom line, the coal is down here, we’re up there. Now let’s get the dirt and move it out of the way,” said Taylor. “Really the most important thing you can do is hire the right people to run the equipment and then provide them with a plan they can accept and follow. In turn, you have to hire the right staff to build that plan. To mine coal at a 14:1 ratio and be competitive, you have to be very careful in your planning.”
One of the biggest factors in successful plan execution is communications. “All of us here in management understand that you’re only as good as your operators and you can’t execute a plan well if there’s no communication between you and them,” said Taylor. Sabine holds at least two meetings a week with the dragline crews as they come on shift where they review the plan for the week. “It’s a formal meeting where we show them what the plans are, what needs to be accomplished, and what situations an operator may encounter. We ask them to bring up any issues that they have. And then throughout the week while they’re on shift, we check in with them often.”
One–on-one time with the operators in the cab is vital for keeping things on track. “The guys in the machines know the dirt the best. They’re the ones that will make or break a plan. You put your faith in them and you include them. Many of our crew has been doing this for decades, they’re the experts. Asking for their input also helps motivate them because they know it’s not just a top down environment. We’re a team and we function best when we’re in that mode of thinking. This is a highly complex operation. We’re mining 3- to 4-ft seams on the thick side and 1- to 2-ft seams on the thin side, and doing that with a 94-cu-yd bucket. It’s tough and you have to always take your time and make sure you do the job right,” said Taylor.
Buchsbaum is a Denver-based freelance writer and photographer specializing in industrial subjects. He can be reached through his Web site at www.lmbphotography.com or by phone at 303-746-8172.
