Halifax Treasure Box Supplies Plants
This article appeared in the company publication, Across the Board, in the fall of 2004 at the time of the Halifax 50th Anniversary. It was written by Nancy Spurlock
National Gypsum purchased plats of land near Halifax, Nova Scotia, in 1954, securing its place in the wallboard industry and underscoring its success in today’s marketplace.
Under the land’s 60 feet of soil lies a treasure box of gypsum rock approximately 250 feet thick. This treasure will supply eight wallboard plants along the eastern seaboard. In addition, the company sells its competitor BPB (CertainTeed) 1500 tons per day for its New Brunswick wallboard plant, and area potato farmers use nearly 20,000 tons to enrich their soil.
“The deep deposit as well as our deep, all-season port give us a strategic advantage over the competition,” said Plant Manager Pat Mills, who has been at the location since 1984. “A typical gypsum deposit is only 40-50 feet thick.”
National Gypsum’s open pit mine, covering approximately 500 acres is located 30 miles from Halifax at Milford Station. The mine is near Canadian National’s rail line, another plus. “Unit trains” carrying 7000 tons of crushed ore travel every day to the port of Dartmouth just across the channel from Halifax.
A complex shipping schedule based on plant demand controls production at the mine. A computer model, which Mills helped design, tells Canadian Ship Lines when to have a vessel at Dartmouth to receive a load for delivery to one or more of the company’s board plants.
Horses combined with hard labor once mined gypsum. Now, the process begins with a computer model of the ore deposit. Information from 3000 core samples was used to produce a three-dimensional view of the mine. The model provides Milford Station with a 20-year mining plan as well as a four-day production plan.
“Our goal is to guarantee rock purity at 89 percent,” Mills said. “To do that, we will bring rock from several areas of the mine and combine it at the crusher. One area might produce rock at 75 percent purity, while another might yield purity in the 95 percent range.”
Actual production starts with a bang and big equipment. After the “overburden” of soil is removed, about 600,000 cubic yards each year, the miners drill 5-inch holes in the gypsum rock and fill them with dynamite. The controlled blasts loosen about 20,000-30,000 tons of rock per day.
The mining is done in benches of about 60-70 feet of rock. Typically miners are working on three levels. Following the blast, loaders fill trucks, and the rock is on its way to the crusher.
“The rock on each truck will produce about 1200 sheets of wallboard,” Mills said. "A truck will haul 40 tons and make about 80 trips to the crusher in a normal day.”
The primary crusher handles rock 6-8 feet in diameter. Any rock measuring over 6 inches after the initial crushing will go to a secondary crusher. Limestone, sand, and hydrate are screened from the gypsum during the crushing process. In total, the crushers will process rock at a rate of 1500 tons per hour. Mills says the height of the rock pile at the end of the crusher is an indication of how well the day went. Conveyors take the ore to the trains which are automatically loaded by computer.
Rock samples from 10 areas of the train are taken to record purity, chlorine content, and rock size. Purity should be 89 percent or higher, chlorine at no more than 100 parts per million (averages 15 ppm), and the rock should easily pass through an 8-inch ring. This information is used for quality control and to place the material on the rock pile at the Dartmouth dock.
Mills likes to have a minimum of two shiploads (80,000-100,000 tons) on the ground at Dartmouth. A radial stacker crane places material on the stockpile which is typically about 400 by 800 feet wide at ground level and 100 feet high.
“Pile management is extremely important,” Mills said. “We place the ore in three lifts. If we did not do this, all the fines would gravitate to the middle. A computerized profile of the pile can tell us where each trainload is located. We shape the pile to allow it to shed water.”
The massive size of the rock pile hides the heart of the operation below. The rock falls into 24 hoppers below the pile to load a belt line. The belt carries the rock to the ship loader.
As the mining operation at Milford Station continues to move eastward, the soil taken from the top is carefully placed back into the pit and grass sown. After another 50 years passes, Milford Station will be a beautiful lake 30 miles from Halifax.