Magnola’s Magic

Noranda is joining the ranks of the world’s low-cost magnesium producers, and doing it with distinction. Behind this effort lie almost 60 years of study, 15 years of intensive R&D, and capital expenditures nearing $750 million.

Interest was first expressed in 1942. It resurfaced in the mid-1980s with a proposal to recover magnesium from a magnesite deposit near the Pamour mine in Timmins, Ont.

Later that decade, champions of the cause arose: Nassef Ghatas and John Peacey. Despite a positive technology audit and market study, Noranda again slowed down the project until current COO Dave Goldman took up the cause and Michael Avedesian became director of magnesium in 1993. By mid-August 1996, a magnesium pilot plant was running at CEZinc and quickly proved a technological success.

Once the go-ahead for a commercial plant was given in the summer of 1997, a site was chosen near Danville, Que. The handsome Magnola Metallurgy plant (80% Noranda owned and operated, and 20% Socit Gnrale de Financement) is nestled next to 300 years of serpentine reserves, tailings from asbestos mining.

Already the casting lines are operational, and by the end of June the first magnesium metal will flow. At the end of 2001, Magnola will be operating at capacity–63,000 tonnes of magnesium metal annually. Reaching full capacity in only 18 months will be a notable achievement.

How to Make Magnesium

The recovery of magnesium from serpentine tailings, which contains 24% Mg, begins with feed preparation. A contractor trucks the serpentine tailings to a gas-fired rotary kiln for drying. The serpentine is screened and crushed before leaching.

From the feed dryer until the ingots are cast, the process takes place in a closed environment. All process circuits are sealed to contain the dust, acid or caustic materials used in the plant. The transformation of rock to molten metal has an air of magic about it; things happen in grey boxes, large containers and enclosed ductwork. Operators continuously monitor the process from two control rooms in the plant.

The feed is leached with hydrochloric acid in reactors, then neutralized with magnesium oxide (MgO) in three tanks. A pair of buffer tanks completes the gravity flow circuit.

The magnesium-containing brine and FeS residue are separated with belt filters. Residue is repulped and pumped to a retention storage pond. As constructed, the residue pond can contain five years of waste, and there is the ability to enlarge it by four more five-year stages. All water from the pond is recycled to the plant.

Brine purification takes place with the addition of caustic soda (NaOH) in two reactors followed by a buffer tank. At this stage metal impurities are removed. Purification residue and brine are separated in filter presses, and the residue is packed in containers for temporary storage at the Magnola site. Once characterized, the purification residue should be sent with the Fe-Si residue to the pond.

The designers had originally planned an additional ion exchange purification step. The pilot plant showed that this expensive installation was not neccesary.

The purified solution is preconcentrated using waste heat from the dryers. Magnesium chloride (MgCl2) is partially dehydrated before brine is sprayed into the top of the fluid bed dryers. Dihydrate magnesium chloride prills are created in the dryers, screened, and the undersize used for seed. Screening also assures good size distribution of nominal-size material. Off-gas is cycloned, scrubbed and cooled before water vapour is released.

Chlorination is the final step in creating a very pure feed for electrolysis. With the addition of high-strength hydrochloric acid (HCl), the remaining water is driven out of the prills and magnesium oxide (MgO) is converted to magnesium chloride (MgCl2). Molten MgCl2 becomes the high purity feed for electrolysis.

The electrolytic cells are of a design not used before in the magnesium industry, and they are among the most energy-efficient and productive on the market.

Each cell is tightly sealed because the same process that creates magnesium metal creates chlorine gas (Cl2). Each cell has a graphite anode on one end and a steel cathode on the other with plates situated between and parallel to the anode and cathode. The salts, at 650C, remain liquid at the bottom of the cells, magnesium floats on top, and the chlorine gas is carried off through a gas-handling system. The gas is fully recycled and returned to the process.

Magnesium is removed from the individual cells by a Techmo truck carrying a 6-tonne crucible. The truck with the loaded crucible is then driven to the casting area and unloads into a holding furnace or either of two alloying furnaces.

Magnola will initially have three metal-casting lines: one for pure Mg ingots and two for alloys. Each line includes automated stacking and strapping equipment. A wide range of ingot shapes, sizes and alloys can be produced.

Where the Gases Go

As much as Magnola is a magnesium production facility, it is also a large gas-handling facility. Chlorine gas (Cl2) is recovered from the electrolysis cells. It is washed with water and compressed. Then HCl gas is synthesized by burning chlorine with hydrogen. The amount of hydrochloric gas it makes gives Magnola the distinction of running the largest such plant in North America. The gas is stripped and distilled so that 99%-HCl gas can be fed into the chlorinator.

The hydrochloric acid coming off the chlorinator undergoes a thermal quench and oxidization. It then passes through activated carbon to absorb any chlorinated hydrocarbons (CHCs) that are present. The clean, 35% HCl acid is used in the leach and neutralization circuit.

Environmental Caretaking

No aspect of the new Magnola operation was overlooked for potential environmental impact. Meticulous baseline studies of local water, plant and animal populations were conducted. After consulting with local farmers, the company has agreed to test their livestock periodically for signs of contamination. Beeswax will also be tested because it is believed to be a very sensitive indicator of air contamination. Potential sources of emissions (all stacks and vents) are continuously monitored.

The plant discharges no liquid effluent; rather, the fluid bed dryers create a negative water balance. Process water is provided by recycling from the residue pond and from collection ponds filled by rain and snowmelt. Additional water requirements are met by pumping from the nearby Nicolet River.

The process of refining magnesium yields some compounds that concern the local environmental community. Magnola will create small amounts of organo-chlorides. Release of dioxins and furans will be minuscule: combined, they will amount to 0.09 gram annually to the atmosphere in the worst-case scenario.

Sulphur hexafluoride gas (SF6) is used in the casting area. It prevents the oxidation of the ingot surface and is widely used in the magnesium industry. However, it is also of concern. Consequently, Magnola and the province agreed to start the plant with SF6 but to stop using it by the end of 2005.

The creation of polychlorinated biphenyls (PCBs) is another concern. They are accidentally generated in the electrolytic cells, explained chief environmental manager Alain Bergeron. PCBs are largely destroyed during hydrochloride gas synthesis and in other thermal oxidation units. A remaining three kilograms will be emitted into the atmosphere, while 1.3 kg will be securely contained in the residue storage pond.

Another compound of concern is hexachlorobenzene (HCB). Annual emissions will be 21 kg, and another 54 kg of HCBs will be stored in the residue pond. These emissions will be monitored as a measure of Magnola’s impact on the environment. When cleaner technologies become available, they will be adopted where possible.

Safety First, Last and In Between

The refining of magnesium requires the use of hazardous materials. Hydrogen is explosive; gases and acids are caustic; magnesium metal burns fiercely. No one enters the plant without viewing the basic safety video. Safety permeates the culture from the corporate to individual levels. The 300 men and women who have taken jobs at Magnola were chosen in part for their positive safety attitudes.

The general training program is lengthy and includes safety. Vice-president and general manager Michel Bedard estimated that 25,000 person-days and $15 million will be spent on the program. Sessions range from an eight-week minimum to as long as 30 weeks.

For the first time in the metallurgical industry a $2.5-million operations simulator is being used. This type of equipment is more common in the petrochemical and aviation industries. Using it has proven very, very useful, Bedard added, giving the operators tremendous confidence.

When word went out that Magnola was hiring, 15,000 applications flooded in. Besides their positive attitude toward safety, employees were chosen for having values such as respect, teamwork and taking ownership of their work. Of the employees, about 25% came from within Noranda, 10% from other magnesium plants and 40% from the aluminum industry. The remaining workers came from the region around the plant.

The last word goes to Bedard: “This is the team we will start up the plant with, and we will succeed.”