Verde Magnesium’s Ilhan Goknel authors for Spot Light Metal on magnesium extraction.

/0 Comments/in /by

Magnesium metal is used in a diverse range of markets and applications exploiting the unique physical and mechanical properties of the element and its alloys. The use of magnesium increases annually as the lightweight properties are used to reduce weight, increase fuel efficiency, and reduce greenhouse emissions. But how is magnesium extracted?

This article describes three magnesium extraction methods: the Horizontal Pidgeon Process, the Vertical Pidgeon Process, and aluminothermic reduction.
This article describes three magnesium extraction methods: the Horizontal Pidgeon Process, the Vertical Pidgeon Process, and aluminothermic reduction.

(Source: gemeinfrei / Pixabay )

Magnesium Extraction: Horizontal Pidgeon Process

In 1941, Dr Pidgeon of Dominion Magnesium in Canada developed the Pidgeon process. Chemical reactions for the thermal process are relatively simple, including dolomite decomposition and magnesium oxide reduction. The dolomite decomposition or calcination involves and endothermic chemical reaction at an elevated temperature.
MgCO3.CaCO3 → MgO + CaO + 2CO2
Depending on the physical characteristics required for the dolime, the calcination temperature can vary from 780 to 1450 °C calcination conditions with influence the reduction reaction.

The calcined dolomite is then mixed with a reductant, typically silicon or aluminum, and heated at approximately 1150 to 1400 °C. At a low pressure of approximately 3 to 10 Pa magnesium oxide in the dolime reduced to metallic magnesium which escaped in the form of gas from the reaction zone and its captured at a reduced temperature.
2MgO + 2CaO + Si → 2Mg + CaO.SiO2

3MgO + 2CaO + 2 l →3Mg + 2CaO.l2O3

Flowsheet and consumptions of Horizontal Pidgeon Process.
Flowsheet and consumptions of Horizontal Pidgeon Process.

(Source: Ilhan Goknel)

The reduction of alkaline earth oxides by means of silicon proceeds theoretically according to the equation:
2MO(S) + Si(s) = SiO2(s) + 2M(g) (I)
The reaction is reversible. Generally, it proceeds from right to left at allowing temperatures with the emission of heat and from left to right only at high temperatures with the absorption of heat. However, this simple equation is not followed, and a secondary reaction always occurs in which the resulting silica combines with a certain amount of unreacted alkaline earth oxide, forming silicate. Hence, when a theoretical reaction mixture is used according to the reaction as described above, only 1 mole of oxide is reduced from 2 moles of metal oxide and 1 mole of silicon and 1 mole of metal is formed and ½ mole silicon remains unused. For a complete utilization of the reducing agent, it is necessary to have twice the amount of oxide shown in equation:
4MO(s) + Si(s) = 2MO.SiO2(s) + 2M(g) (II)

In general, the reaction cases when a dibasic silicate 2MO.SiO2 has been formed. It has shown that in a system of alkaline earth oxide-silica, the dibasic silicate is preferably formed. Consequently, in the case of MgO, the reaction follows equation (II) where Mg replaces M. This alkaline earth oxide reduction method by reducing agents produces a volatile metal and non-volatile oxide known as early 1878.

Thermal reduction process proceeds as follows:

  • 1. Calcination of dolomite to dolime,
  • 2. Blending of the dolime with a reductant, such as ferrosilicon,
  • 3. Heating of the dolime-reductant mixture is a vessel under vacuum,
  • 4. Condensation of the liberated magnesium vapor,
  • 5. Melting, alloying and casting of the condensed magnesium.

PRIMARY MAGNESIUM PRODUCTION: PROCESSES & MATERIALS

Want to learn more about magnesium production and the Pidgeon Process? Then download our free whitepaper now.
The whitepaper contains the following aspects:
1. Horizontal Pidgeon Process
2. Dolomite
3. Calcination
4. Reduction
5. Refining
6. Life cycle assessment

Get Free Whitepaper!

Magnesium Extraction: Vertical Pidgeon Process

The Vertical Pidgeon Process is a relatively new technology, with many advantages over the Horizontal Pidgeon Process. Several institutes in China have taken the vertical process into an industrial scale. With this method the retort is mounted into the furnace in a vertical position, load and unload of the material functions automatically by gravity, equipped with a matched machine, taking modernize control technology to monitor the material turning into raw magnesium and unload the slag. Meantime, it uses the slag’s waste heat to preheat the material; the vertical process is considered environmentally friendly with high heat application rate, which also ensures the automation, mechanization, and reduced labor intensiveness of magnesium production.

Compared with the traditional horizontal process, the vertical process has the following advantages:

  • The production capacity for unit retort increases from 25 – 30 kg per retort to 65-70 kg. The vertical retort has a good result by heating, and all materials are evenly enriched in the retort. The temperature field in the retort is uniformly distributed, which is conducive to the reduction process, and it could be appropriately expanded to retort volume.
  • The auxiliary production cycle is shortened by min. 1 hour per cycle, which is a total of min. 600 hours per year. This means an increase of approx. 8 % in capacity and energy efficiency, consequently.
  • The gravity work has upper feeding and bottom slagging, equipped with appropriate equipment to realize the slag discharge and raw magnesium for mechanical transfer. It automatically discharges the slag, broke vacuum, open retort, and takes magnesium mold by modern control technology, which can greatly shorten the auxiliary production period and reduce workforce needs.
  • The Vertical Pidgeon Process improves the service life of reduction retort. The time of charging and slagging is shortened, and the material is preheated. The phenomenon of sharp rise and fall of tank temperature is alleviated, and the service life of reduction retort is improved. By improving the alloy composition of the retort, minimum maintenance can be achieved. As a result, an additional workshop for recycling the retorts can be replaced by contracting third parties for recycling.
  • Material load, slag unload, raw magnesium taken out could be done by machinery equipment and gravity; thus, the labor intensity could be reduced.

Magnesium Extraction: Aluminothermic Reduction

The new process of dolomite clean magnesium smelting has two parts. The first is a new process for the comprehensive utilization of calcium and magnesium separation of dolomite. This process produces magnesium carbonate, which is a raw material of magnesium smelting. This stage solves the problem that the Pigeon method reduction slag cannot be comprehensively utilized. The second is the use of aluminothermic reduction, a new magnesium smelting process, which solves the waste gas residue of traditional magnesium smelting and uses the most advanced microwave heating technology today.

Part 1: New Technology of Comprehensive Utilization of Calcium and Magnesium Separation of Dolomite

New process of this plant layout was optimized based on the secondary pressurized carbonization method. First, the dolomite calcination equipment was improved, and then the subsequent process conditions were optimized and improved, and the secondary pressurized carbon chemical section was eliminated. To simplify, the carbonization efficiency is greatly improved, and the product quality is better. At the same time, the whole process has fully realized a new clean production process without waste gas, waste residue and wastewater discharge.

Part 2: New Technology of Thermite Reduction Magnesium Smelting

Thermite reduction method: The principle of magnesium production technology, under vacuum conditions, using metallic aluminum as a reducing agent to reduce magnesium oxide to prepare metallic magnesium, and by-product magnesium aluminum spinel. The preparation process includes a raw material mixing process and vacuum smelting process.

Compared with the Pigeon method, the total amount of reduction materials in this technology is reduced by more than 50 %. Energy consumption is reduced by about 54 %, and CO2 tail gas emissions are close to zero; spinel powder as a by-product can be sold as a product, and no waste residue is generated. The process is fully in line with the requirements of high efficiency, energy saving, clean and environmental protection.

Magnesium Extraction: Conclusion

For the last two decades, world production of magnesium metal has enjoyed a period of strong growth, despite a few interruptions. Since 2002, total global primary and secondary production has increased from an estimated 630 kt to slightly over 1.1 Mt or a cumulative annual growth rate (CAGR) of 5%. Recent primary production increases have taken place almost exclusively in China, with output declining or ceasing in several Western countries.

The cost of raw materials, energy, and labor within China is also rising, so Chinese Pidgeon plants’ operating expenses increase, thus pushing the cost of Chinese magnesium production closer to those of plants elsewhere in the world.

According to the Chinese government’s 12th Five-Year Plan, the Chinese magnesium industry’s targets are developing production, energy-saving and emission reduction, and more input in technological innovation and structural adjustment.

Due to significant capital costs involved, new electrolytic plant capacity is likely to remain minimal. Construction of new thermal plant capacity is more likely, achieved either through Pidgeon or alternative process production plants or by innovations in feedstock production methods for an optimized Pidgeon process plant.

Catalyzing Commercialization: Clean, Low-Energy Method for Primary Magnesium Metal Production

/0 Comments/in /by


Magnesium metal is a critical infrastructure material commonly associated with structural lightweighting in automotive, aircraft, and portable electronic segments. Currently
, China uses the silicothermic Pidgeon process to produce 85% of global Mg metal. This crude process, which emits on average 28 kg CO2eq/kg Mg, is inefficient and polluting, but its low labor and energy costs translate to the lowest metal price. “Although it supported the survival and development of the global magnesium industry, the Pidgeon process has no future due to its carbon footprint,” says Alexandru Rosu, the business development lead for private equity investment firm Amerocap LLC’s Romanian magnesium project.

 

Another way to produce Mg metal relies on electrolytic technologies. However, this method has high capital costs and produces toxic dioxins and chlorine compounds, and thus is not a major contributor to world production.

To reduce the cost, emissions, and waste associated with primary production of Mg metal, Colorado-based Big Blue Technologies (BBT), with funding from the National Science Foundation (NSF), is developing a new carbothermal reduction (CTR) method. CTR is a thermochemical approach in which MgO reacts with carbon, liberating Mg gas along with byproduct carbon monoxide. This method is analogous to the silicothermic Pidgeon process where silicon, produced by reducing silicon dioxide with carbon, is then used to reduce MgO to Mg. CTR, in comparison, is a direct production route with fewer steps that consumes about half the energy. Using renewable electricity resources and select raw materials, BBT’s process can be nearly net-zero in emissions from ore-to-ingot (0.5–15 kg CO2eq/kg Mg).

images

▲ Big Blue Technologies’ carbothermic process for magnesium production relies on a high-temperature condenser in which molten chloride salt is used as the cooling fluid (>350°C).

Previous attempts to commercialize CTR failed due to difficulties with material handling, refining, and the re-oxidation of metal product during condensation, a problem known as reversion. BBT’s process overcomes these hurdles and prioritizes safety.

BBT’s process technology relies on metal deposition in a high-temperature condenser with subsequent in situ vacuum distillation. In the first step, crude Mg deposits onto the sidewalls of the condenser. Subsequently, the temperature of the condenser is increased (700–750°C) to vaporize the Mg and collect a 99.9% pure metal product. The product magnesium does not require further refining and can be directly alloyed and/or cast into ingots. During this process, fines are captured and oxidized immediately, eliminating the significant safety hazards posed by conveyance of pyrophoric materials.

Process design was driven by three key considerations from prior work at Univ. of Colorado at Boulder: condense at low, but economic, vacuum conditions; condense at relatively high temperatures to facilitate large crystal growth and mitigate pyrophoric fines; and incur relatively short exposure times of the condensate to the CO atmosphere. While the condenser is critical, engineering an integrated system to exploit these principles has been the focus of all prototyping over the past six years.

Today, BBT operates a 50-kW custom-built vacuum arc furnace at 1 kg-Mg/hr and continues to increase production rates. BBT has built out MgO-to-ingot processing capabilities, and is launching a cyclic batch process to demonstrate continuous operation. Using BBT’s process data as input, a techno-economic analysis suggests that a commercial system would produce Mg at a cost of <$2/kg Mg with energy consumption of <15 kWh/kg, which far exceeds current commercial capabilities. In addition, MgO can be sourced from various primary ores as well as industrial waste products.

Mac McCreless, CEO of Garrison Minerals, says, “BBT’s process has major implications for reducing emissions in transportation sectors. We’ve seen the successes of this team and the technology and believe in the capacity to scale this process to meet current market needs in North America, filling a major supply gap.”

This technology was funded through the NSF Small Business Technology Transfer (STTR) Program.

This article was prepared by the National Science Foundation in partnership with CEP.