The process involves the treatment of the fly ash by chemicals to reduce sulfur, iron and silicon to acceptable levels so that the beneficiated material can be used as a feedstock in the thermal reduction process.
In September 2013, an Australian patent was approved on the hydromet process. In 2018 patents have been granted in USA, China, EU and Indonesia. India is expected to grant a patent in 2019.
In 2018, LMG also adopted its hydromet process by developing an acid based system to dissolve the hard mineralogy in some fly ashes.
THERMAL REDUCTION PROCESS
The thermal reduction process is a simple and proven process which was invented in early 1940’s by Dr. Lloyd Montgomery Pidgeon of the Canadian National Research Council. The first plant was built in 1941 in Haley, Ontario, Canada. This plant operated for 66 years, most recently by Timminco Metals. In the past 10 years, the Pidgeon process has come to dominate the world magnesium production. China is the dominant magnesium metal supplier, relying almost exclusively on this method.
Magnesium is produced from calcined dolomite under vacuum and at high temperatures using silicon as a reducing agent.
The following is an outline of the normal thermal reduction process.
In the process, the finely crushed dolomite (magnesium/calcium) carbonate is fed to rotary kilns where it is calcined, and where the carbon dioxide is driven off leaving a product – dolime.
Normally, the dolime is then pulverized in a roller mill prior to mixing with finely ground ferrosilicon and fluorspar. The fine dolime, ferrosilicon, and fluorspar are weighed in batch lots and mixed in a rotary blender.
This mixture is then briquetted in briquetting presses. Briquettes are then conveyed to the reduction furnaces.
The reduction operation is a batch process releasing magnesium in vapour form, which condenses in the water cooled section of the retort outside furnace wall. The residue from the reduction charge, which is inert, is removed from the retort and will be sent back to the existing tailings dams.
After removal from the furnace, the magnesium “crown” is pressed from the sleeve in a hydraulic press.
Vertical Retort Technology
Whilst LMG is using the thermal reduction process it has also investigated the use of automated vertical retorts. At this time, the development of this technology is not far enough advanced to replace the proven horizontal retorts.
This page updated: 22/10/2014