Sintermagnesia production for the manufacturing industry
30 May 2013
Anthony McEneaney MBA, M.Sc, CChem MRSC, technical services co-ordinator, Premier Periclase Ltd
Sintermagnesia, also known as periclase or deadburned magnesia or magnesium oxide, is a major raw material in the manufacture of refractory bricks used for lining the furnaces used in the steel, cement and glass industries. Magnesium oxide (MgO) has a high strength, mechanical stability and chemical inertness at high temperatures, i.e. greater than 1,400°C.
The melting point of magnesium oxide is 2852°C, which is almost half the surface temperature of the sun. This high melting point makes it highly resistant to chemical attack by the basic slags, alkalis and high lime fluxes found in the steel-, cement- and glass-manufacturing processes.
Premier Periclase produces two grades of large crystal sintermagnesia – Premier LC and SKLS. These products are used in the most severe wear areas of steel convertors, ladles and other steelmaking vessels. The company also produces magnesium hydroxide suspension (Premier ECOMAG), which is an alternative to sodium hydroxide for neutralisation and removal of heavy metal from acidic wastewater streams, and Premier TechMag, a calcined magnesium oxide powder that is used in the animal feed and chemical industries.
The two main raw materials used are seawater and limestone (calcium carbonate, CaCO3). There is also a significant energy input into the process in the form of natural gas and petroleum coke. Seawater contains approximately 3.5% dissolved salts, of which about 0.5% are soluble magnesium salts – magnesium chloride (MgCl2) and magnesium sulphate (MgSO4). About 7,000m3/hr of seawater is used in the process. Approximately 500 tonnes of seawater are required to produce one tonne of magnesium oxide.
Seawater is pumped from the mouth of the River Boyne twice daily into a large polder, with a capacity of approximately 300,000m3. As the river is tidal, pumping only takes place for about eight hours per day, i.e. two hours before and after each high tide. The seawater is screened to remove any debris and then chlorinated to prevent the growth of marine organisms in the pipelines taking it to the factory located a couple of kilometres upstream. Once in the factory, the seawater is degassed to remove hardness (dissolved calcium carbonate) before being pumped to the reactor.
High purity limestone is taken from quarries located about 8km southwest of the plant. It is heated to temperatures in excess of 1,600°C in a rotary kiln fitted with a pre-heater to convert it to lime or calcium oxide (CaO). The kiln, which is fired by a combination of natural gas and petroleum coke, has a capacity of approximately 150,000 tonnes per annum. The reaction is described by the following chemical equation:
CaCO3 + Heat (approx. 1,600°C) → CaO + CO2 (gas)
The lime is then mixed with freshwater to convert it to lime slurry or calcium hydroxide (Ca(OH)2). This process, which is highly exothermic, is known as slaking or hydration:
CaO + H2O → Ca(OH)2 + Heat
After slaking, the lime slurry is pumped to the reactor tank, where it meets the degassed seawater. The reactor is a 30.5m diameter tank with an internal dilution and mixing system that ensures thorough mixing of the seawater and lime slurry. Precipitation of magnesium hydroxide (Mg(OH)2) is very fast, almost instantaneous and occurs as per the following reaction:
MgCl2 + MgSO4 + 2H2O + 2Ca(OH)2 → 2Mg(OH)2 + CaCl2 + CaSO4.2H2O
In addition to seawater and lime slurry, the reactor also contains a recirculating load of already precipitated magnesium hydroxide. This acts as a ‘seed’ for the newly precipitated Mg(OH)2 crystals to adhere to. On average, the magnesium hydroxide load is passed through the reactor ten to 12 times before it achieves the required characteristics to pass onto the next stage of the process.
After the reactor, the magnesium hydroxide slurry is pumped into two large, 99m diameter tanks known as primary thickeners. The thickeners separate the solid magnesium hydroxide from the spent seawater, using gravity and aided by an organic polymer (or flocculant).
An excess of lime is used in the precipitation reaction to control boron levels in the magnesium hydroxide solid. Boron is an impurity that has a detrimental effect on the performance of the final refractory when it is in service in steel furnaces. The excess alkalinity, or overlime, is necessary to provide a balance between calcium oxide and boron in the final sintermagnesia product.
Spent seawater, which contains excess dissolved lime from the overliming, is pumped to another vessel called the effluent clarifier. Here it is reacted with seawater to produce a magnesium hydroxide precipitate that is used for production of the magnesium hydroxide suspension for treatment of acidic wastewater. After this, the spent seawater is returned to the Irish Sea through a pipeline extending approximately 1.5km out on the seabed.
In the next stage of the process, the settled and thickened magnesium hydroxide slurry is pumped to the secondary thickeners and then to three rotary vacuum filters, which are located in the multi-hearth building. Here, the slurry is filtered and washed with freshwater to remove chlorides, which come from the seawater. The washed material, filtercake, is then pumped to two pressure filters. These filters squeeze more water from the cake to increase the solids content further, to approximately 70% magnesium hydroxide solids. This reduces the amount of ‘free’ water that has to be burned off in the next stage of the process, which is calcination.
Premier Periclase operates two natural gas-fired multiple hearth furnaces (MHF) running in parallel. Each furnace has 16 hearths or floors. The material is pushed from one floor to the next in a zig-zag pattern by a rotating rake on each hearth. Excess moisture in the magnesium hydroxide filtercake is boiled off in the first few hearths and as the temperature profile increases to a maximum of about 1,000°C down through the furnace, the magnesium hydroxide is thermally decomposed to an active magnesium oxide (MgO) powder, also known as caustic calcined magnesia or CCM. The thermal decomposition of magnesium hydroxide is described by the following chemical equation:
Mg(OH)2 + Heat (~1,000°C) → MgO + H2O (vapour)
FINAL STAGE OF PRODUCTION
The resulting magnesium oxide powder is cooled to about 300°C in the lower hearths of the furnaces before being sent to the briquetters where its pressed into stick shaped pellets or green briquettes. These pellets are the feed to the final part of the process – sintering in the shaft kilns.
The high temperature heat-treatment process that converts the green briquettes to sintermagnesia is known as sintering. It is done in two vertical natural gas-fired shaft kilns. Each kiln has a rated output of 50,000 tonnes per annum.
Green pellets are fed to the shaft kilns, where they are heated to approximately 2,300°C. There is a countercurrent flow of green briquettes and secondary or cooling air in the kiln. Natural gas and primary air are injected into the kilns at one of four different levels. The intense heat in the kilns encourages the magnesia crystals to grow from about 400 angstroms (Å) to approximately 120-160 microns (μm). The briquettes also shrink as the density increases above 3.40g cm-3. The final product – sintermagnesia, periclase, deadburned magnesia or magnesium oxide – is a dense, chemically inert, low boron, refractory grade material.
Premier Periclase sells its products to customers located around the world in Austria, Germany, India, Italy, Spain, Poland and the UK. Material is despatched from the plant in loose bulk from the company’s own wharf on the River Boyne, by bulk bags in containers and trailers and bulk liquid and powder tankers. The production process is illustrated in the flowsheet (see Fig 1).
Anthony McEneaney is the technical services co-ordinator with Premier Periclase Ltd. Employed in Premier Periclase since 1985, with a background in research and development and product development, he currently manages sales of non-sinter products and also monitors greenhouse gases and environmental compliance.
The company is a wholly owned subsidiary of RHI AG, the world’s leading refractory producer. RHI is headquartered in Vienna, Austria and has more than 100 operations in 33 plants located worldwide. It employs about 7,800 people and is listed on the Vienna stock exchange.
Premier Periclase began as a jointly owned company between Hepworth Ceramic Holdings Ltd and Cement-Roadstone Holdings Ltd (subsequently CRH plc). Construction of the plant began in 1977 on the site of an old wet-process cement factory. A severe downturn in the demand for steel in the early 1980s, which in turn led to a decrease in demand for refractories, resulted in Hepworth Ceramic Holdings pulling out of the joint venture.
CRH assumed 100% ownership of the company and it survived the adverse market conditions and prospered. Since then, the company has survived many similar downturns in the steel and refractory markets and remained as part of the CRH Group until it was sold to RHI AG at the end of September 2011. See www.premierpericlase.com for more.