Spinel’s spring

By Mike O'Driscoll
Published: Monday, 25 July 2011

Steelmaking, cement, and glass refractory applications bloom for the synthetic magnesia-alumina mineral, although world production remains limited

 

RHI’s sintered spinel plant at Radenthein, Austria. RHI AG 

There are certain synthetic industrial minerals that have taken time to evolve, but once established after much research and development, they have carved their niche in the market.

This is certainly the case with spinel, which only began to emerge as a force in the early 1990s, and is now established in the refractories market as a premium grade combining magnesia and alumina (see panel for brief background).

As with most synthetic minerals, by the nature of their manufacturing process and use of high purity feedstock raw materials, they do not come cheap, ranging between $800-1,800/tonne ex-works for spinel (see panel), but they do offer superlative performance characteristics.

In the refractories industry, the use of synthetic materials is increasing as the various end market manufacturing technologies demand better performing raw materials for their refractory brick and monolithic applications (mainly steel, followed by cement, glass, ceramics etc.).

In this issue’s Trading Faces feature, Ashok Sen, director of Indian refractory minerals trader Magus Marketing Pvt Ltd told IM: “Opportunities are in the production of synthetic raw materials, based on locally available low quality or inappropriate minerals, on a commercially viable basis.”

Another driver for increased use of synthetic materials is the shortage or inconsistent availability of large volumes of natural minerals. This has mainly stemmed from either legal and environmental barriers to exploitation of natural resources or, as in the case of key minerals from China, constraints in export volumes.

Spinel also demonstrates other primary characteristics of synthetic industrial minerals: reliance on specific feedstock raw materials; a limited number of global producers; and continuous work on product development and new applications.

Limited supply sources

Fused spinel refractory market share 

 

Source: Motim 
As explained in the accompanying panel, spinel may be produced as a fused or sintered grade, tailor-made to specific applications with varying concentrations of magnesia and alumina, and even with additions of zirconia.

A glance at the table of producers indicates that world sources of supply for spinel are quite limited. This is not surprising given the technical commitment required for production of quality grades and the relative size of the spinel market in refractories, estimated at some 40-50,000 tpa outside China.

The main factor common to these companies is that they are not spinel-only producers, but manufacture a portfolio of synthetic refractory minerals, such as fused and calcined aluminas and sintered mullites.

Thus they already have access to raw material supply chains and, crucially, refractory market customers.

Naturally, there is a division between sintered and fused spinel producers, based on the companies’ core production technology prowess and utilisation of existing plant capacity, which can be flexible to produce a portfolio of products. Owing to the latter, it is therefore frequently difficult to ascertain accurate production capacities for spinel only.

For example, Almatis GmbH, the world’s leading producer of speciality aluminas, produces sintered spinel at its Ludwigshafen plant in Germany.

At the end of 2011, Almatis is expecting to complete a new production line with the installation of new crushing and sizing equipment, including an impact crusher.

However, it is only possible to estimate a total production capacity for the plant with this new installation at about 75,000 tpa of combined alumina and spinel grades.

Likewise, fused spinel producers are most likely to quote total fusion capacities producing a range of fused alumina, magnesia, mullite, and spinel grades, whose relative output depends on market demand.

Mike Pierce, vice president sales at C-E Minerals explained to IM: “C-E produces fused spinel in our facility in Puerto Ordaz, Venezuela. This is also where we produce fused white alumina for the refractory and investment casting industries.”

“The cullet material is then shipped to our plant in Andersonville, Georgia [where sintered mullite is also produced] for processing and packaging. Our capacity to make spinel depends on how much fused white we produce.” Pierce added.

Producers each have their own brand names for their spinel products, and the figures used in the brand name denote mostly the alumina (Al2O3) concentration (eg. Itochu’s NSP70 - see specifications table), although occasionally the magnesia (MgO) content of the grade (eg. C-E Mineral’s Spinel 25).

Margit Toth, commercial director, Motim Electrocorundum Ltd, Hungary, told IM: “Our current production level of fused spinel is around 10,000 tpa, which is flexible according to the market environment.”

“We are producing three standard grades of fused spinel: FS-10, FS-27 and FS-33. The figures refer to the MgO percentage in the spinel.” said Toth.

“What is a challenge for us is the ever rising raw material and energy prices.” Toth added.

Another characteristic is captive production. This is where leading refractory product manufacturers have existing plant facilities to process and produce refractory minerals, from either purchased raw materials or from their own integrated resources, for their in-house consumption.

Both RHI AG, Austria, and Magnesita Refratarios SA, Brazil have these capabilities, and each have made firm commitments in recent months to increase their vertical integration of raw materials and decrease their dependence on mineral supplies from third parties.

RHI produces three grades of sintered spinel MA67 (67% Al2O3), MA78 (78% Al2O3), and MA90 (90% Al2O3).

In Brazil, Magnesita Refratarios uses a mixture of spinels. Dr Paschoal Bonadia Neto, head researcher at Magnesita Refratarios told IM: “We use fused magnesia rich, and sintered alumina rich, spinels. We produce our own fused spinel, which accounts for our largest consumption. Our total spinel consumption is around 10,500 tpa.”

ANH Refractories Co., of the USA consumes about 2,500 tpa of both fused and sintered spinel in various magnesia and alumina rich grades for cement and steel applications.

Jason Borgesi, senior manager, Global Procurement, ANH told IM: “Our supply currently comes mainly from China and Germany. Fused spinel seems to be stable, there have been periodic supply issues with alumina rich sintered spinel.”

With such a small pool of producers, the spinel market is naturally very competitive, and Chinese exports are making their mark in Europe, and are improving their grades (see below).

Margit Toth, commercial director, Motim, told IM: “We are facing significant competition from China. On the other hand we are able to offer a good quality material with a reliable supply chain and reasonable commercial conditions.”


World spinel producers*

Company Plant location Grades/Spinel capacity where known (tpa)
Almatis GmbH Ludwigshafen, Germany Sintered: AR78, AR90, MR66 (% Al2O3)
C-E Minerals Puerto Ordaz, Venezuela Fused: Spinel 25 (% MgO); processed at Andersonvile, Georgia
Cermatco Ltd Aylesham, Kent, UK Fused: CerMagALS (90% Al2O3), CerMag FS (72% Al2O3), CerMag FS67
Daehan Ceramics Co. Ltd Youngam-kun, Chollanam, South Korea Sintered: SS-83, SS-90, SS-95 (% Al2O3)
Elfusa Sao Joao da Boa Vista, Sao Paulo, Brazil Fused: ; MAE-10, MAE-26, MAE-28, MAE-32 (% MgO); 6,000
Henan Mianchi Great Wall Corundum Co. Ltd Tiantan, Mianchi, Henan, China Fused: AM-70, AM-85, AM-90 (% Al2O3)
Huayin Group Corp. Haicheng, Liaoning, China Sintered: MAS-50, MAS-66, MAS-76, MAS-90 (% Al2O3)
Itochu Ceratech Corp.** Seto, Aichi Sintered: NSP-70, NSP-95 (% Al2O3)
Jiangsu Jinghui Refractory Materials Co. Ltd Zhenwu, Jiangdu, Jiangsu, China Sintered: ZMA, JMA-66, JMA-78 JMA-90 (% Al2O3)
Magnesita Refratarios Contagem, Minas Gerais, Brazil Fused; captive production
Motim Co. Ltd Mosonmagyarovar, Hungary Fused: FS-10, FS-27, FS-33 (% MgO); 10,000
Passary Minerals Ltd Bijabahal, Rourkela, Orissa Fused/Sintered: 77-79% (% Al2O3)
RHI AG Radenthein, Austria Sintered: MA67, MA78, MA90 (% Al2O3); 19,500; captive production
Washington Mills Electrominerals Corp. Niagara falls, Ontario, Canada Fused: SP-27 (% MgO), Bauxite Spinel (64.5 % Al2O3); 27,000
White Circle Oxides Ltd Samalkot, East Godavari, Andhra Pradesh Sintered: Spinwhite 65, 78, 90 (% Al2O3); 6,000


* ie. the main producers for refractory grades; there are other smaller, and captive, producers, especially in China that claim to produce various grades of spinel.


Feedstock raw material


The two primary ingredients to produce spinel are a source of magnesia and a source of alumina. Higher grade spinels require high purity magnesia sourced from seawater or brines, and will use Bayer alumina as the alumina source.

Other grades of spinel can utilise caustic calcined magnesia sourced from magnesite and use bauxite as the alumina source.

Mike Pierce, of fused spinel producer C-E Minerals said: “We have a local source [to Puerto Ordaz, Venezuela] of alumina and purchase the magnesia outside of Venezuela.”

In the USA, Washington Mills Electrominerals Corp. produces two fused spinel grades, each from high purity (ie. synthetic) magnesia, but the SP-27 grade uses Bayer alumina and the Bauxite Spinel grade uses bauxite.

An RHI spokesperson told IM that it uses “calcined alumina and calcined magnesia”.

Andreas Buhr, global technical director refractories of Almatis GmbH, told IM: “Almatis uses high purity Bayer process alumina, 99% Al2O3, and synthetic sourced magnesia.”

In Hungary, Margit Toth, commercial director, Motim said: “For fused spinel production we purchase appropriate quality calcined alumina and caustic calcined magnesia. We maintain several channels for both key raw materials to keep the quality, cost and supply security on reasonable level.”

“The demand for quality purity is very high in some fields of application, that is why we are always looking for new suppliers for raw material and we are always revising our production technology to reach this requirement” Toth added.

Daehan Ceramics Co. Ltd of South Korea, produces a sintered spinel by calcining a mixture of high purity alumina and magnesium hydroxide in a rotary kiln at 1,850¡C.

In China, Henan Ruishi Special Refractory Co. Ltd of Yichuan, Henan, supplies a fused spinel produced by mixing alumina with no less than 98.5% Al2O3, caustic burned magnesia with no less than 96% MgO, and “an appropriate amount” of high purity magnesite, >47% MgO, and fusing in an electric arc furnace.

Jiangsu Jinghui Refractory Materials Co. Ltd, of Zhenwu, Jiangsu province produces a sintered zirconia-containing magnesia alumina spinel which is claimed to offer better thermal shock resistance, corrosion resistance, and erosion resistance.

The grade has 5-6% of high purity zirconia which forms cubic ZrO2 as a priority and monoclinic ZrO2 as an accessory phase, uniformly distributing among the magnesia alumina spinel crystals.

Although western consumers of spinel use established grades sourced from Europe and the Americas, spinel grades, especially magnesia-rich grades, are also imported from China, which is seeing increased production of higher grade spinels.

Prof. Zhou Ningsheng, of Henan University of Science and Technology and Director of the High Temperature Materials Institute, told IM: “The overall supply of [Chinese] spinel tends to more high purity or high performance grades, evidenced by the supply of bauxite based sintered spinel declining, while the supply of fused bauxite based spinel (Al2O3+MgO³98%) increasing.”

Minimising the impurity levels in spinel is critical to achieving higher performance. Huayin Group, of Haicheng, Liaoning, China has been developing a new source of magnesite in Tibet which it has used to make a sintered spinel grade (see specifications table).

In 2007, the company established a mine at Kamaduo, Tibet, to exploit a high purity aphanitic magnesite deposit after a global search for new sources.

Huayin was expecting to complete construction of a 300 tpd caustic calcined magnesia (CCM) plant in Tibet by mid-2011, and another plant by 2012.

Zhou explained: “Huayin compared the quality of sintered spinel using caustic magnesia made from Tibet cryptocrystalline magnesite versus Liaoning magnesite. They are able to produce higher grade spinel [with Tibetan magnesia] with significant reduced impurity at a very competitive cost/performance effectiveness.”

There are more Chinese producers of spinel than those indicated in the table, mostly for the domestic market, and these are located mainly in Henan and Liaoning provinces, host to important sources of bauxite/alumina, and magnesite, respectively.

“In China, an increased tendency for high purity spinel is clear, simply because it leads to higher performance and less contamination of steel.” said Zhou.

Refractory market trends

Japanese spinel consumption in refractories 2008-10 (tonnes) 
2008 2009 2010
Spinel consumption 12,435 8,229 12,020
Spinel brick output 15,032 8,736 11,744
 

Source: The Japan Refractories Association 

Broadly speaking, magnesia-rich spinel has found its niche in cement kiln refractories while alumina-rich spinel is used in steelmaking refractories.

Overall, the outlook for spinel is promising with increased demand for the use of synthetic refractory minerals.

“The market is steady to increasing as the trend to high quality raw materials continues.” reported Mike Pierce, C-E Minerals.

Refractory leader, RHI confirmed to IM that there was “strong demand” for spinel in refractories and “We are trying to keep up our capacities with the increasing demand.”

Margit Toth, commercial director, Motim said: “Spinel applications are always growing. We see new possibilities in the glass industry and in the steel industry too.”

Regarding the Chinese market, Zhou commented: “So far, growing areas of refractory applications for high grade spinel can be seen for large ladles, refining ladles, sliding plates, the transition zone of cement kilns, and burner bricks for lime/cement kilns.”

Spinel selection

The choice of spinel selection - ie. fused or sintered, magnesia-rich or alumina rich, pre-formed or in-situ - remains a topic for debate, although the choice of grades is really is down to the intended refractory application and conditions of use.

Independent minerals consultant, Ted Dickson told IM : “Both [sintered and fused] have their place in the market. As far as in-situ spinel is concerned there is still a very important market specifically in steel ladles.”

“Initial development in Japan was for monolithic formulations, although in Europe the trend tended to be in spinel-forming bricks and shapes.” Dickson added.

ArcelorMittal Refractories, Poland, which consumes about 800-1,000 tpa spinel sourced from Europe and China, uses a mixture of grades. Sanjiv Bushan, vice president of the board, ArcelorMittal, said: “The materials change depending on applications. For castables, the sintered spinels are both types, MgO rich or Al2O3 rich. For brick applications, both fused and sintered spinels are used; sintered spinel for burned bricks and fused spinel for tempered bricks.”

Fused vs sintered

Typical specifications of sintered and fused spinels 

 

Source: Almatis, C-E Minerals; Itochu Corp.; Washington Mills Electrominerals Corp.; Prof. Zhou Ningsheng 

Andreas Buhr, of Almatis, considers that a sintered spinel grade provides for a more homogenous spinel phase, and is thus superior to a fused grade.

Often, however, it is a case of compromising one property when improving on another.

Bob Drew, technical and marketing director - base metals at RHI AG, UK, commented: “The advantage of the fused grain is improved density and resistance to chemical degradation due to the reduction of grain boundaries.”

“However the cleavage of the fused grain through the crystal lattice presents a non-reactive face to the brick matrix reducing bonding. So if high hot strength is required, a sinter grain can be beneficial, however, the fused grain will improve thermal shock resistance.”

Ricardo Mosci, chief executive officer, RefrAmerica LLC, USA, agrees with the philosophy of compromise in refractory material selection, and defines refractory science as being “...very similar to politics sometimes: it is the science of compromises. Perhaps the major difference is that refractories never lie.”

Fused spinel, if slowly cooled, present the largest crystals and the least reactive ones. Therefore, they are the best in resisting chemical attack and dissolution at elevated temperatures.

Sintered spinels, usually produced in rotary kilns, have their alumina and magnesia components fully reacted. The crystals are much smaller than fused spinel crystals, the grains are more porous and their chemical activity is higher, but they cost less than fused spinel.

“In moderate temperatures, in the absence of lime-rich liquid phase, sintered spinels do an excellent job.” remarked Mosci.

Garrette Bai, research and development at a refractory raw materials company, considers that the differences between sintered and fused spinel are comparable to the differences between tabular alumina and white fused alumina.

“The intragrain pores in sintered spinel or tabular could improve the thermal shock resistance over fused ones. But, the chemistries in the sintered products could be more homogeneous.” said Bai.

Pre-formed vs in-situ

As well as production of pre-formed spinel by fusion or sintering, some spinels are designed to be formed in-situ in refractory matrices from the reaction of free magnesia and alumina during use.

The resulting properties in using in-situ and pre-formed spinel in castables used in steelmaking are significantly different, but both types of castables have their advantages in the lining of steel ladles.

When selecting the spinel type, the different requirements on refractory lining materials for the different zones of a steel ladle must be taken into account.

In the past ten years, castables used for steel ladle side walls have progressively changed from alumina-spinel castables to alumina-magnesia types.

One reason for this has been that a finer spinel, like the in-situ formed spinel in alumina-magnesia castables, would bring better corrosion and penetration resistance than a pre-reacted spinel.

Cost pressures also forced the use of cheaper raw materials like alumina and magnesia to form spinel in-situ during use instead of using pre-formed spinel in castables.

Castables formulated with high quality pre-formed spinel exhibit high thermo mechanical stability, high mechanical strength and erosion resistance as well as an improved penetration resistance (Schnabel et al 2010).

Spinel forming mixes, ie. in-situ spinel, impart good corrosion resistance but have a low hot strength and consequently low erosion resistance. The softening of the castable is a drawback when high thermo mechanical stability is required but is advantageous when stress relaxation (elastic behaviour) is important.

However, pre-formed spinels also have their place.

When produced at high production temperatures, up to 1,900¡C, the alumina-rich spinel phase can be maintained by rapid cooling. Owing to these vacancies in the crystal structure, pre-formed alumina-rich spinel has the capability to absorb FeO and consequently retard the infiltration of slag. This is an important feature and a major factor in the advantage of spinel containing mixes for resistance to steelmaking slag (Schnabel et al 2010).

Dr Paschoal Bonadia Neto, head researcher at Magnesita Refratarios told IM: “In-situ spinel seems to be a promising technology, as more and more knowledge is gained on its performance and reaction control versus pre-formed spinel, giving interesting results in terms of corrosion resistance.”

A combination of in-situ and pre-formed spinel products can also be beneficial.

Jason Borgesi, senior manager, Global Procurement, ANH, told IM: “ Spinel forming products in combination with in-situ products provide enhanced performance in some steel making applications.”

However, there are issues to pay attention to when assessing the use of in-situ or pre-formed spinels in steel ladle castables.

The use of magnesia, as one of the reactants for in-situ spinel formation in castables, often causes difficulties such as poor flow or quick setting because of the hydration of the magnesia (Schnabel et al 2010).

Furthermore, the volume expansion owing to the hydration may lead to cracking during the drying of the castable which is especially critical when producing pre-cast shapes.

The in-situ formation of spinel is also linked with a strong volume expansion that needs to be controlled. A too high expansion would lead to mechanical stresses and thus spalling of the lining.

Work has been conducted to develop methods to control the expansion behaviour of alumina-magnesia castables. This has included selecting the right grain size for alumina and magnesia, and the addition of 0.5-1.0% fumed silica (microsilica) to counterbalance the expansion and to achieve good workability of the castables.

Castables containing pre-formed spinel do not have such expanding behaviour nor any unwanted hydration and can thus be formulated without fumed silica addition (Schnabel et al 2010).

In China, research has indicated that in-situ spinel is favouring refractory performance, such as is in alumina-magnesia castables for steel ladles.

But Prof. Zhou Ningsheng warned: “Nevertheless, the hydration of magnesia remains a concern, or a problem. A co-system of spinel-bearing and spinel forming is a good compromise, not only in Al2O3-MgO ladle castables, but in spinel basic bricks for cement kilns as well.”

Ricardo Mosci, RefrAmerica LLC, also underlined some disadvantages of in-situ spinel use: “While pre-formed spinels can be added to magnesia spinel bricks up to 20%, the amount of in-situ spinel that a brick tolerates is limited, because the spinelisation reaction occurs with volumetric change. Less spinel equals higher elastic modulus, lower resistance to thermal cycling, and less coating in cement kiln applications.”

However, in-situ spinel remains a good option for lime recovery kilns, pebble lime kilns, and others. “The use of mag-spinel bricks keeps increasing in cement and lime kilns, replacing the magnesia-chrome spinel bricks still in use by some nations.” said Mosci.

Japan demand down

The Japanese refractories market is normally a reasonable barometer of trends in refractory raw material consumption.

According to Eiji Motoki (2010), Japan’s consumption of spinel for refractories was around 3,500 tpa from 1979 to 1986, after which it rapidly increased to 17,000 tpa by the early 1990s and to almost 20,000 tpa by 1995, its peak.

After the mid-1990s, spinel consumption started to decline in line with decreased specific consumption of refractories in steelmaking, and from the late 1990s to the present plateaued out around the 12-14,000 tpa level.

In recent years, from the accompanying table it can be seen that the recession of 2009 took its toll on Japanese spinel consumption and brick production, but that at least regarding spinel refractory consumption, it was recovering back to near 2008 levels during 2010, ie. a little over 12,000 tpa.

However, the Japanese market is perhaps not as strong as it used to be.

Japanese synthetic mullite producer, Itochu Ceratech Corp ., used to produce sintered spinel, but now imports spinel from South Korea for the Japanese market.

Yoshi Muraki, ceramic sales department of Itochu Ceratech Corp. told IM: “At present, we don’t produce [spinel] at our Seto plant because the Japanese market is small. We gave up our NSP50 grade for cement applications several years ago since demand was low”.

“When demand returns and is strong in the future, for example more than 10,000 tpa, we will produce again here.” said Muraki.

However, Itochu still sells NSP70 and NAP95 grades for the Japanese alumina spinel castable market for steel ladle applications, although Muraki noted that the Japanese demand trend in this application for alumina-magnesia and alumina-spinel remained low.

“We are supplying spinel to limited customers in Japan, and our sales volume is about 2,000 tpa” said Muraki.

Cement refractories

The most extensive use for magnesia-rich spinels in refractories has been as a replacement of chromite-containing bricks used in cement rotary kilns, owing to the environmental hazard of magnesia-chrome brick waste disposal (ie. the threat of carcinogenic Cr6+ entering the water system).

This trend has been well established in developed countries, although magnesia-chrome bricks are still used in many developing countries’ cement kilns. Buhr commented to IM: “This market will continue to grow in the future.”

Margit Toth, commercial director, Motim, Hungary said: “The application in the cement industry is rather seasonal and it is influenced by the concrete economical situation of the construction industry.”

In China, fused bauxite based spinel has dominated applications for cement rotary kiln bricks in the transition zone.

Prof. Zhou Ningsheng explained: “If China produces 10,000 tpa magnesia spinel bricks, at least 1,500 tpa need to be replaced. Cement rotary kilns are fired mainly by coal in China. Fused bauxite spinel is sufficient for this application with less alkalis attack, while sintered spinel basic bricks are mainly used in small kilns of 2,500 tpd clinker.”

One of the most important refinements of magnesia spinel bricks used in cement kilns has been the improvement of the cement coating property which occurs when in use. Initially, magnesia spinel bricks were inferior to magnesia-chrome bricks in this characteristic.

The problem was overcome by the addition of small amounts of iron oxide in solid solution to the magnesia raw material used to make the magnesia spinel brick (Obana 2010).

Steelmaking refractories

Alumina-rich spinel castables are widely used in steel ladle linings below the slag line and as pre-cast shapes such as purging plugs and well blocks.

Shanmugam Vridhachalam Pichai, head of research and development at Saudi Refractory Industries said: “Addition of spinel at around 25% in the castable system ultimately improves the life of well blocks in the ladle. If you get 40 heats in the ordinary castable , the addition of spinel definitely increases the life from 40 to 55 heats.”

“These applications for spinel are growing in demand. They are standard in Europe, and in less developed countries they are evolving to be standard.” said Buhr.

In steel ladle bottoms where the volumetric stability under high temperature and pressure is most important, and high erosion resistance is also important, castables containing pre-formed spinel are preferred. In-situ spinel formulations have reduced erosion resistance.

The requirements for steel ladle side walls are different, however. Ladle sidewalls are less subject to erosion compared to the bottom, and thus the lower hot strength of the in-situ spinel castable is problematic.

The ability for stress relaxation is of high importance to avoid stress peaks which may lead to cracking. Therefore castables with spinel formation, imparting 1-2% permanent linear change, provide advantages in ladle side walls as they can close cracks in the lining.

However, in the event of higher sidewall erosion resulting from poor tapping practices or extensive stirring a combination of pre-formed spinel and in-situ spinel is considered the best option.

Regarding pre-cast shapes which are exposed to high erosion, these use pre-formed spinel castables which have high hot strength and high thermal stability.

Dr Paschoal Bonadia Neto, head researcher at Magnesita Refratarios told IM: “With the recent advances in the engineering of spinel, pre-formed and in-situ, castables, including the use of nano-spinel, some new fields of application may appear, especially in steel ladles, if the new technologies prove to be cost effective.”

However, spinel has not had its own way all of the time. One of the areas that first prompted the development of alumina spinel technology was as a replacement for magnesia-chrome in slagging applications.

Ruth Engel, of Refractory Consulting Services, in a recent article said: “Even though it [spinel] achieved this goal in many areas, alumina spinel refractories have not been successful in very severe environments, where magnesia chrome is still preferred as its life is significantly longer.”

Engel went on to comment: “The use of magnesia chrome refractories has declined over time, but, for the last 10 years or so, their consumption has remained fairly stable and they continue to be used in very severe environments.”

Glass refractories

Spinel, already used in glass refractories, is expected to see increased demand from the solar glass manufacturing sector.

In a presentation at the Annual Meeting of the International Commission on Glass (ICG) in Shenzhen China, Chris Windle, technical manager, at UK-based DSF Refractories & Minerals Ltd, said: “Conversion to oxy-fuel firing technology and increased interest in solar glass production has resulted in significant attention to new refractories that can withstand harsh environments at increasingly elevated melting temperatures for longer campaigns.”

Magnesia spinel provides alkali and boron resistance, excellent thermo-mechanical stability, and the ability to withstand high temperatures up to 2,000¡C.

“These properties mean that spinel can be utilised in melter superstructure and crown applications for a wide range of glass compositions including float, solar, E, C and lighting. Campaigns of spinel melter superstructure have now reached 12 years with no visible wear and oxy-fuel melter crowns have been installed with great success culminating in float melter spans.” said Windle.

Spinel products can make conversion to oxy-fuel melting economically viable which in turn improves productivity, reduces NOx emissions and has a positive impact on glass quality.

References

Engel, R. (2010), Chrome bearing refractories: is there a future?, The Refractories Engineer, May 2010, p.12.

Motoki, E. (2010), Trend in refractory raw materials in Japan, Journal of the Technical Association of Refractories, Japan, Vo. 30, No.4 December, p.234

Obana, T. (2010), Trend of chrome-free refractory materials, Journal of the Technical Association of Refractories, Japan, Vo. 30, No.4 December, p.243.

Schnaebel, M., Buhr, A., Exenger, R., & Rampitsch, C. (2010), Spinel: In situ versus Preformed Ð Clearing the Myth, Refractories WorldForum, 2, 2010, p.87.



Spinel spotlight

Definition & occurrence

The spinel group includes minerals with a crystal structure of a divalent metal oxide and a trivalent metal oxide Ð it includes chromite, magnetite, and other minerals.

The definitive form is true spinel, which is a magnesium aluminium oxide (MgAl2O4) , with variations including chrome spinel (Fe,Mg)(Al,Cr)2O4. Spinel has a stoichiometric ratio of 71.8% Al2O and 28.2% MgO.

Natural spinel, or members of the spinel group, are found as minor constituents of alkali basalts, while gem spinels are often found in marbles (metamorphosed limestones) and pegmatites. Spinel also occurs in the olivine rich rocks of the mantle, and chrome spinel is a diamond indicator mineral in kimberlites.

There is no commercial development of spinel deposits, although natural spinels are often traded as gemstones.

Production & grades

Synthetic spinel is produced either by fusion in an electric furnace or by sintering in a shaft or rotary kiln.

Feedstock raw materials are high purity seawater or brine sourced magnesia and Bayer process alumina for the higher end grades, and caustic or dead burned magnesia and bauxite for lower grades.

As well as production of pre-formed spinel by fusion or sintering, some spinels are designed to be formed in-situ in refractory matrices from the reaction of free magnesia and alumina during use.

This application of secondary spinel applies to both bricks and castables. This can impart added attributes to the product, such as resistance to chlorine and sulphur attack in cement kilns.

The development and addition of synthetic spinel grains allows for a greater spinel content in the brick in conjunction with secondary spinel.

A strong feature of all spinels is the tendency to substitutional solid-solutioning, where large percentages of one or both of the spinel components may be substituted by others of the group.

For the magnesium-alumina spinel both magnesium and aluminium cations can be replaced by others with similar size.

Additionally, the MgAl2O4-structure shows an increasing phase region with increasing temperature, especially towards higher alumina contents. This allows the production of alumina-rich spinels.

Fused and sintered spinels may be separated into magnesia-rich (>30% MgO,<70% Al2O3), and alumina-rich spinel grades (<30% MgO, >70% Al2O3), and the producers’ spinel grade brand names usually indicate the relative typical alumina, and less commonly, its magnesia content.

Typical specifications of refractory spinel grades range 21-33% MgO and 66-80% Al2O3.

Particle sizes in descending order can range from up to 3-4mm, 1-2mm, 0.5-1.0mm, 0-0.5mm, down to 75 and 45 microns.

Uses

With a melting point of 2,135¡C, the most important industrial use for synthetically manufactured spinel is in the refractories industry, in both bricks and monolithics, where the material imparts improved thermal shock resistance.

Synthetic spinel’s main uses are as a replacement for magnesia-chrome brick in cement and lime kilns (using MgO-rich spinel), and as a 15-30% component of castable monolithics for steel ladle linings, and in pre-cast shapes such as well blocks (using mostly Al2O3-rich spinel).

Glass refractories also utilise spinel.

Price examples

Spinel grade Price/tonne
Fused spinel, US ex-works $1,400-1,600
Fused spinel, FCA Hungary €700-1,500
Sintered spinel, FOB  ex-works Seto, Japan $1,700-1,800
Fused spinel, high purity, ex-works China $1,000-1,200
Fused bauxite based spinel, ex-works China $800-900