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.
RHIs sintered spinel plant at Radenthein,
Austria. RHI AG
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
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
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
In this issues 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
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
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.
|Fused spinel refractory market
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
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
worlds 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
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
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. Itochus
NSP70 - see specifications table), although
occasionally the magnesia (MgO) content of the grade (eg. C-E
Minerals 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
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
What is a challenge for us is
the ever rising raw material and energy prices. Toth
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
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%
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
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
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*
||Grades/Spinel capacity where known (tpa)
||Sintered: AR78, AR90, MR66 (% Al2O3)
||Puerto Ordaz, Venezuela
||Fused: Spinel 25 (% MgO); processed at Andersonvile,
||Aylesham, Kent, UK
||Fused: CerMagALS (90% Al2O3), CerMag FS (72% Al2O3),
|Daehan Ceramics Co. Ltd
||Youngam-kun, Chollanam, South Korea
||Sintered: SS-83, SS-90, SS-95 (% Al2O3)
||Sao Joao da Boa Vista, Sao Paulo, Brazil
||Fused: ; MAE-10, MAE-26, MAE-28, MAE-32 (% MgO);
|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 (%
|Itochu Ceratech Corp.**
||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)
||Contagem, Minas Gerais, Brazil
||Fused; captive production
|Motim Co. Ltd
||Fused: FS-10, FS-27, FS-33 (% MgO); 10,000
|Passary Minerals Ltd
||Bijabahal, Rourkela, Orissa
||Fused/Sintered: 77-79% (% Al2O3)
||Sintered: MA67, MA78, MA90 (% Al2O3); 19,500; captive
|Washington Mills Electrominerals Corp.
||Niagara falls, Ontario, Canada
||Fused: SP-27 (% MgO), Bauxite Spinel (64.5 % Al2O3);
|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
An RHI spokesperson told
IM that it uses calcined alumina and
Andreas Buhr, global technical
director refractories of Almatis GmbH, told
IM: Almatis uses high purity Bayer
process alumina, 99% Al2O3, and synthetic
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
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
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
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
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
Broadly speaking, magnesia-rich spinel has found its niche in
cement kiln refractories while alumina-rich spinel is used in
|Japanese spinel consumption in
refractories 2008-10 (tonnes)
|Spinel brick output
Source: The Japan Refractories Association
Overall, the outlook for spinel is
promising with increased demand for the use of synthetic
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
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
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
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
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.
|Typical specifications of
sintered and fused spinels
Source: Almatis, C-E Minerals; Itochu Corp.; Washington
Mills Electrominerals Corp.; Prof. Zhou
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
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
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
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
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
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 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
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
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
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),
Japans 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
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
Yoshi Muraki, ceramic sales
department of Itochu Ceratech Corp. told IM:
At present, we dont 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.
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
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).
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
These applications for spinel
are growing in demand. They are standard in Europe, and in less
developed countries they are evolving to be standard.
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
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
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
Magnesia spinel provides alkali and
boron resistance, excellent thermo-mechanical stability, and
the ability to withstand high temperatures up to
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
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.
Engel, R. (2010), Chrome bearing refractories: is there a
future?, The Refractories Engineer, May 2010,
Motoki, E. (2010), Trend in
refractory raw materials in Japan, Journal of the Technical
Association of Refractories, Japan, Vo. 30, No.4 December,
Obana, T. (2010), Trend of
chrome-free refractory materials, Journal of the Technical
Association of Refractories, Japan, Vo. 30, No.4 December,
Schnaebel, M., Buhr, A., Exenger, R., & Rampitsch, C.
(2010), Spinel: In situ versus Preformed Ð Clearing the
Myth, Refractories WorldForum, 2, 2010, p.87.
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%
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
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
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.
MgAl2O4-structure shows an increasing
phase region with increasing temperature, especially towards
higher alumina contents. This allows the production of
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
Typical specifications of
refractory spinel grades range 21-33% MgO and 66-80%
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.
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 spinels 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
Glass refractories also utilise spinel.
|Fused spinel, US ex-works
|Fused spinel, FCA Hungary
|Sintered spinel, FOB ex-works Seto, Japan
|Fused spinel, high purity, ex-works China
|Fused bauxite based spinel, ex-works China