Burning up: Refractory clays face an unclear future

Because refractory clays are undoubtedly linked to refractory markets, which are in turn affected by markets such as construction, there has been a fall in demand for their use on the back of the global economic downturn. It was because of the downturn that the housing market plummeted in 2008 (see p14).

This was initially thought to be a short-lived slump, but market sources told IM that they believed the slump may continue further into 2012 and even run into 2013.

“During 2010 and first half 2011 the market recovered [and saw] good levels but after September 2011 the market decreased again and the forecast is not very promising,” Fernando Lopez, managing director of Spain-based Arciresa Refractarias SA, said.

“I would say the global refractory clay market is slowly improving but there are still many bumps in the road because of the global economic situation,” Robert Virta, mineral commodity specialist, US Geological Survey (USGS) told IM.

“Obviously, the economy is still the leading influence on the market. This is especially true with the slowing of most of the global economies in 2011 and 2012.”

US markets for refractory clays

In the US in particular, there are signs of life following the recession, Virta said.

“The US market is recovering after the recession. Fire clay sold or used for refractory products are almost where they were in 2006 but still less than sales prior to 2005. The same is true of kaolin sold or used for refractory products,” he continued.

Shane Bower, managing director of Christy Minerals, which is based in Missouri, US and owned by O’Brien Industrial Holdings, LLC, agreed that the US market had softened this year.

“The market has softened through the third quarter in basically all of our end-use markets,” he said.

However, Bower remained optimistic to a point saying that overall the market was “good”, despite “softening of 5-10% seen in relation to Q1 and Q2 of this year”.

Factors affecting the US market undoubtedly reflect a fallout from the global economic crisis, which has made buyers more cautious.

Also, Bower alluded there was a “general uncertainly” and the US elections are currently taking place as people are unsure which direction the economy will follow post-election.

In 2011 production in the US of refractory clays, was at 240,000 tonnes, up from the 2010 level of 216,000 tonnes, but down from the high in 2007 of 565,000 tonnes, according to the USGS.

Christy Minerals, which produces both raw and calcined clays with an alumina range of 35-60% can produce 100,000tpa of calcined aggregate, Bower said.

The company’s main markets are in North America namely the US, Mexico and Canada although the company does ship products outside of North and Central America, Bower explained.

For Japan’s Krosaki Harima Corp., meanwhile, demand for its products has increased this year as the domestic steel industry has undergone a slow recovery following the devastating earthquake and tsunami in March 2011.

Krosaki Harima is the world’s fourth largest refractories producer after EU-based Vesuvius and RHI AG, and Magnesita Refratarios SA, based in Brazil.

It has, for the last year, been building a refractory clay plant in India, which will produce taphole clay.

Taphole clay is a type of refractory monolithic product which is capable of being shaped. It is specifically used for the filling and sealing of blast furnace (BF) tapholes.

New trends

Substitution is key in most markets, and in refractory clays new products are being developed all the time in order to make costs go further.

In the past, there has been a move away from refractory grade bauxite, which has seen a rise in the use of clays.

The decline in consumption of bauxite in refractories has been due in part to technological changes, which have reduced the consumption per tonne of steel produced, and an increase in alternative alumina sources. When the price of refractory grade bauxite began to rise steeply, alternative alumina sources became more viable.

This reverses a trend from 20 years ago when cheaper bauxite was available and replaced the alternative alumina-silicate sources in refractories. Substitutes such as andalusite, mullite, fused alumina, and recycled materials are now eating into bauxite’s refractory market share.

“We have seen an interesting shift in refractory customers, interest in other alumina materials such as clays for their products,” Shane Bower told IM.

“This has been occurring over the last three to four years as uncertainty in supply and pricing with bauxite has caused refractory producers to look at alternatives,” he added.

Substitution

“It’s getting difficult for refractory companies to source bauxite,” Carlos De Ferrari, CEO of Andalusita SA, told IM in July this year. “Andalusite has its own market but this will grow further as it becomes more difficult to source bauxite.”

Andalusite is more likely to attract interest, especially as the Peruvian producer Andalucita SA recently announced it will be doubling production and launching a new product next year Ñ a move which the industry has taken stock of.

“New player Andalucita SA from Perœ is introducing a good material [and will] increase of production in the market,” L—pez said.

The Lima-based company, which started operations three years ago, produces premium refractory grade andalusite, Al₂O₃, at 59-60% alumina, with 0.60%-0.85% iron.

Elsewhere, Andalusite Resources, based in South Africa, said earlier this year that it is still shipping products to European, African and Japanese customers.

Mullite

Mullite 60/70 and 78, in particular, has seen an increase in demand, Lopez told IM, as some are using it in place of bauxite.

Mullite can be manufactured by calcining high alumina kaolin and bauxitic kaolin (see panel, overleaf).

Commercial synthetic mullite comes in two forms. Mullite also occurs naturally, in the Scottish Isle of Mull.

It has other uses also Ñ a scientific study showed that pollution levels caused by diesel-powered vehicles could be reduced by as much as 45% by using mullite instead of platinum in combustion engines.

Dr Kyeongjae Cho, materials science, engineering and physics specialist at the University of Texas in Dallas, US, who conducted the study, said that the synthetic oxide performed as effectively and more efficiently as a catalyst than the expensive rare metal.

Cho published the findings in the 17 August issue of the research journal Science where he explained how computer-modelling was used to test an oxygen-based composition of mullite as a consumer of toxic nitric oxide and nitrogen dioxide (known as NOx pollutants).

Prices, outlook and indicators

Things will get harder before they get easier, Fernando Lopez, managing director of Spain-based Arciresa Refractarias SA, told IM.

“Everybody thinks the market will improve in 2014,” Lopez said. “The outlook for 2013 is not good, but we consider it similar to 2011.”

“We do not see the panic reactions, just a more cautious approach to orders and forecasting on a quarterly basis,” Bower said, adding that of all the refractory clays chamottes 40-45% has seen a decrease in sales.

FOB prices of the clays are stable, sources told IM (see table), but what has impacted pricing is fluctuations in exchange rates and an increase in freight costs.

These changes have meant that variations on pricing can be as high as 15% in some grades this year, Lopez revealed.

Sources agreed that, since the financial crisis of 2008-09, business has been sustained at higher levels.

US exports of refractory clay were at 390,000 tonnes, in 2011, although this includes some refractory-grade kaolin. This is down from 404,000 tonnes in 2010.

The export price has fallen in the last five years (see graph). And the number of people employed by the industry has also dipped.

But Virta said that, although in the short term prices have fallen, they are up from ten years ago. And, he added, he believed that prices were likely to increase.

“If you look at the unit value of US shipments of fire clay and unshaped clay for refractory manufacture, as reported by the US Census Bureau, it has increased from 2001 to 2010,” he said.

Bower said the refractory clays market was “currently stable.”

“There is always pressure to provide the most economical materials to industry,” he added.

“We work very hard internally to drive down cost and through efficiencies are able to maintain an economical solution for our customers. I do believe this will continue into the future.”

Sources: Philippe Blanchart, professor, Ecole Nationale Superieure de Ceramique Industrielle, Limoges, France; Dictionary of Mining Terms, American Geological Society; and Gerald Routshka, Pocket Manual Refractory Materials



Refractory clays broken down

Refractory clays consist essentially of hydrated aluminium silicates with minor proportions of other minerals. The general formula for these aluminium silicates is Al₂O₃.2SiO₂.2H₂O, corresponding to 39.5% alumina, 46.5% silica, and 14% water. Kaolinite is the most common member of this group.

There are a number of terms used to describe such clays, perhaps the most common are fire clay, flint clay (also known as hard clay), plastic clays (also known as ball clays), and kaolins. These clays contain kaolinite (and maybe other clay minerals and quartz) as the main mineral phase, and can contain 20-45% Al₂O₃, <3% Fe₂O₃, <3% Na₂O +K₂O. These are used in fire clay refractory products containing <45% Al₂O₃.

On calcination at temperatures of 1,200-1,500¡C depending on alumina and alkali oxide content, the main mineral phases are transformed to mullite, 3A_l2O₃.2SiO₂, cristobalite, and a glassy phase.



Using high alumina kaolins and bauxitic kaolins as raw material, calcined refractory clays can be produced with 45-70% Al₂O₃, and with a higher degree of phase transformation to mullite. Commercial grades of such calcined clays generally fall into three categories based on alumina content: 45-50%, 60% and 70%. Lower alumina grades, around 38-44% are also widely traded. These calcined clays are used in high alumina refractory products, containing >45% Al₂O₃.

Mullite is a significant component of fire clay and high alumina products. For high alumina products , the mullite content can fluctuate greatly depending on raw materials, content of fluxes (K₂O, Na₂O, CaO, MgO, and shares of Fe₂O₃ + TiO₂), amount of cristobalite, and firing temperature.

Consequently, the type and amount of existing flux can supersede the importance of Al2O3 content. The shape and arrangement of mullite crystals also has an effect on end product properties.

Terms used to describe these burned refractory clays include chamotte (also high alumina chamotte, bauxite chamotte), calcined kaolins, alumino-silicate calcines.

However, fire clay and flint clay which has been mined and then calcined, is frequently still unchanged in name, ie. still referred to as fire clay and flint clay.

The term sintered mullite is sometimes also used to describe a high quality calcined refractory clay. However, strictly speaking, this product is more usually manufactured (by sintering or fusion) from a batch of raw materials which include clays, kaolin, and calcined alumina.

Applications

Refractory clay grades have applications in both refractory bricks (eg. fire clay bricks <45% Al₂O₃, such as super - duty, high duty, medium duty and semi-silica and high alumina bricks, classes 50%, 60%, 70%, and 80%), and monolithics (eg. grogs, mortars and mixes, ramming and gunning mixes, castables).

Refractory clays are used to make bricks that are used in blast furnaces in reheat furnace and soaking pit alls, roof, and sub-hearths. High alumina bricks are widely used as back-up linings in several furnace types, eg. horizontal channel induction furnaces, pressure pore channel induction furnaces, and vertical channel induction furnaces.

Fire clay and super-duty plastics are used in soaking pits, linings of heating furnaces, boiler settings, forging furnaces, annealing ovens and furnaces, cupolas, incinerators, and bake ovens. High alumina plastics (60-95% Al₂O₃) are used in aluminium upper side walls, iron ladles and reheat furnaces. Higher alumina products (80% Al₂O₃) are used for electric furnace roofs, burner ports, and blocks and reheat furnace hearths.

Fire clay castables are used in high abrasion environments and for chemical applications that react with lime or magnesia cells. High alumina castables are used in tundish back-up linings, iron transfer, foundry and steel ladles, soaking pit, and reheat furnace bottoms and hearths.

Some definitions

Chamotte

1. The refractory portion of a mixture used in the manufacture of firebrick, composed of calcined clay or of reground bricks (Dictionary of Mining Terms, 1997, American Geological Society).

2. A purpose made fired alumino-silica aggregate. (RHI Refractories America Online Refractory Glossary)

Chinese clays

<45% Al₂O₃: mild clay (also includes ball clay), fire clay, kaolin.

>45% Al₂O₃: flint clay, kaolin, high alumina ore

75-90% Al₂O₃: high alumina chamotte (China Fire Clay, Liu Hongquan, 1999)

Diaspore clay

A rock consisting of essentially diaspore bonded by fire clay. Commonly diaspore clay of the purest grade usually contains between 70% and 80% alumina after calcination.

Fire clay

1. A siliceous clay rich in hydrous aluminium silicates, capable of withstanding high temperatures without deforming, disintegrating, or becoming soft and pasty.

It is deficient in iron, calcium, and alkalis, and approaches kaolin in composition, the better grades containing at least 35% Al₂O₃ when fired. (Dictionary of Mining Terms,1997, American Geological Society)

2. An earthy or stony mineral aggregate which has as the essential component hydrous silicates of aluminium with or without free silica. (RHI RefractoriesAmerica Online Refractory Glossary)

3. A plastic, kaolinitc clay with sufficient Al2O3 to be refractory. The clay usually occurs as an underclay. (Industrial Minerals & Rocks, 1994, SME)

Flint clay

1. A smooth, flint like refractory clay rock composed dominantly of kaolin, which breaks with a pronounced conchoidal fracture and resists slaking in water. It becomes plastic upon prolonged grinding in water. (Dictionary of Mining Terms, 1997,American Geological Society).

2. A hard, non-plastic, kaolinitc claystone that breaks with conchoidal fracture but does not disperse in water. (IndustrialMinerals & Rocks, 1994, SME).

Grog or fire clay mortar

Raw fire clay mixed with calcined fire clay, or with broken fire clay brick, or both, all ground to suitable fineness. (Dictionary of Mining Terms, 1997, American Geological Society).