End User Focus: Minerals’ flame gain

Two types of flame retardant (FR) additive can be recognised (primary and secondary) and both types are included in the discussion.

Primary FR additives are those that are capable of significantly reducing the flammability of polymers in their own right. The main commercial products of this type are:

• Metal hydroxides and related products, principally aluminium hydroxide;
• Phosphorus and organic and inorganic phosphates;
• Halogen compounds (brominated and chlorinated organics);
• Expandable graphite.

There is also significant use of chloro-phosphates, which confuses the statistics, as they sometimes get counted with phosphates and sometimes as separate entities.

Secondary FR additives have little or no effect on their own, but significantly improve the performance of primary FR types. The main products of this type are:

• Antimony trioxide;
• Metal borates;
• Organo-clays;
• Zinc, tin and molybdenum salts.

The estimated share of the principal products in the world market is given in Figures 1 and 2. The metal hydroxides are used at significantly higher loadings than most of the other types, and this accounts for their relatively high tonnages. Another feature worthy of comment is the large change in ranking for brominated compounds and antimony trioxide compared to metal hydroxides when looking at tonnage and value. This reflects the relatively high price for the former additives.

The European market differs from the world one in having less use of halogens and antimony and greater use of metal hydroxides, as concern about the smoke and related issues associated with halogenated products is more prevalent.

While increasing safety, the use of flame retardants generally increases product costs and compromises end use properties. Because of this, their use is more prevalent in the developed world economies, especially NAFTA, the European Union and Japan. Their use in the emerging markets is now expected to increase rapidly. Indeed some analysts predict double digit growth figures in China and possibly India.

Figure 1: Estimated world consumption of main FR types (by tonnage)

On the downside, the mechanism results in high levels of smoke and corrosive gases, which has led to concerns about the generation of toxic products. Despite these limitations they continue to dominate much of the market.

The chlorine used is ultimately derived from deposits of salt (halite), which is usually solution mined and then electrolysed to produce the chlorine. Bromine is generally produced from bromide-rich brines by treatment with chlorine gas. Various chemical processes are used to turn the elemental chlorine and bromine into suitable organic compounds.

Metal hydroxides

The main products of this type are aluminium and magnesium hydroxides and basic magnesium carbonates. These function by decomposing endothermically, with the release of water and/or carbon dioxide, at temperatures close to where the host polymer itself degrades. This both cools the article and dilutes the combustible fuel.

This type of product is regarded as environmentally friendly compared to most other technologies, as it gives low levels of smoke and corrosive or toxic gases under fire conditions. Already well established, these products are growing faster than the overall market due to their green credentials.

Currently the European market for metal hydroxides is the most developed, due to greater concerns over halogenated alternatives compared to other regions.

Figure 2: Estimated world consumption of main FR types (by value)

The production method used is essentially the same as that operated to make Bayer alumina; indeed fire retardant grades were originally coproduced on the same plant and significant amounts are still made this way today.

In the process bauxite is dissolved in sodium hydroxide to produce sodium aluminate. This solution is then purified (the origin of the notorious red mud often associated with alumina production). After purification, the solution is seeded and cooled to precipitate the aluminium hydroxide. This gives a relatively large sized particle which can be milled down to more suitable sizes. A significant part of the flame retardant grades is now made by re-dissolving Bayer product, followed by further purification and precipitation. This gives greater purity and better control of particle size and shape.

There is also a niche market for the monohydrate boehmite [AlO(OH)]. Again, while this occurs as a natural mineral, the flame retardant product is made synthetically with the ultimate mineral source usually being bauxite.

The main limitation of aluminium hydroxide is its decomposition temperature, which is about 200ºC. This is uncomfortably close to the processing temperature of some polymers and this is where magnesium hydroxide comes in.

Magnesium hydroxide

Magnesium hydroxide has a very similar fire retardant effect to aluminium hydroxide but a significantly higher decomposition temperature (about 300ºC). It is generally more expensive to produce than equivalent grades of aluminium hydroxide and this has restricted its market penetration.

Just as with aluminium hydroxide, there is a direct mineral equivalent, known as brucite. In this case some deposits of brucite are pure enough to enable effective FR products to be produced by grinding, beneficiation and drying. Such deposits are mainly found in China, but are exported to most world markets. In some cases the brucite is mixed with significant amounts of calcite, but such mixtures still have some utility.

The natural brucite products have limitations, however, and a significant amount of synthetic product is also produced. Strangely, given the much smaller size of the market, these synthetic methods are much more varied than for aluminium hydroxide. They use a variety of raw materials and also result in a wide variety of product qualities.

The highest grade products are made from concentrated and purified solutions of magnesium chloride. This magnesium chloride solution can come from many sources. It can be a natural brine or produced by solution mining (eg. such as the method used by Kyowa in Veendam, Netherlands, ICL in Israel and in the planned production facility of Nikochem in the USSR), from sea water desalination (as at least one plant in Japan), or by dissolving a magnesium mineral in hydrochloric acid.

The main mineral in use today is serpentine (magnesium iron silicate hydroxide). This is used by Albemarle at a plant in Austria, where the serpentine is leached with hydrochloric acid to produce a magnesium chloride solution, which is then purified and subjected to hydropyrolysis to give magnesium hydroxide and hydrochloric acid. The acid is concentrated and recycled to the leach stage. The leach co-produces silica gel which is also marketed. Other magnesium silicates could probably be used in this process.

A number of other processes have been seriously examined. Most use precipitation by adding lime or dolomitic lime to sea water, a variant of a long established process used to make magnesia for refractories. Another option is to hydrate magnesium oxide produced by calcination of magnesite (magnesium carbonate).

Worldwide consumption of all magnesium hydroxide types is estimated at 50-60,000 tonnes, with about half of this being used in Western Europe. The market is about evenly split between the synthetic and natural types. Use of natural brucite is expected to significantly increase in China as its market develops, due to the country’s indigenous resources. Use of synthetic products in Eastern Europe is also expected to increase significantly if Nikochem’s planned production facility in Russia is realised (see IM March 2010: Russia to start magnesium hydroxide production).

Basic magnesium carbonate

There are several forms of basic magnesium carbonate and one, known as the mineral hydromagnesite [Mg₅(CO₃)₄(OH)₂.4H ₂O], has particularly useful properties for use as an FR additive - it has been commercially developed by Minelco Group. Hydromagnesite releases both water and carbon dioxide on heating, with a decomposition temperature a little higher than aluminium hydroxide.

As with brucite, there are good deposits suitable for use after grinding, beneficiation and drying. These almost invariably co-exist with the mineral huntite [Mg₃Ca(CO₃)₄] which is difficult to remove but does not appear to seriously compromise the fire retardant performance, even if present at up to 50% w/w.

The main deposits being exploited are in Greece and Turkey. There is also a small amount of synthetic product made, but the present precipitation processes do not give a totally satisfactory particle form.

Phosphorus and compounds

A number of phosphorus-containing products are used as fire retardant additives. The simplest is phosphorus itself, in the elemental form known as red phosphorus. This is quite surprising at first sight, given the flammable nature of this element, but is has proven to be a very effective product for use in oxygen or nitrogen-containing polymers, especially polyamides. In other polymers it has to be used with a synergist.

Ammonium polyphosphate is also a very effective flame retardant under the right conditions, giving useful intumescent effects; again it is often used with synergists.

The other large class of phosphorus-containing flame retardants is the phosphate esters, widely used in plasticised PVC, when high levels of flame retardance are required. In some cases chlorophosphate esters are used.

All of the above products are ultimately derived from phosphorus-containing ores. The main deposits of these ores are to be found in North Africa and the USA, although China is the main producer today, followed by the USA (IM December 2009: Phosphate face-off).

Ammonium polyphosphate and the phosphate esters are produced from phosphoric acid, which can be derived from the rocks either by leaching them with sulphuric acid or by heating with sand and coke to produce white phosphorus and then hydrolysing this in water.

The white phosphorus route has the advantage that the phosphorus is removed in the vapour phase and so is obtained in a pure form, thus allowing quite low grade ores to be used. Red phosphorus is produced from the white phosphorus.

Expandable graphite

Expandable graphite (EG) is a very effective flame retardant in some polymers, especially polyurethane foam. It is made from natural graphite, which is converted to flake graphite by grinding and flotation followed by chemical purification. The flake graphite is then treated to intercalate an acid between the graphite layers (sulphuric, nitric or acetic acid are commonly used). This acid provides the mechanism for expansion and the resulting flame retardancy. China and India have large reserves of natural graphite.

Antimony trioxide

This is the main secondary flame retardant in commercial use. It has little effect alone, but as a synergist it very significantly increases the efficiency of halogen-containing systems.

The main source of antimony trioxide is the mineral stibnite (Sb₂S₃) and this can be directly converted to the trioxide by reaction with air or oxygen. Sublimation is used to purify the oxide, especially to remove the toxic arsenic oxide (most stibnite ores contain arsenic impurity). Some companies prefer to produce the metallic element first and then oxidise it.

Most of the world production of antimony metal and the oxide is carried out in China today, but some metal is exported and converted elsewhere.

Borates

Borates, especially zinc borate, are another important class of secondary flame retardants, most often used in conjunction with halogenated systems. They are derived from various borate ores of which there are four main ones; the sodium borates, tincal and kernite, the calcium ore, colemanite, and the mixed sodium calcium ore, ulexite. The largest deposits are in Turkey, the USA and Russia. Turkey has about two thirds of the world’s known reserves at over 1bn tonnes.

Others

There are a number of minor additives, largely of the secondary type. The main ones are organo-clays (mainly based on montmorillonite), molybdenum compounds, ferrocenes and zinc stannates. Many of these are mainly seen as alternatives to antimony trioxide which has had a very volatile price history (see Mineral Pricing 101).

The organo-clays are mostly used as synergists with metal hydroxides and some premixed versions of aluminium or magnesium hydroxide are commercially available from Albemarle.

Supply

Many of the flame retardants mentioned above account for quite small tonnages compared to their mineral resources and are also used in other, much larger, applications; so supply should not be an issue in the medium or even long term.

The high volume FR products are the halogenated ones, aluminium hydroxide, phosphorus-based grades and the borates, but even here supply does not seem to be an issue. Halogen-based flame retardants are derived from salt deposits or brines and these are in abundant supply. Bauxite reserves are plentiful and well spread, so supplies for aluminium hydroxide production should also be adequate, despite the high tonnages consumed. Both borate and phosphate reserves are also plentiful, although of more limited geographical distribution.

Table 1: Selected leading producers of flame retardant additives