The thermal decomposition of huntite and hydromagnesite—A review

Abstract

Naturally occurring mixtures of hydromagnesite and huntite are important industrial minerals. Their endothermic decomposition over a specific temperature range, releasing water and carbon dioxide, has lead to such mixtures being successfully used as fire retardants, often replacing aluminium hydroxide or magnesium hydroxide. The current understanding of the structure and thermal decomposition mechanism of both minerals and their combination in natural mixtures is reviewed. The crystalline structure of both minerals has been fully characterised. The thermal decomposition of huntite has been characterised and is relatively simple. However, the thermal decomposition mechanism of hydromagnesite is sensitive to many factors including rate of heating and the composition of the atmosphere. The partial pressure of carbon dioxide significantly affects the decomposition mechanism of hydromagnesite causing magnesium carbonate to crystallise and decompose at a higher temperature instead of decomposing directly to magnesium oxide.Keywords: hydromagnesite, huntite, fire, flame, retardant, mineral This review critically examines the sometimes conflicting reports in the published literature. It draws together current knowledge of the structure and thermal decomposition of hydromagnesite and huntite, in order to provide an insight into the thermal behaviour of mixtures of these minerals and to optimise their selection and applications. Industrial use of Mineral Fillers as Fire Retardant AdditivesThe largest group of mineral fire retardants are metal hydroxides. Their endothermic decomposition and associated release of inert gasses or water vapour, above the processing temperature but below the thermal decomposition temperature of polymers suppresses the ignition, while the accumulation of a solid inert residue on the surface of the burning polymer reduces the heat release rate. Aluminium hydroxide (ATH) and magnesium hydroxide are the most widely used [1]. Globally aluminium hydroxide is the highest tonnage fire retardant [2,3]. It decomposes according to the following reaction:The endothermic loss of water resulting from the thermal decomposition of ATH has been variously reported [3][4][5] between 1170 and 1300 Jg Magnesium hydroxide is used less widely than ATH. It decomposes through a similar endothermic mechanism to ATH, giving off water. Mg(OH) 2(s) → MgO (s) + H 2 O (g)The endotherm for this reaction is quoted at values between 1244 to 1450 Jg -1 by various authors [3,[5][6][7]. It starts to decompose at about 300 -330°C giving off water [5]. L.A. Hollingbery, T.R. Hull / Thermochimica Acta 509 (2010) 1-112 Although ATH and magnesium hydroxide are the most well known mineral fire retardants, Rothon [5] has identified a number of minerals (Table 1) that could be of potential benefit in polymers. Each decomposes endothermically with the evolution of either carbon dioxide, water or both. Of these minerals, hydromagnesite is the one that has probably seen most commercial interest. Hydromagnesite is naturally occurring...

The fire retardant behaviour of huntite and hydromagnesite – A review

“…The decomposition mechanism of hydromagnesite and huntite was discussed earlier in this paper and in greater detail elsewhere [40]. However, it is worth considering the decomposition mechanisms within the confines of a polymer matrix.…”

“…A recent article [40], by the current authors, reviews this body of work in detail. However a brief summary is presented here in order to explain the fire retardant behaviour of these minerals in polymer formulations.…”

Fire retardant action of mineral fillers

“…In addition to the two most important fire retarding mineral fillers, Al(OH) 3 and Mg(OH) 2 , various mixed magnesium and calcium hydroxides and carbonates have been considered. In addition to quantifying their individual suitability as fire retardants, huntite and hydromagnesite are available as naturally occurring mixtures containing different proportions of the two components, which are not easily separated 25 . This approach allows simple estimation of the fire retardant contributions of different mixtures which are available.…”

The fire retardant effects of huntite in natural mixtures with hydromagnesite

“…This paper forms the final part in a recent series of papers by the current authors highlighting the thermal decomposition and fire retardant behaviour of natural mixtures of huntite and hydromagnesite [1][2][3][4]. Mixtures of huntite and hydromagnesite form naturally and are commercially mined and processed as an alternative to the commonly used mineral filler fire retardants, aluminium hydroxide (ATH) and magnesium hydroxide (MDH).…”

“…Previous authors have discussed the decomposition of huntite [1,3,4,[8][9][10][11] and hydromagnesite [1,3,4,8,[12][13][14][15][16][17][18][19][20][21][22][23][24]. Natural hydromagnesite particles have a blocky morphology [1,2] and once processed the majority of the particles are usually between 1 and 10 µm in diameter depending on the processing.…”