Dr. Ralf Giskow, The Variety of Phosphates Jörg Lind, Erwin Schmidt Chemische Fabrik Budenheim KG, for Refractory and Technical D-55257 Buden- heim www.budenheim- Applications by the Example cfb.com of Aluminium Phosphates

In 1669 the chemical element phos- Phase-I-content influences, for phorus was discovered by H. Brandt, example, the dissolution property of an alchemist. In 1694 Boyle made STPP in water. The choice of the the first phosphoric acid by dissolv- right phosphate can already be the ing phosphorus pentoxide (P2O5) in crucial point even with such a “sim- water. This was the start of the phos- ple“ phosphate like sodium tripoly- phorus chemistry. Since this time phosphate. phosphoric acid and its salts have a The next step into the direction firm place in chemistry and technical of complexity is a phosphate called applications. Phosphates are part of sodium hexametaphosphate (SHMP) our life and are used in a variety of in colloquial language. In fact, the ways also in industrial fields like chemical name is a mistake. The refractories, glass, ceramics, con- name sodium hexametaphosphate struction industry and for a lot of would stand for a sodium phosphate other technical purposes. with a ring structure (“meta”). But SHMP has a chaintype structure as can be proved. These melted glassy polyphosphates (SHMP) with pH-val- Fig. 1 Phosphates in Refractory phosphates are polyphosphates with ues between three and nine, most of FABUTIT 734, a modified STPP and Technical Industries different chain lengths. The average the products also in an instantised against a standard The most common and well known chain length and thus the properties quality (Fig. 2). STPP phosphates for technical applica- of the phosphate, like the pH-value The mentioned examples illustrate tions are sodium tripolyphosphate for example, are controlled by the the multiplicity of the phosphates. production process. Additionally the Beside STPP and SHMP, ammonium (Na5P3O10), sodium hexametaphos- final product can be refined, for phosphates, aluminium phosphates, phate [(NaPO3)n] or monoalumini- example by the process of instantis- boron phosphates, calcium phos- um phosphate (Al(H2PO4)3). These phosphates are used as defloccu- ing. This leads to sodium polyphos- phates, alkaline phosphates, magne- lants, additives or binders. Sodium phates which have an improved solu- sium phosphates or zinc phosphates tripolyphosphate, abbreviated STPP bility and dissolution speed. At the are used in different technical areas. (sodium tripolyphosphate), is a same time the hygroscopicity is re- To explain the variety of possible uses commonly used deflocculant for clay duced. The Chemische Fabrik for phosphates in the field of refracto- or materials containing clays in the Budenheim manufactures sodium ries, additives, glass or enamel, the ceramics industry, in refractories or in oil and gas prospect drilling. Already in these applications, phos- phates show their variety. STPPs are not all the same. Modifications exist, which have a remarkable influence on the properties. FABUTIT 734 is an example for a chemical modified sodium tripolyphosphate. This pro- duct has an improved solubility, which effects the liquefaction in a positive way. Fig. 1 shows the result of a comparing test between a modified product like FABUTIT 734 against a commercial standard STPP. In addition, STPP exists in two differ- ent crystal structures, a low and a high temperature phase. According Fig. 2 to the production process the BUDIT 3 to 9 as possible variations amount of the high temperature of sodium poly- phase can be adjusted. This so called phosphate cfi/Ber. DKG 81 (2004) No. 5 E 1 and modifications. Starting point is This intense exothermic reaction always the reaction between phos- must be controlled by the proper phoric acid and a source of alumini- manufacturing process. The result um, like aluminium hydroxide, is an aqueous solution marketed which leads to a monomer phos- with a concentration between 40 phate. The next step is the polymeri- and 60 %. sation process by temperature (Fig. The raw materials for the production 3). Polymerisation is the union of of monoaluminium phosphate are two or more molecules of given already important for the final quali- structure to form a new compound, ty. Chemische Fabrik Buden- usually accompanied by a thermal heim uses only a clear, white phos- loss of water which results into the phoric acid of food quality. In the formation of di-, tri- or poly-phos- production process care is taken to phates. ensure the correct basicity, the rela-

In 1975 Tsuhako, Japan, summarized tion of Al2O3 to P2O5, which has to the formation of aluminium phos- be nearly exactly 1:3. This guaran- phates by thermal treatment (Fig. 4). tees that no free phosphoric acid is The following illustrates the relevant present in the monoaluminium aluminium phosphates in the areas phosphate. Fig. 3 group of aluminium phosphates has of refractories, ceramics, glass and Principle of poly- been chosen and will be described in merisation/types of accompanying technical applica- the following abstract. Monoaluminium Phosphate phosphates tions. The most well-known type of alu- – All Liquids are not Alike minium phosphate is of course the The use of the raw materials, the cor- monoaluminium phosphate. But the Monoaluminium rect manufacturing process and family tree of aluminium phosphates chemical modifications leads to is comprehensive and the possible Phosphate many chemical binders, which are all uses of aluminium phosphate are The manufacturing process of mono- covered by the generic term of extensive. aluminium phosphate from phos- monoaluminium phosphate. Mono- phoric acid and aluminium hydroxide aluminium phosphate produced Aluminium Phosphates – can be described by the following with technical phosphoric acid equation of reaction: Chemistry and Application always lead to technical monoalu- minium phosphates, which usually Aluminium phosphates exist in a 3 H3PO4 + Al(OH)3 are unstable solutions of bad stora- variety of stages of polymerisation --> Al(H2PO4)3 + 3 H2O bility, where impurities tend to floc- culate and sediment. A wrong basic- ity for example gives either a very unstable solution or very “reactive” solutions with a high amount of free phosphoric acid. Beside this, many technical monoaluminium phos- phate liquids contain impurities like sulphur, heavy metals or fluorine (!), which are according to experience detrimental to any installations and facilities. This is the main reason why the Chemische Fabrik Buden- heim only uses high-quality raw materials. The torture of choosing applies also to monoaluminium phosphate. The first monoaluminium phosphate liquid was produced in the roaring twenties and used as binder for chamotte. The outrider for the industrial use was the Metallge- sellschaft AG (later the Chemetall GmbH) with the development of a binder called “Feuerfestbinder 32”. This binder was based on a clever idea, covered by a publication with the headline “Mineralleim zur Bindung von Chamotten” (= engl. mineral glue to bind chamotte). This Fig. 4 “mineral glue” had a similar compo- The formation of sition as monoaluminium phos- aluminium phos- phates by thermal phate. The Metallgesellschaft turned treatment [Tsuhako] this proposal to advantage and

E 2 cfi/Ber. DKG 81 (2004) No. 5 developed the chemical binder FFB Additional modifications are possi- 32 using a sludge from amblygonite, ble. Today many different concen- a lithium-aluminium-phosphate, trations of liquid monoaluminium and phosphoric acid. This was the phosphates are found, or the birth of phosphate-based bonding amount of free phosphoric acid is agents in Germany. adjusted. The production of a mono- FFB 32 still exists. It is a clear, pure aluminium phosphate in powder 50 % monoaluminium phosphate type form is also possible. The varied liquid for ceramics and refractories, use of such orthophosphate is re- produced by Chemische Fabrik markable. Budenheim, which took possesion of The most common and well known the binders activities of Chemetall at application for monoaluminium the end of 1999. Today monoalu- phosphate is the bonding of refrac- minium phosphate, like FFB 32, is tories. Besides raw materials based mentioned in the literature, but their Fig. 5 The effect of an widely used as bonding agent or on Al2O3, also basic raw materials existence has not yet been proved. It inhibited phosphor- coating. Monoaluminium phos- can be used, which form quick set- is to be accepted that such a dialu- ic acid coating on phate reacts with oxidic and non ting refractories. Among other minium phosphate is an unstable steel plates after oxidic raw materials by the forma- things the strong exothermal and intermediate stage of the polymeri- storing over night tion of phosphate salts, even at low very fast reaction between MgO and sation process. temperatures. This is one of the main monoaluminium phosphate is used Aluminium tripolyphosphate how- advantages of this sticky chemical to produce cold setting systems. ever exists in two different forms, an binder. Starting at about 100 °C, Examples for such self setting mate- anhydrous product and a dihydrate. bonded materials already have a rials are repair mixes or modern During the manufacturing process high strength, which constantly materials like self setting inorganic intermolecular water from the used inceases until the ceramic bond foams, which isolate and which are monoaluminium phosphate is dri- comes into effect. From 300 to fire proof (Fig. 6 ). ven off. The resulting form is decid- 350 °C the phosphate changes to an Monoaluminium phosphate is also ed by the production temperature insoluble form. This is the reason needed for the oxidation protection. and the after-treatment. why this temperature is often re- In this case the phosphate is used as The following equation of reaction commended when using monoalu- coating to form a protective layer on describes the principle of polymeri- minium phosphate. As a matter of graphite materials to reduce the sation: principle the usage of monoalumini- deflagration of carbon. Another um phosphate is followed by lower application for monoaluminium Al(H2PO4)3–> AlH2P3O10 + 2 H2O firing temperatures as well as by a phosphate is so called electro insu- cost reduction in energy. lating plates. In this case the proper- By firing at about 250 °C an alumini- But chemistry of course found ways ty of monoaluminium phosphate to um tripolyphosphate anhydrous is to modify monoaluminium phos- have a strong affinity to metal is produced, a material very rarely phate advantageous. Also the use as binder used in technical applications. But for fibre materials is known. The this material is the basis for alumini- Example: bond with monoaluminium phos- um tripolyphosphate dihydrate. The phate is brittle and hence not flexi- Inhibited Solutions lost intermolecular water can be ble, but the binder can withstand restored using thermal energy and FFB 705 is an example for such a very high temperatures. This is a water, which is done in a subsequent solution containing an inhibitor. In remarkable advantage compared second manufacturing step. This the manufacturing process an with organic binders, like phenolic leads to a phosphate, which is hard- inhibitor is added and integrated resins. ly soluble in water and acids. The into the chemical structure. Despite Other applications and uses are not main application for this phosphate the very low content of the inhibitor only possible, but are found in prac- is the use in corrosion protection, Fig. 6 included in the structure, the effect tice. Monoaluminium phosphate when aluminium tripolyphosphate is Inorganic foam penetrates applications which are far bonded with of the inhibitor is remarkable. The used to manufacture corrosion pro- monoaluminium inhibitor is added to prevent or away from the classic ceramics or tective pigments. phosphate reduce undesired reactions with refractories industry. The Chemische metallic iron, which are often includ- Fabrik Budenheim always tries to find ed in minerals and raw materials, new fields of application, according caused by the processing or by the to the slogan “ Off the beaten track”. deposit. The reaction of the metallic iron and the acid binder causes an Aluminium undesired gas development. Additionally the inhibitor protects Tripolyphosphate the facility and aggregates, because Monoaluminium phosphate is the it reduces the attack on steel and base for the production of alumini- iron. The effect of an inhibitor is um tripolyphosphate (AlH2P3O10) shown by a test with steel plates, and additional polymeric aluminium which were stored over night in phosphates. Aluminium tripolyphos- phosphoric acid. Fig. 5 illustrates the phate is the next step of stable alu- attack of phosphoric acid and the minium phosphates. Other interme- protection of an appropriate diate stages like dialuminium phos- inhibitor. phate for example, are sometimes cfi/Ber. DKG 81 (2004) No. 5 E 3 AlPO4: berlinite Aluminium Metaphosphate/ <=> (705 °C) tridymite Aluminium Polyphosphate <=> (1 025 °C) cristobalite <=> (~ 1650 °C) melt Aluminium metaphosphate and alu- minium polyphosphate are manu- The inversion temperature at 575 °C factured from monoaluminium between the low temperature quartz phosphate by a polymerisation and high temperature quartz is also process. The boundary conditions well known. Also in this case the during the polymerisation deter- behaviour is similar to trialuminium mine the characteristic of the final phosphate. But the inversion tem- aluminium phosphate, whether it perature of berlinite with 586 °C is a will become a ring-type aluminium little higher. Meanwhile at least metaphosphate or an aluminium six types of AlPO4 do exist, in ac- polyphosphate with a chain struc- cordance to the main variants of ture. Both products are nearly com- Fig. 7 But also aluminium tripolyphos- quartz. pletely dehydrated, but due to the Phosphate glass phate is a worthwhile candidate for After all this theoretical description distinctive structures, the properties products application in new uses and to the question arises for what purpos- of the final phosphate are quite dif- encounter new properties. At the es the industry uses such phos- ferent. Both phosphates are insolu- moment the use of aluminium phates. The berlinite type material ble in water and acids, but soluble in tripolyphosphate is insignificant in does have so called piezoelectric a strong alkaline environment. Due refractories or the construction in- properties and is used for electronic to its more stable ring structure alu- dustry, but Chemische Fabrik Buden- devices. minium metaphosphate dissolves heim is looking for possible applica- AlPO4 has a three dimensional struc- much slower than the chain struc- tions and possible modifications, ture. Its hardness and insolubility tured aluminium polyphosphate. either of chemical or of thermal kind renders possible other applications. The use and applications of the to modify this phosphate in a useful The structure and the apparent phosphates are closely assigned to way. An interesting field for example porosity, especially of the hydrated these properties. might be the incorporation into materials (Zeolite structure) make it Aluminium metaphosphate is main- waterglass bonded systems to adjust possible to use such phosphates for ly used in glass and enamel. The very

the set time and to guarantee hard- the chemical catalysis, the desicca- high amount of P2O5 of about 80 % ening of such mixes. tion of gas or additional chemical makes this phosphate an ideal phos- processes, like adsorption or ion phorus source for glassy materials. Trialuminium Phosphate exchange. The variety of aluminium phosphate All these areas have nothing in com- glasses (Fig. 7) combined with

Trialuminium phosphate AlPO4 mon with ceramics and adjacent the high demand in quality regard- exists in many different variants with applications. But also here the trialu- ing colouring elements and impuri- different crystal structures. minium phosphate is suitable. The ties stretches from laserglass, optical On the basis only of the manufactur- use for example in glass is known. glasses, filtering glasses to a glassy ing process two forms exist. The first The structural relationship of AlPO4 ceramic with a very low coefficient of is a thermally treated variant (trialu- to SiO2 is already pointing into this expansion used as carrier for minium phosphate, heavy, anhy- direction. Both materials are known telescope mirrors as well as in drous), the second one is prepared by as materials having the capability of the production for heat resistant precipitation and forms a lightweight, forming a glassy network by melting glasses.

hydrated product. The variants pre- or in a glass melt. In the enamel industry the high P2O5 pared by a thermal process are insol- According to this property trialu- content is used mainly as a fluxing uble in water and acids. The precipi- minium phosphate is used for exam- agent because of the glass network

tated variant on the other hand tends ple in glassy ceramic for dental building property similar to SiO2. to dissolve in distinct acids. In both application as well as in glassy Contrary to SiO2 the phosphate cases the trialuminium phosphates coatings in ceramics or enamel. increases the coefficient of expan- dissolve in an alkaline environment Glasses based on aluminiumphos- sion of an enamel and also reduces and form aluminates and/or alkali phates are characterised by a very the melting point. The alumina con-

phosphates. AlPO4 has a very high good resistance against hydrofluoric tent of the aluminium metaphos- melting point of approx. 1 850 °C acid (HF). Therefore this types of phate improves the resistance of the and its crystal structure is isomor- glasses are used for so called glassy coating and also has a net-

phous to quartz (SiO2). This signifies ionomeric glasses. Such glasses are work building functionality. that trialuminium phosphate also employed in dental care, for exam- Aluminium metaphosphate is already exists in the crystal structure of tri- ple in ionomer glass cements below widely used in the industry, in con- dymite and cristobalite. The conver- tooth-bridges or as filling for dental trast to aluminium polyphosphate sion temperatures of quartz in com- applications. Such organic/inorgan- with the chain structure, which, for

parison to AlPO4 clarify the relation- ic cements often contain fluorine, the moment, has just a minor use. ship between these mineralogical which are needed for healthy teeth. The manufacturing process of alu- phases. The fluorine is slowly released from minium polyphosphate is much more the cement, but without destroying difficult and requires very accurate

SiO2: quartz the cement itself. The trialuminium production steps. Nevertheless, alu- <=> (867 °C) tridymite phosphate in the glass prevents minium polyphosphate has very <=> (1 470 °C) cristobalite the destruction of the ionomer interesting properties which make it <=> (1 713 °C) melt cement. suitable for specific technical applica-

E 4 cfi/Ber. DKG 81 (2004) No. 5

tions. Mainly the fact, that aluminium phates in particular. It is intended to phosphates is the driven force of polyphosphate is more easily dis- give an impression about the chem- Chemische Fabrik Budenheim. solved in an alkaline environment istry and the use of phosphates and allows this phosphate to be used in should give an idea for what kind of some areas of the construction indus- applications this products might be References try. useful. It is always worthwhile to look On the other hand it is noticeable into the complex inorganic phos- (1) Über das Abbinden von keramischen Rohstoffen mit Monoaluminiumphos- that polymer aluminium phosphates phate chemistry when it comes phat-Lösung (Feuerfestbinder 32) Teil 1 arise in monoaluminium phosphate to questions regarding chemical & 2; Ber. DKG 37 (1960) [8], 40 (1963) bonded refractories. This brings us binders, additives or raw materials [7] back to the beginning and we have based on P2O5. (2) Aluminiumoxid und seine Bindung mit come to full circle. The development and even the Monoaluminiumphosphat; R. Appelt; knowledge of aluminium phos- Sprechsaal 116 (1983) [9] (3) Inorganic Phosphate Materials, phates and other phosphates are far Summary T. Kanazawa (ed.), Materials Science from being complete. New manu- Monographs 52 (1989) This paper is an overview about facturing processes, new modifica- (4) Phosphorus, D.E.C. Corbridge, Elsevier phosphates and aluminium phos- tions and new ideas how to use 1978 und 2000

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