Refractory Binding Agent--Types, Preparation Method And Application Of Sodium Polymerized Phosphates

Phosphoric acid and orthophosphate (such as aluminum phosphate) will be alkaline refractory raw materials in the alkaline oxides such as MgO, CaO, etc. to produce a violent neutralization reaction of instant solidification and difficult to construction, at the same time, solidification is too fast is difficult to form a dense structure. Therefore, alkaline materials containing magnesium sand, calcium sand and magnesium-calcium sand can not use phosphate as binding agent. Alkaline amorphous refractories and unburnt bricks mostly use polymerized phosphate (mainly polymerized sodium phosphate) as a binding agent. In addition, polymerized phosphate can also be used as a dispersant (water reducing agent) for various materials of refractory castables. In the ceramic industry and the wet preparation of refractory mud, sodium polymerized phosphate is also a good mud water reducer.

01 Classification of Sodium Polymerized Phosphate

Sodium polymeric phosphates can be classified into sodium polyphosphate, sodium metaphosphate, and sodium ultraphosphate based on the Na₂O/P₂O₅ molar ratio (R), as shown in Table 1.

Various types of polymerized phosphates can be further subdivided according to the degree of polymerization (n). For sodium polyphosphate, when n = 2 (i.e., Na₄P₂O₇), it is called sodium dipolyphosphate (also known as sodium pyrophosphate); when n = 3 (i.e., Na₅P₃O₁₀), it is referred to as sodium tripolyphosphate. For sodium metaphosphate, when n = 6 (i.e., Na₆P₆O₁₈), it is called sodium hexametaphosphate. The compositions of some common polymerized sodium phosphates are shown in Table 2, among which sodium dipolyphosphate and sodium hexametaphosphate are the most commonly used binding agents for alkaline refractories.

02 Sodium Tripolyphosphate

(1) Preparation Method

Sodium tripolyphosphate is produced through a neutralization and polymerization reaction using orthophosphoric acid and soda ash as raw materials. The production process is divided into three stages:

① Phosphoric acid reacts with soda ash to produce a mixture of sodium monohydrogen phosphate (Na₂HPO₄) and sodium dihydrogen phosphate (NaH₂PO₄). The reaction formula is:

3H₃PO₄ + 2.5Na₂CO₃ + nH₂O → 2Na₂HPO₄ + NaH₂PO₄ + (n + 2.5)H₂O + 2.5CO₂↑

② The degree of neutralization of the mixture is controlled (i.e., the molar percentage of sodium monohydrogen phosphate in the total amount of sodium monohydrogen phosphate and sodium dihydrogen phosphate). According to theoretical calculations, when condensing Na₂HPO₄ and NaH₂PO₄ into sodium tripolyphosphate, the degree of neutralization should be maintained at 66.67%. At this ratio, the theoretical contents of sodium tripolyphosphate and P₂O₅ in the product are 100% and 57.9%, respectively.

③ The resulting sodium phosphate mixture is dehydrated and heated to above 300 °C, where it condenses into sodium tripolyphosphate. The reaction formula is:

2Na₂HPO₄ + NaH₂PO₄ → Na₅P₃O₁₀ + H₂O

To produce higher-purity sodium tripolyphosphate, industrial processes commonly use spray drying or film drying methods. The purity of sodium tripolyphosphate produced from thermal phosphoric acid and soda ash is generally higher than that produced from extractive phosphoric acid.

(2) Properties and Technical Conditions

Sodium tripolyphosphate is a self-colored powder that appears slightly yellow or gray when the purity is low. It has a bulk density of 0.48–0.72 g/cm³ and a melting point of 622 °C. Sodium tripolyphosphate is almost insoluble in water at 0 °C. Between 0–50 °C, its solubility ranges from 14.5 to 16.5 g per 100 g of water, with little variation. However, its solubility increases rapidly as the temperature rises above 50 °C. The pH value of its aqueous solution is between 9.4 and 9.7.

Sodium tripolyphosphate exhibits some hygroscopicity in humid environments, but this is significantly lower than that of sodium hexametaphosphate.

In industrial-grade sodium tripolyphosphate, the content of sodium tripolyphosphate ranges from 85% to 96%. It also contains 4% to 15% sodium pyrophosphate, along with small amounts of sodium orthophosphate and sodium metaphosphate. Table 3 shows the codes, specifications, and technical conditions of industrial sodium tripolyphosphate according to the Chinese standard GB9983-88.

Sodium tripolyphosphate dissolves in water to form sodium phosphate and sodium dihydrogen phosphate. These two compounds react with alkaline refractory materials containing MgO to produce sodium and magnesium phosphates, resulting in a bonding effect. It is used as a binder for alkaline refractory materials. Whether used in spray fillers or non-burning products, it provides fast hardening and high strength.

The reaction products formed between sodium tripolyphosphate and alkaline refractory raw materials also have high melting points. For example, Mg(H₂PO₄)₂, Mg(PO₃)₂, MgHPO₄, and Mg₂P₂O₇ have melting points of 1165 °C and 1382 °C.

Sodium tripolyphosphate undergoes polymerization when heated, which enhances the material's strength and prevents structural loosening due to phase changes. As a result, the bonded material maintains high strength from room temperature to medium temperatures.

After the appearance of a liquid phase at high temperatures, the material's strength remains significantly higher than that of materials bonded with magnesium sulfate, magnesium oxide, or water glass, even though some reduction in strength occurs under thermal conditions. Additionally, magnesium-based materials using sodium tripolyphosphate as a binder exhibit good thermal shock stability.

03 Sodium Hexametaphosphate

Sodium hexametaphosphate is one of the glassy sodium phosphate salts and is also known as "Graham's salt" because it was first discovered by Graham. It is produced by first reacting soda ash with orthophosphoric acid to form sodium dihydrogen phosphate. This intermediate is then subjected to heating, dehydration, and polycondensation to form sodium hexametaphosphate. The reactions are as follows:

2NaH₂PO₄ → Na₂H₂P₂O₇ + H₂O (at 150 °C)

Na₂H₂P₂O₇ → 2NaPO₃ + H₂O (at 270 °C)

6NaPO₃ → (NaPO₃)₆ (at 620 °C)

The resulting sodium hexametaphosphate melt is glassy, with a Na₂O/P₂O₅ molar ratio (R value) of 1 (the molar ratio for glassy sodium phosphate salts ranges from 1.0 to 1.7).

Sodium hexametaphosphate appears as a flaky or lumpy vitreous material and becomes a white powder after crushing. It is hygroscopic, extremely soluble in water, and can be mixed with water in any ratio. The aqueous solution is weakly alkaline, with a pH of 6.0 to 8.6.

In water, sodium hexametaphosphate hydrolyzes into sodium dihydrogen phosphate, and this hydrolysis is accelerated as the temperature increases. The presence of metal ions also promotes hydrolysis, in the following order of effectiveness:

Al³⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ > Li⁺ > Na⁺ > K⁺

Industrial sodium hexametaphosphate contains 65%–68% P₂O₅ and less than 0.15% water-insoluble matter. It dissolves slowly in water when in block or flake glass form and should be crushed into a powder to accelerate dissolution when used as a binding agent for refractory dressings.

When used as a binding agent, sodium hexametaphosphate hydrolyzes into sodium dihydrogen phosphate (NaH₂PO₄) upon contact with water. NaH₂PO₄ reacts with alkaline-earth metal oxides, such as magnesium sand, to form Mg(H₂PO₄)₂ at room temperature. Mg(H₂PO₄)₂ then transforms into MgHPO₄ shortly after drying. MgHPO₄ has strong adhesive properties and allows the mixture to harden rapidly. Although Mg(H₂PO₄)₂ is used as a binder in refractory dressings, it is not used in fireproofing materials.

Both Mg(H₂PO₄)₂ and MgHPO₄ condense into magnesium polyphosphates-[Mg(PO₃)₂]ₙ and [Mg₂P₂O₇]ₙ, respectively-when heated to around 500 °C. This condensation significantly improves bond strength, which remains high over a wide temperature range (up to 800 °C), before the appearance of a liquid phase.

Sodium hexametaphosphate is mainly used as a binding agent in magnesium and magnesium-chromium unfired bricks, castables, and alkaline spray patching materials. For castables, the concentration of the aqueous solution should be 25%–30%. The amount added is generally 8%–18%, and it should be kept as low as possible while ensuring ease of mixing, in order to maintain the material's high-temperature performance. Coagulant promoters can include aluminum cement or other calcium-containing materials, such as lime powder.