Phosphate solutions and their use as binders

This invention concerns aqueous phosphate solutions and their uses.

Aqueous Mono Aluminum Phosphate solutions have been described for use as binders, cements, or coatings in a variety of applications where thermally stable or quick setting properties are desired. Thus mono aluminum phosphate solutions have been described as a binder in refractory mortars/plastics, and moulded articles and most particularly, high alumina refractories, a coating for silicon steel to provide interlaminar electrical resistance for transformer cores (U.S. Patent No. 2,745,203), a component of a matrix for coating and binding refractory fibers (PCT application No. W085/01676), a reactive component for bonded aggregate structures (U.S. Patent No. 4,419,133), a reactive component of an inorganic resin system utilized as a cement in a variety of applications (U.S. Patent No. 4,141,744 and US 4504555), or a reactive component of a foamed ceramic (U.S. Patent No. 3,148,996). Aqueous Mono magnesium phosphate solutions have also been described for use as binders in refractory applications and for coating silicon steel. Both the mono aluminum phosphate solutions and the mono magnesium phosphate solutions as sold contain free phosphoric acid.

Many of the compositions comprising mono aluminum phosphate solutions are prepared and/or applied out-of-doors, and may require shipping, storage, and/or blending of the mono aluminum phosphate solutions out-of-doors. Mono aluminum phosphate solutions have relatively high crystallization and freezing points, e.g., a 50% solution has crystallization point of +5°F (-15°C) and a freezing point of -10°F (-23⁰C) resulting in difficulty in shipping, storage, blending, or application of aqueous mono aluminum phosphate containing compositions out-of-doors in cold climates. In particular, heated storage facilities, and heated transfer lines are required if temperatures are anticipated below 10°F (-23°C).

We have discovered aqueous phosphate solutions comprising mono aluminum phosphate having depressed crystallization and freezing points, thereby reducing the difficulties involved in the shipping, storage, blending, and application of aqueous mono aluminum phosphate containing compositions in cold climates.

The present invention provides an aqueous phosphate solution, comprising component (i) mono aluminum tris(dihydrogen phosphate) and component (ii) a salt of a metal selected from the group consisting of alkali metals, alkaline earth metals and aluminum and mixtures thereof and an acid at least as strong as phosphoric acid, with the proviso that if the said metal consists of aluminum, then said acid is stronger than phosphoric acid, and component (iii) phosphoric acid. the total weight amount of the components (i), (ii) and (iii) being 3O-55%, the molar percentage of said metal in component (ii) to the total of said metal in component (ii) and aluminum in component (i) being 1-65% and the molar percentage of phosphoric acid (iii) (expressed as H₃PO₄) to total of said metal in component (ii) and aluminum in component (i) being 0.5-100%.

In the solutions of the invention the metal in component (ii) is an alkali metal e.g. lithium, sodium or potassium, or an alkaline earth metal e.g. calcium, strontium, barium or magnesium, or aluminum, or a mixture thereof. The metal salt of component (ii) is derived from an acid at least as strong as phosphoric acid (ie. with a pKa at most that of the pKa for the first dissociation of phosphoric acid) e.g. phosphoric acid itself, or sulfuric, nitric or hydrochloric acids, with the proviso that when the metal in component (ii) is aluminum, the acid is stronger than phosphoric acid e.g. sulfuric. nitric or hydrochloric acids or a mixture thereof. When the metal is an alkali metal or alkaline earth metal, the salt is preferably derived from phosphoric acid. Most preferably the salt in component (ii) is magnesium or calcium bis(dihydrogen phosphate) especially the magnesium salt. Examples of alkali metal salts are sodium and potassium dihydrogen phosphates. Other salts for component (ii) include magnesium sulfate and aluminum sulfate. In a special embodiment of the invention the salt in component (ii) is a mixture of magnesium bis(dihydrogen phosphate) and aluminum sulfate and the solution of the invention is one wherein components (i) and (ii) contain aluminum and magnesium ions and dihydrogen phosphate and sulfate ions.

The molar percentage of metal in component (ii) to the total of metal in component (ii) and aluminum in component (i) is 1-65%, e.g. ₁-60% such as 1-45%, e.g. 2-30%. The percentage is most preferably 6-20% such as 10-20%, e.g. about 14%, giving a solution having a combination of significantly reduced crystallization point coupled with properties as a binder substantially similar to those of the aluminum phosphate solution without adding component (ii). Solutions of the invention can advantageously have a crystallization point as herein after defined of -26.1⁰C to -34.4^oC (-15°F to -30⁰F). For solutions of highly reduced crystallization point for very low temperature use, the molar percentage of metal in component (ii) may be 20-60%, e.g. 20-50%, especially with alkaline earth metals particularly magnesium.

The aqueous solution of the invention usually contains 40-55%, by weight components (i) and (ii) in total especially 45-55% by weight such as about 50 weight %; solutions of 45-53% total concentrations tend to have higher stability compared to ones of higher concentration. A preferred solution contains 40-52%, e.g. 40-50%, by weight of component (i) and 1-15% e.g. 5-15% by weight of component (ii), especially ones with 40-52% of component (i) and conversely 10-1% of component (ii), e.g. mono magnesium dihydrogen phosphate giving a total weight of 47-53% by weight of components (i) and (ii). Thus solutions may contain 1-9%, e.g. 3-7% such as about 5% by weight of component (ii) and 41-49%, e.g. 43-47% such as 45% by weight of component (i). In preferred solutions there is 40-52 wt % of the mono aluminum phosphate, 1-15% wt % of magnesium bis (dihydrogen phosphate) with the molar percentage of magnesium to total of magnesium and aluminum being 5-15%.

Component (iii) in the solutions of the invention is phosphoric acid which is present in a molar percentage based on the total metal in component (ii) and aluminum in component (i) of 0.5-100%, preferably 5-60%, e.g. 5-50%, and especially 10-50% such as 10-25%, e.g. about 20%. The phosphoric acid (iii) (herein after called "free phosphoric acid") is that in excess of the acid which fully reacts with the metals in components (i) and (ii) to form metal (dihydrogen phosphates); no distinction is made between acid present free in the solution and that (if any) which may be complexed with the metal dihydrogen phosphates. The molar percentage of component (iii) phosphoric acid to the metal in component (ii) is usually 0.5-200%.

The above solutions of this invention which comprise mixtures of mono aluminum tris(dihydrogen phosphate), an alkali metal or alkaline earth metal dihydrogen phosphate and phosphoric acid contain an amount of phosphates (e.g expressed as P₂0₅) which is more than that stoichiometrically needed to form the mono metal salts. Thus the P₂0₅ content of the solution is related to the content of metal and aluminum so that the number of equivalents of P₂0₅ is greater than the total number of Equivalents of Al and Number of equivalents of alkali or alkaline earth metal, e.g. as in the formula :where % P₂0₅ is the weight content of phosphate in the solution, A, B and C are the % by weight of A1₂ O₃, alkali metal oxide and alkaline earth metal oxide respectively and Mg and M_c are the molecular weights of the alkali metal oxide and alkaline earth metal oxide respectively. Preferably the P₂0₅ weight content of the solution fulfils the following function.

The solutions especially contain 1-5% by weight e.g. 2-3% weight excess of P₂0₅ over that stoichiometrically required for the amount of aluminum and metals present in the solution.

Because of this content of extra phosphoric acid these solutions are differentiated from earlier described solutions of sodium (or potassium) aluminum phosphate such as those used as food additives of formula Na A1₃ H₁₄ (P0₄)₈ 4H₂0 (see eg. USP 3311448 and USP 4375456) or the corresponding potassium salts crystals of which have also been isolated from wet phosphoric acid sludge solids (see J. Agr. Food Chem. 1966, Vol 14 No 11 pp 27-33). A similar differentiation is also clear from the mixtures of the above sodium aluminum phosphates with calcium dihydrogen phosphates described for use in Baking (see Kirk Othmer Encyclopaedia of Chemical Technology Vol 3 Second Ed. 1964 Page 58 and USP 4375456). The solutions of the inventions are also differentiated from the hot concentrated solutions of USP 2550490 and BP 1586583 from which the food additive sodium aluminum phosphates may be crystallized. Such solutions of the invention are different from the reaction products of magnesium oxide with monoaluminum phosphate binders which give insoluble cements and do not contain the excess of phosphoric acid (see J.E. Cassidy, Ceramic Bulletin Vol 56, No.7 pp 640-43 and J. Ando et al Yogyo-Kyokai Shi Vol 82, No.12, 1974 pp 644-9, Chem. Abs. Yol 82, 1975 pp 102271z).

The amounts of component (i) mono aluminum tris (dihydrogen phosphate), component (ii) mono alkali metal or alkaline earth metal dihydrogen phosphate and water in the solutions may also be expressed in terms of their relative molar percentages, i.e. excluding the free phosphoric acid. In these terms the solutions may contain 2-7 mole % component (i), 0.1-4.5 mole %, e.g. 0.1-4 mole % component (ii) and correspondingly 97-93 mole % water. Preferred mole % of component (i) are 3-7%, e.g. 4-6 mole %. Preferred mole % of component (ii) especially of alkaline earth metal dihydrogen phosphates such as mono magnesium phosphate are 0.1-2%, e.g. 0.2-1.5%. Preferred mole % of water are 96.5-93%, e.g. 95-93.9%. The preferred molar percentages of components (i), (ii) and water are especially preferred in combination with molar percentages of free phosphoric acid of 0.5-2% (expressed as H₃PO₄ on the basis of moles of components (i), (ii) and water) or 10-50 mole % based on the total metal components (i) and (ii). Addition of more phosphoric acid at constant mole % components (i) and (ii) decreases the crystallization point further.

The solutions of the invention may be made by mixing the three components (i), (ii) and (iii), usually by adding metal salt component (ii) to component (i) while component (iii) may be present with either component (i) or (ii) or both, or may be added subsequently. Thus an aqueous solution of monoaluminum phosphate and phosphoric acid may be mixed with an aqueous solution of mono magnesium phosphate and phosphoric acid, both solutions being sold by Albright & Wilson Inc. of Richmond, Virginia. When the salt component (ii) is derived solely from phosphoric acid then it may be convenient to prepare the solution by adding basic compounds of aluminum and metal of component (ii) to an appropriate excess of phosphoric acid. Adding the basic compounds to the acid rather than the reverse increases the stability of the solutions of dihydrogen phosphates obtained. Examples of such basic compounds are alumina, hydrated alumina, aluminum hydroxide, and oxides, hydroxides, carbonates and bicarbonates of alkali metals and alkaline earth metals e.g. magnesium oxide or hydroxide. The corresponding metals may be used instead of the basic compounds. Thus magnesium oxide and alumina may be dissolved in an appropriate excess of phosphoric acid either consecutively or concurrently.

In the case of solutions in which component (ii) is a metal salt of an acid other than phosphoric acid, component (ii) as such may be added to a mixture of component (i) and (iii), or the basic compounds of aluminum and the metal of component (ii) may be added to a mixture of phosphoric acid and the other acid forming the anion in the metal salt, the amount of the other acid being usually equivalent to the metal in component (ii). Thus the basic compounds may be added consecutively or concurrently to a mixture of phosphoric acid and the other acid e.g. sulfuric or nitric acid or a mixture of both; the mixture may be obtained by solvent purification of wet process phosphoric acid e.g. when the acidic solution to be treated with the base is to contain a molar sulfuric to total phosphate percentage of 0.0001-1%. Advantageously a source of such a mixture of acid is the rinse and used liquors from acidic aluminum polishing baths, in which aluminum surfaces are treated with treating mixtures of phosphoric, sulphuric acid and nitric acid to give the surface a bright finish. Such treating mixtures are sold by Albright & Wilson Inc. under the Trade Mark "PHOSBRITE". The rinse and used liquors contain phosphoric acid, sulphuric acid and aluminum, together optionally with small amounts of nitric acid and/or copper. Such liquors (or the solvent purified phosphoric acid) may be mixed with extra phosphoric acid e.g. "thermal" acid if needed to produce an acid solution of the requisite strength for producing the solutions of the invention. Such liquors may for example contain by weight 20-40% phosphoric acid, 2-10% sulfuric acid and 0.1-5% aluminum. Thus overall in these solutions of the invention the molar percentage of sulfate ion to phosphate in components (i), (ii) and (iii) is usually 0.0001-20%, e.g. 5-15%.

Thus in a special embodiment the solution of the invention contains aluminum and magnesium and dihydrogen phosphate and sulfate ions and phosphoric acid, with the proportions as specified above, the molar percentage of sulfate to total phosphate (i.e. dihydrogen phosphate and phosphoric acid) being about 1-29% e.g. 5-15% such as about 10%. The crystallization point of such solutions with the added sulfate is unexpectedly low, and lower than that of the corresponding solutions with sulfate replaced by dihydrogen phosphate.

Analyses of the solutions of the inventions may be for (a) the specific metals Al and e.g. Mg (b) total phosphate e.g. by color or gravimetric methods and (c) free acidity. The amount of free acidity can be found by determination of total acidity (including the acid values of the dihydrogen phosphate salts) and subtraction therefrom of total phosphate. The amount of free phosphoric also can be be found by difference from the total phosphate content and that combined with the specified metals. Solutions of mono aluminum dihydrogen phosphate in the absence of excess phosphoric acid usually have a pH of 4-4.5, while the pH of solutions of the invention is usually 1-3.5, especially 2-3 for solutions of the invention diluted to 1% total solids content.

The solutions of the invention may be used as binders for refractory mortars, e.g. with alumina as aggregate or for quick setting cements with a divalent or trivalent metal oxide e.g. magnesium oxide and/or alumina as curing agent and a wide variety of aggregates varying from ones of low density e.g. of 64-320 kg/m³ (4-20 pounds per cubic foot) or 80-400 kg/m³ (0.08-0.4 g/cc) such as perlite or vermiculite or mixtures thereof for use in insulation or of intermediate density e.g 320-960 kg/m³ (20-60 pounds per cubic foot) or 400-1000 kg/m³ (0.4-1 g/cc) to ones of high density e.g. 800-2400 kg/m³ (50-150 pounds per cubic foot) or 1000-2500 kg/m³ (1-2.5 g/cc).such as sand and gravel, for uses where high compressive strengths are needed e.g. for grouts for use in cracks in roadways. In these uses the curing agent may be mixed with the solution in the presence of aggregate and formed into the desired shape e.g. trowelled into cracks or applied to surfaces. The present invention also provides a two-component pack comprising in the first pack a solution of the invention and in the second pack separated therefrom a mixture of a metal oxide curing agent and an aggregate, the first and second packs being such that they form a set cement on intimate mixing. There is also provided a method of producing a set cement in which an aluminum hydrogen phosphate solution of the invention reacts with a metal oxide curing agent in the presence of an aggregate, e.g. perlite. The solutions of the invention comprising less than 30% molar mono magnesium or calcium dihydrogen phosphate as component (ii) usually give cured products with substantially similar thermal resistance to those without the added component (ii), as do those with less than 20% of mono alkali metal phosphates as component (i). Those solutions with more than 30% molar mono alkaline earth metal phosphate as component (ii) usually give cured products of higher flexibility but lower strength than those without added component (ii).

In this specification the term "crystallization point" of the solution is used to mean the temperature at which crystals are first observed in the solution which is stirred in a vessel which is immersed in an acetone/carbon dioxide bath at about -50°F (about -46⁰C). The term "freezing point" of the solution in this specification means the temperature at which the above solution all crystallizes. The terms are therefore different from those obtained from freezing point curves obtained isothermally.

The invention is illustrated in the following Examples. All parts and percentages in this specification are by weight. The mono aluminum dihydrogen phosphate solution used in the Examples was a commercial product sold by Albright & Wilson Inc. of Richmond Virginia, having the following properties. Analysis A1₂0₃ (specification range 7.8-8.2 typically 7.8%). P₂0₅ (specification range 33.4-34.O%, typically 33.7%. Specific gravity 80⁰F/60⁰F, (27°C/16°C) (specification range 1.47-1.50 typically 1-.485) and free phosphoric acid (pH 1). Crystallization point 5⁰F (-15°C), Freezing Point -10°F (-23°C) and boiling point 220°F (104⁰C). The crystallization and freezing points were determined as specified above.

Example 1-8 and Comparative Example A

Magnesium oxide, phosphoric acid, and water were added to the 50 percent monoaluminum phosphate solution to obtain solutions with from 1 to 25 percent monomagnesium phosphate, respectively. Each solution contained a total of 50 percent solids and an excess of phosphoric acid in amount based on the weight of the solution of 2.5% (expressed as P₂0₅ or 3.5% expressed as H₃P0₄).

Each solution was cooled in the carbon dioxide-acetone bath and the temperature at which crystallization first occurred was recorded in degree Fahrenheit. The molar proportions of the above ingredients can be expressed in terms of relative molar % mono aluminum phosphate (MAP), mono magnesium phosphate (MMP) and water (i.e. excluding the free phosphoric acid). Results were as follows:

X denotes no crystals produced at all.

Examples 9-11

Phosphoric acid water and (i) calcium oxide,(Ex.9) (ii) disodium hydrogen phosphate Ex.lO, or (iii) mono potassium dihydrogen phosphate the (Ex.11) were added to the 50 percent monoaluminum phosphate solution to produce solutions which contained 45 percent monoaluminum phosphate, 5 percent of the respective alkali metal or alkaline earth metal dihydrogen phosphate, and an excess of phosphoric acid in amount (based on the weight of solution) of 2.5% expressed as P₂0₅ or 3.5% expressed as H₃ P0₄.

Each solution was cooled in the carbon dioxide-acetone bath and the temperatures at which crystallization first occurred was recorded.

Results were as follows:

Example 12

A mixture was prepared of 80 grams of the 50 percent monoaluminum phosphate solution and 20 grams of a 37 percent monomagnesium phosphate solution containing MgO : 6.6-6.9% and P₂0₅ : 32.3-34.3% [corresponding to an excess of phosphoric acid of 10% weight (as P₂0₅)] and specific gravity 80°F/60°F, (27/16⁰C) of 1.475-1.500.

The mixture thus contained 40 wt% monoaluminum phosphate and 7.4 wt% monomagnesium phosphate (15.6% weight based on total of metal phosphates or 21.2% molar based on total of Mg and Al) and extra phosphoric acid (21.7% molar based on the total of Mg and Al). The mixture, which was a solution at room temperature was cooled in the carbon dioxide acetone bath and the temperature at which crystallization first occurred was recorded. The results were as follows:

The temperatures at which the original component solutions first crystallized and then completely froze were measured to be:

Example 13

Magnesium oxide of 99.99% purity and phosphoric acid of 85.2% H₃P0₄ concentration and specific gravity 1.686-1.692 were added to the 50 percent monoaluminum phosphate solution to obtain a solution containing monoaluminum phosphate 45%, monomagnesium phosphate 5% and excess phosphoric acid in amount of 2% (expressed as P₂0₅ or 2.8% expressed as N₃PO₄).

The solution contained in molar percentages (based on the total of Mg and Al) 13.9% magnesium dihydrogen phosphate and 17% excess phosphoric acid. The solution was cooled in the carbon dioxide-acetone bath and the temperature at which crystallization first occurred was recorded.

Results were as follows: Crystallization point -18°F (-28°C), Freezing point -29°F (-34⁰C).

Example 14

A refractory composition was made mixing 37.4 parts of calcined alumina, 15.9 parts of magnesium oxide and 46.6 parts of the solution of Example 3. The composition set rapidly at room temperature to give a solid cured refractory product.

Example 15, 16 and Comparative Example

In a similar manner to that of Ex.1, solutions were prepared of mono aluminum tris (dihydrogen phosphate MAP) and free phosphoric acid and variable amounts of magnesium bis dihydrogen phosphate (_MMP). The crystallization point (CP) of these solutions (°F) was determined. The results were as follows:

In terms of the molar percentages of MAP, MMP and water based on the total of MAP, MMP and water and excluding the phosphoric acid content, the above results can also be expressed as follows:

Examples 17-20

In a similar manner to that of Ex.1, aqueous solutions were prepared containing MAP, MMP and variable percentages of phosphoric acid. The crystallization point CP of the solutions in °F was determined. The results were as follows.

In terms of the relative moles % of MAP, MMP and water (excluding phosphoric acid), the results may be expressed as follows:

Examples 21-25

A solution was prepared containing 45 wt % MAP, 5% wt MMP and free phophoric acid 3.4% and its crystallization point was determined. The solution was progressively diluted with water to concentration as listed below and the crystallization points determined. The results were as follows, including the relative molar % MAP, MMP and water (excluding the free phosphoric acid).

Example 26

An aqueous feedstock acid solution of phosphoric acid, sulfuric acid and aluminum obtained as wash and rinse liquor from an aluminum bright-polishing process contained by weight

• 28.5% H₃P0₄, 6.5% H₂SO₄, 0.8% Al (as A1₂0₃)
• 0.4% nitric acid, 0.04% benzotriazole, 244 ppm copper and 326ppm nitrogen (as ammonium salt).

To a mixture of this feedstock acid (51.7 parts) and 85% thermal phosphoric acid (37.5 parts) was added with stirring 0.92 parts MgO and then to the hot solution obtained was added with stirring 9.86 parts hydrated alumina (A1₂0₃.3H₂0) to give a solution which contains aluminum and magnesium ions and hydrogen phosphate and sulfate ions in weight amounts of 7.4% Al (expressed as A1₂0₃), 0.95% Mg (expressed as _MgO), 33.6% phosphate (expressed as P₂0₅) and 3.6% sulfate (expressed as H₂S0₄), together with the small amounts of nitrate, ammonia, copper and benzotriazole. The solution had a total concentration of 53.0 wt %, contained a molar % of Mg to total of Mg and Al of 13.8%, a molar % excess of H₃P0₄ to total of Mg and Al of 46% and a molar % of sulfate to total phosphate of 7.8%. The solution also contained in relative molar terms monoaluminum dihydrogen phosphate 5.03%, monomagnesium acid sulfate 0.64%, monomagnesium di.hydrogen phosphate 0.17% and water 94.15% excluding the free phosphoric acid.

The crystallization point of the above solution was measured as described in Ex. 1 and found to be <40°F (-40°C) (no crystals were observed).

In a comparative experiment, in which the magnesia was replaced by a corresponding weight % of alumina trihydrate to give a final solution containing 7.8% A1₂0₃ instead of 7.4% A1₂0₃ and 0.95% MgO, the crystallization point of the final solution was found to be -20⁰F (-28.9⁰_C).