The present invention is concerned with an improved process for the production of an aqueous suspension of hydroxylapatite from calcium hydroxide or calcium oxide, and phosphoric acid.
Hydroxylapatite (Ca.sub.5 (OH)(PO.sub.4).sub.3), hereinafter also referred to as HAP, is used to a very large extent as a teeth cleaning agent, as a bone replacement material, and as a carrier material in the pharmaceutical industry. The production of this product, which is very sparingly soluble in water, usually takes place by the precipitation of aqueous solutions or suspensions of calcium compounds, for example calcium hydroxide, calcium carbonate, or calcium chloride, with a stoichiometric amount of phosphoric acid or of a soluble phosphate at a pH value of from 9.5 to 11. For economic reasons, the preferred process is thereby the reaction of quicklime in aqueous suspension with phosphoric acid which is as highly concentrated as possible according to the following reaction: EQU 5 Ca(OH).sub.2 +3 H.sub.3 PO.sub.4 .fwdarw.[Ca.sub.5 (OH)(PO.sub.4).sub.3 ]+9 H.sub.2 O
It is a problem in the case of this reaction that, on the one hand, the concentration of the lime milk is very difficult to exactly determine since, as a technical product, the lime milk is not normally exactly definable and contains, for example, variable amounts of calcium carbonate and water. On the other hand, the precipitating out of the hydroxylapatite, unreacted calcium hydroxide can easily be entrained or, due to an excess of phosphoric acid, tricalcium phosphate can be co-precipitated as impurity.
Furthermore, it is desirable that the hydroxylapatite is obtained as a finely divided suspension and is not obtained in a lumpy condition due to too rapid precipitation, since only then is a satisfactory further working up ensured. However, this finely divided suspension cannot be filtered or can only be filtered with difficulty. Thus, dry powder must be obtained therefrom by spray drying or in vacuum driers. All impurities contained in the solution are thereby passed over into the end product. Therefore, it is necessary to start from starting materials which are as pure as possible and to carry out the reaction exactly up to the theoretical end point.
U.S. Pat. No. 4,324,772 discloses that a finely divided hydroxylapatite can be produced by passing 80% phosphoric acid into an approximately 30% solution of lime milk with vigorous stirring until a pH value of 9.5 to 11, especially of 10 to 11, and preferably of 10.5, is achieved and subsequently, in a second step, by the further addition of phosphoric acid, there is adjusted a pH value of 7.0 to 7.4 and preferably of 7.2 to 7.3. In this way, in the first step, there is only achieved a reaction of from 70 to 90%, the reaction being completed in the second step of the reaction, a finely divided hydroxylapatite thereby being obtained. The control of the pH value and the speed of stirring are thereby stated to be extraordinarily critical since, due to slight deviation of the pH value from 10.5 or due to incomplete stirring, calcium hydroxide can easily be included in the precipitated product and then, in the case of further reaction with phosphoric acid, can no longer react. The precise maintenance of the pH conditions thereby proves to be difficult because the pH measurement in the suspension obtained is very difficult. Furthermore, the two-step process is not economical.
U.S. Pat. No. 4,274,879 describes a continuous process for the production of hydroxylapatite in which an approximately 20% solution of lime milk is reacted with a 90% phosphoric acid solution at a temperature of 80.degree. to 85.degree. C. and at a controllable pH value of 9.5 to 10.5, whereby, in the case of a residence time of about 1 hour in a flow-through reactor, there results a hydroxylapatite with "less than 5% orthophosphate" (more correctly .beta.-tricalcium phosphate). Here, too, the precise pH measurement is problematical and a product results which cannot be used for many technical processes.
Therefore, there is a need to find a simpler and better controllable process for the production of pure hydroxylapatite.