Process for producing hydroxyapatite

A continuous process for producing hydroxyapatite which comprises a two-stage reaction in which pH control and reactant flow are carefully regulated.

BACKGROUND OF THE INVENTION 
This invention relates to the production of hydroxyapatite. 
The batch preparation of TCP by the reaction of lime and phosphoric acid is 
well known. Due to the inherent nature of batch reaction, however, the 
volume of TCP produced thereby is necessarily less than could be produced 
by a continuous process. For example, U.S. Pat. No. 3,387,925 discloses a 
batch reaction of lime and phosphoric acid wherein the particle size of 
the TCP is controlled in order to obtain a precipitate of fine particles. 
In accordance with the invention of the aforementioned patent, it was 
found that a smaller particle size may be produced by controlling the 
final pH, the reaction temperature and the rate of addition of the 
reactants. Thus, the temperature of the reaction is kept within a range of 
from about 20.degree. to 50.degree. C., the lime slurry is continuously 
agitated to prevent localized acid build-up and the 20% aqueous phosphoric 
acid which is used is added to the lime slurry as quickly as possible 
until the lime slurry is neutralized to a pH of between about 6.7 and 8.5. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is disclosed a process for 
producing hydroxyapatite in large volumes which comprises continuously 
charging to the first stage of a two-stage reactor a slurry of calcium 
oxide in water and a solution of phosphoric acid in water. The lime slurry 
and the solution of phosphoric acid in water are then allowed to react in 
the first stage under vigorous agitation at a pH such that the viscosity 
of the reaction mass is near minimum viscosity, i.e. a pH of from about 
9.5 to about 11. The reaction is then continued in the second stage of the 
reactor under vigorous agitation while continuously charging additional 
solution of aqueous phosphoric acid in an amount sufficient to maintain 
the pH in the second stage at about 7.0 to about 7.4. The hydroxyapatite 
reactant product is continuously removed from the reactor. 
SPECIFIC EMBODIMENTS OF THE INVENTION 
The primary reaction of the present invention is as follows: 
EQU 10 Ca(OH).sub.2 +6 H.sub.3 PO.sub.4 .fwdarw.3[Ca.sub.3 (PO.sub.4).sub.2 
].Ca(OH).sub.2 +18 H.sub.2 O 
This reaction gives a 100% yield of product. 
The reaction is conducted in a two-stage reactor wherein the rate of flow 
of phosphoric acid and lime slurry is adjusted so that from about 70 to 
about 94% of the lime slurry is converted in the first stage and the 
remaining portion is converted in the second stage. It is preferred that 
approximately 90% of the reaction be completed in the first stage for best 
quality product. Above about 94% completion of reaction in the first stage 
affords a product which is unsatisfactory because of high bulk density. 
When phosphoric acid is added to lime slurry at a pH above about 7, 
hydroxyapatite precipitates as a solid phase. In order to precipitate 
hydroxyapatite with low bulk density and high surface area, the 
precipitation must be carried out at a pH above about 10. Accordingly, a 
two-stage reactor is required to produce hydroxyapatite on a continuous 
basis. Both stages must have vigorous agitation. The phosphoric acid is 
added in the first stage in a manner suitable to disperse the acid so that 
high local concentrations of acid are avoided. 
The major process variables are (1) the lime/acid ratio, (2) pH and (3) 
agitation. The rates of acid flow and lime flow must be adjusted such that 
about 90% of the lime is converted to hydroxyapatite in the first stage 
and the remaining 10% is converted in the second stage. The ratio is 
controlled indirectly by avoiding high local acid concentration or high 
local lime concentration. 
Control of pH is vital to the successful control of the process. It is 
important to hold the pH in stage one between about 9.5 to about 11. A pH 
of 9.5-10 indicates that the reaction in stage one is approaching 100% and 
the viscosity of the reaction mass approaches a maximum where agitation 
becomes less efficient. A pH above about 11 indicates that the extent of 
reaction is too low in stage one. Deviation from a pH of from about 9.5 to 
about 11, preferably from about 10 to about 11 and, more preferably, at 
about 10.5 indicates that either acid or lime flow is not correct and must 
be adjusted. Phosphoric acid delivery to the second stage is controlled in 
such a manner as to hold the pH at about 7.0 to about 7.4, preferably from 
about 7.2 to about 7.3. It is important to have accurate pH control at 
this point to produce quality product. 
The most critical variable in the process is agitation. Agitation in both 
stages is very difficult because of the high viscosity of the slurry. 
Excellent agitation is required in the first stage to form a quality 
product. Poor agitation in the first stage affords an overflow product to 
the second stage containing entrapped, unreacted lime which cannot be 
reacted in the second stage. This causes the pH of the dried final product 
to go up to pH 8-9 when reslurried in water. Alternately, high local 
concentrations of phosphoric acid must be avoided to prevent the formation 
of dicalcium phosphate which is impossible to convert to the desired 
hydroxyapatite. Efficient agitation is very important in the second stage 
to obtain equilibrium so that the pH of the final product slurry will not 
drift. The viscosity in the second stage is higher than it is in the first 
stage and thus more efficient agitation is necessary. Insufficient 
agitation will result in a rapid, wide change in pH as unmixed portions of 
different pH are carried through the process. 
The temperature at which the reaction is carried out has little effect on 
the product quality or on the efficiency of the process. Accordingly, 
temperatures from 35.degree. to 90.degree. C. are satisfactory.

EXAMPLE 
To a two-stage reactor having a 26.51. stainless steel first stage and a 
glass lined 79.51. second stage equipped with acid and lime feed lines, an 
overflow trough from the first stage to the second stage, product outlet 
in the second stage and high efficiency agitators, was added lime slurry 
(30% suspension in water) and 80% phosphoric acid at a rate such that 31.3 
kg. of lime and 70.8 kg. of acid were added per hour. The flow rate 
provided an acid/lime ratio such that 90% reaction at a temperature of 
about 70.degree.-85.degree. C. occurred in the first stage. A pH of about 
10.5 was held in the first stage by means of flow controllers coupled to 
pH probes. After 15 min., the suspension overflowed to the second stage 
where phosphoric acid (80%) was automatically fed by means of flow 
controllers coupled to pH probes which were calibrated to maintain a pH of 
about 7.2-7.4. After 45 min. in the second stage, a 30% suspension of 
hydroxyapatite in water overflowed to a flash dryer. The suspension was 
flash-dried to produce 45.4 kg./hr. of submicron, powdered hydroxyapatite.