EFFECT OF PHOSPHATE AND SILICATE ON THE CRYSTALLIZATION OF GIBBSITE FROM OH-AL SOLUTIONS

Samples were prepared by adding varying amounts of NaH2PO4or Na2SiO3to a hydroxy-aluminum solution (0.02 M in Al; NaOH/Al mole ratio = 2.6) and diluting to 0.002 M in Al. Gibbsite was observed in the control 2 weeks after preparation, and its amount increased with time, accompanied by a decrease in pH. Gibbsite was also observed after 7 months in the sample containing 0.125 eq SiO32−per mole of Al. With the addition of 0.25 eq or more SiO32−or 0.1 eq or more H2PO4−per mole of Al, no crystalline Al(OH)3was observed in any sample for at least 3 years. The results in this report and the literature suggest that the nature and concentration of the counteranions play a critical role in governing the crystallization of Al(OH)3from OH-Al polymers. Different anions can be arranged in a series in accordance with their effectiveness in inhibiting the crystallization of aluminum hydroxide: Phosphate > silicate > sulfate > chloride > nitrate > perchlorate. © 1979 The Williams & Wilkins Co.