Method for the purification of zinc oxide controlling particle size

A continuous method for the recovery of zinc oxide from waste material streams which comprise zinc compounds, comprising the steps of:

• a. roasting said waste material at an elevated temperature and in a reducing atmosphere;
• b. treating said waste material with an ammonium chloride solution at an elevated temperature to form a product solution which comprises dissolved zinc and dissolved zinc oxide whereby any iron oxide in said waste material will not go into solution;
• c. separating said product solution from any undissolved materials present in said product solution including any of said iron oxide;
• d. adding zinc metal and a dispersant to said product solution whereby any lead and cadmium ions contained within said product solution are displaced by said zinc metal and precipitate out of said product solution as lead and cadmium metals and said dispersant is selected from the group consisting of dispersants which will prevent the aggregation of said zinc metal;
• e. separating said product solution from the lead and cadmium metals;
• f. lowering the temperature of said product solution thereby precipitating a mixture of crystallized zinc compounds;
• g. separating said precipitated zinc compounds from said product solution;
• h. washing said precipitated zinc compounds with wash water thereby solubilizing certain of said precipitated zinc compounds;
• i. separating remaining precipitated zinc compounds that have not solubilized from said product solution;
• j. drying said remaining precipitated zinc compounds at a temperature of at least 100° C. whereby a resulting product is zinc oxide of 99% or greater purity by weight;
• k. dissolving said resulting product in a concentrated sodium hydroxide solution;
• l. filtering out any undissolved materials;
• m. dispersing said sodium hydroxide solution into droplets between 100 and 300 microns in size;
• n. combining said droplets with a sufficient amount of 70° C. to 100° C. water to dilute the sodium hydroxide solution by a factor of 3 to 30 by volume, thereby precipitating zinc oxide crystals;
• and
• o. filtering out said zinc oxide crystals.