Method for preparing zinc orthosilicate phosphor

A method for preparing a manganese-activated zinc orthosilicate fluorescent lamp phosphor having the empirical formula:

• Zn.sub.(2.00-x-y) Mn.sub.x SiO.sub.(4.00-y) (WO.sub.3).sub.z
• wherein:
• 0. 04.ltoreq.x.ltoreq.0.15;
• 0.ltoreq.y.ltoreq.0.05;
• and
• 0.ltoreq.z.ltoreq.0.002,
• wherein all of said manganese activator is present as manganese (II) and occupies zinc (II) sites whereby said phosphor has improved white bodied coloration and improved brightness due to the absence of unreacted manganese containing second phases which result in discoloration and reduced brightness, comprising the steps of:
• blending a zinc source, a manganese (II) source, a silicon source, and a tungsten source in amounts in accordance with said formula with NH.sub.4 Cl and NH.sub.4 F in an amount of up to about 2 weight percent NH.sub.4 Cl and up to about 0.2 weight percent NH.sub.4 F to form a relatively uniform admixture, said zinc source consisting essentially of zinc oxide or zinc carbonate, said manganese source consisting essentially of manganese (II) oxide or manganese (II) carbonate, said silicon source consisting essentially of silicic acid, and said tungsten source consisting essentially of tungsten trioxide;
• firing said admixture in a closed container under an inert non-oxidizing atmosphere to promote retention of manganese in the plus two valance state whereby oxidation of manganese to a higher valence state is avoided, the NH.sub.4 Cl and NH.sub.4 F being in an amount sufficient to purge the closed container of residual air and/or O.sub.2 and the NH.sub.4 F being in an amount sufficient to effect a larger size phosphor particle than said method, absent NH.sub.4 F, said firing comprising increasing the temperature from about 700 degrees Centigrade to about 1,250 degrees Centigrade over time period of about 30 minutes, heating the admixture at a temperature of about 1,250 degrees Centigrade for about 2 to about 3 hours, and decreasing the temperature to about 700 degrees Centigrade over a time period of about one hour to form a precursor lamp phosphor containing substantially all of said manganese in the plus two valance state and including unreacted manganese;
• milling and washing said precursor lamp phosphor in an aqueous citric acid solution to remove substantially all of said unreacted manganese to form said fluorescent lamp phosphor, said citric acid solution comprises from about 0.2 to about 0.4 weight percent citric acid;
• rinsing the citric acid washed lamp phosphor with ammonium hydroxide;
• drying the rinsed phosphor;
• and sieving the dried phosphor through a 200-mesh to 400-mesh screen.