The
pre-reaction calcination process resulted in a significant increase in
whiteness with increasing phosphorous content, up to 87% whiteness
when phosphorous content is higher than 20%. However, at phosphorous
content below12%, itswhiteness is far belowthe post-reaction calcination
sample. The post-reaction calcinations process, on the other
hand, reached 73% maximum whiteness at 4.5% phosphorous content
and then slightly reduced or unchanged with further increasing phosphorus
content. The observed decrease in whiteness for samples from
the post-reaction calcination was due to the remaining phosphoric
acid in product increasing with increasing the addition of phosphoric
acid. This remaining phosphoric acid carbonizes the organic matter
into fine carbonaceous particles during calcination, which needs a
higher calcination temperature for further removal.