A phosphorous pentoxide producing method includes forming pre-feed agglomerates containing phosphate ore particles, carbonaceous material particles, and silica particles and heating the pre-feed agglomerates in a reducing or inert atmosphere to an induration temperature from above 900 C to less than 1180 C and maintaining the induration temperature for 15 minutes or more. The method includes forming feed agglomerates and increasing a compression strength of the feed agglomerates to above 25 lbf using the heating, the feed agglomerates exhibiting a calcium-to-silica mole ratio less than 1 and a silica-to-(calcium+magnesium) mole ratio greater than 2. A reducing kiln bed is formed using the feed agglomerates, kiln off-gas is generated, and phosphorous pentoxide is collected from the kiln off gas.