NOVELTY - Recyclable esterification or transesterification catalyst system comprises kiln dust or Portland cement, and an (1-5C)alkanol, where the surface area of the kiln dust or Portland cement is 0.05-10 m2/g, and the kiln dust or Portland cement has been activated by contact with (1-5C)alkanol at 20-100 degrees C.    USE - The catalyst system is useful to catalyze esterification and/or transesterification reactions, e.g. for preparation of biodiesel. The kiln dust (cement/lime kiln dust) is useful for converting a variety of feedstock acids and/or esters to biodiesel such as methyl soyate (all claimed).    ADVANTAGE - The esterification or transesterification catalyst system is recyclable. In the method, the kiln dust is recovered and reused in a subsequent method for preparing the fatty acid (1-5C)alkyl ester. The formation of the ester is carried out without added solvent other than (1-5C)alcohol. The catalyst system is efficient, inexpensive and environmental friendly. The kiln dust converts a variety of feedstock acids and/or esters to biodiesel in high yield under mild conditions. The catalyst system does not have the problems of solubility, separation and recyclability. The kiln dust when dispersed in alcoholic solvent such as methanol and when heated for a period of time provides a robust catalyst composition that can be recycled for several times. The catalyst can maintain the catalytic activity after more than five times (preferably more than 20 times) of using and recovering the catalyst. The kiln dust acts as an efficient catalyst with not only vegetable oils but also with oils with high free fatty acid contents (e.g. chicken fats and cooking oils), which is a significant problem with most of the currently used catalysts. The kiln dust can be easily separated from the reaction mixture without washing or neutralization and is derived from the waste that is generated from cement production. The activation process is extremely low in cost and the catalyst material is stable even after repeated use as a catalyst in reactions. The catalyst is highly active, and produces biodiesel from oils rapidly (often in less than one hour) and under mild conditions.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a method for preparing a fatty acid (1-5C)alkyl ester comprising contacting a glyceride-containing vegetable or animal oil with the kiln dust and (1-5C)alcohol to provide a reaction mixture, under conditions so that the kiln dust catalyzes the formation of the corresponding vegetable/animal oil-derived, fatty acid (1-5C)alkyl ester and glycerol;    (2) a method for preparing fatty acid (1-5C)alkyl esters from a feedstock that comprises one or more fatty acids and optionally one or more fatty acid glycerol esters comprising: combining the feedstock, the catalyst system, an acid, and molecular sieve particles under conditions such that the kiln dust catalyzes the formation of fatty acid (1-5C) alkyl esters, and glycerol when a fatty acid glycerol ester is present;    (3) a method for preparing fatty acid methyl esters from the feedstock that comprises one or more fatty acids and optionally one or more fatty acid glycerol esters comprising: preparing a suspension that comprises kiln dust, methanol and molecular sieve particles; heating the suspension to a temperature above 40 degrees C; combining the suspension with the feedstock to provide a reaction mixture; heating the reaction mixture to provide the fatty acid methyl esters; and separating the fatty acid methyl esters from the reaction mixture;    (4) a method for preparing fatty acid methyl esters from a feedstock that comprises poultry fat comprising: combining poultry fat comprising free fatty acids with a solid acid so as to immobilize free fatty acids on the solid acid; combining the poultry fat with molecular sieve particles to provide dried poultry fat; optionally filtering the solid acid and immobilized free fatty acids from the poultry fat, and optionally filtering the molecular sieve particles from the dried poultry fat; combining the poultry fat with the catalyst system comprising methanol to provide a reaction mixture; heating the reaction mixture to a temperature above 40 degrees C to provide the fatty acid methyl esters; and separating the fatty acid methyl esters from the reaction mixture; and    (5) a method for producing methyl soyate comprising contacting soybean oil, methanol, and kiln dust, under conditions such that the kiln dust catalyzes the formation of glycerol and the methyl soyate, where the kiln dust is cement kiln dust or lime kiln dust.