acid under strong stirring in order to reduce the small amount of oxidized rifaximin that forms during the reaction and finally the ph is brought to about 2 0 by means of a further addition of concentrated aqueous solution of hydrochloric acid in order to better remove the excess of 2 amino 4 methylpyridine used in the reaction the suspension is filtered and the obtained solid is washed with the same solvent mixture water ethyl alcohol used in the reaction such semi finished product is called u201c raw rifaximin u201d the raw rifaximin can be directly submitted to the subsequent step of purification alternately in case long times of preservation of the semi finished product are expected the raw rifaximin can be dried under vacuum at a temperature lower than 65 u00b0 c for a period of time between 6 and 24 hours such semi finished product is called u201c dried raw rifaximin u201d the so obtained raw rifaximin and or dried raw rifaximin are purified by dissolving them in ethyl alcohol at a temperature between 45 u00b0 c and 65 u00b0 c and by crystallizing them by addition of water preferably in weight amounts between 15 and 70 in respect of the amount by weight of the ethyl alcohol used for the dissolution and by keeping the obtained suspension at a temperature between 50 u00b0 c and 0 u00b0 c under stifling during a period of time between 4 and 36 hours the suspension is filtered and the obtained solid is washed with.