1. IN A CONCENTRATING PROCESS IN WHICH A PULP OF WATER AND FINELY DIVIDED SOLIDS FEEDS INTO A PUMP BOX, ADDITIONAL WATER IS INTRODUCED TO THE PUMP BOX TO DILUTE THE PULP AND THEREBY DECREASE ITS DENSITY, THE VOLUME OF THE ADDITIONAL WATER IS REGULATED TO MAINTAIN A SUBSTANTIALLY CONSTANT LEVEL OF THE DILUTED PULP IN THE PUMP BOX, A PUMP TRANSFERS THE DILUTED PULP AT CONSTANT VOLUME TO A BANK OF PARALLEL SPIRAL CONCENTRATORS, AND THE CONCENTRATOROS SEPARATE SOLIDS IN THE PULP INTO CONCENTRATE AND TAILING PRODUCTS, THE COMBINATION THEREWITH OF A CONTROL METHOD COMPRISING MEASURING THE DENSITY OF THE DILUTED PULP BEFORE IT REACHES SAID CONCENTRATORS GENERATING A SIGNAL REPRESENTATIVE OF THE DENSITY MEASUREMENT, RETURNING TO SAID PUMP BOX A PORTION OF THE CONCENTRATE PRODUCT FROM SAID CONCENTRATORS TO COUNTER THE DECREASE IN DENSITY OF THE PULP IN SAID BOX BROUGHT ABOUT BY THE ADDITIONAL WATER, AND VARYING THE PORTION OF RETURNING CONCENTRATE IN RESPONSE TO CHANGES IN SAID SIGNAL TO MAINTAIN THE DILUTED PULP AT A RELATIVELY CONSTANT DENSITY, A RELATIVELY CONSTANT WEIGHT OF SOLIDS IN EACH CONCENTRATOR, AND ALL CONCENTRATORS IN THE BANK IN CONTINUOUS OPERATION.