A four unit helical conveyor type mixer with cut and folded elements built by L. C. Peery at Virginia Polytechnic Institute in 1939 was tested. Preliminary investigations, made before tests to obtain pertinent data were begun, showed the advisability of straightening these folds. The purpose of this was to prevent all of the mixing action from occurring in the first unit and to show an incomplete picture of the mixer operation. These preliminary investigations also showed the maximum particle size usable in the mixer without obtaining excessive size reduction during mixing. In addition to the above, the investigations showed that in tests where a difference in the density of these materials existed as well as a difference in their crystalline structure, irregularities in the mixing data obtained under these conditions could be attributed more to the difference in density than to the difference in the crystalline structure of the materials. On this basis, tests made on the screw conveyor mixer with the available materials were titled as studies of the effect of a difference in density on mixing. The first tests on the mixer were made with chert and copper sulfate to determine the effect of speed of elements, feed rate, feed proportions, and particle size on mixing efficiency. These materials were relatively inexpensive and the copper sulfate was readily leached from the samples to form a basis for weight analysis of samples. In the tests to determine the effect of a difference in density on mixing efficiency the materials used were barytes, andalusite and chert with copper sulfate, which is the only soluble material of the four listed. The procedure followed is as follows: the particle size, density, and weight of the two materials to be used were recorded. These materials were fed into the mixer and at given time intervals samples were taken at the sampling ports and discharge opening. The feed time, time of the run, rpm. of the mixer elements, average time between samples and weight of material in the system were taken. These samples were analyzed and families of curves drawn to show the relationship of variations in the variable factors to mixing efficiency. Results show that a mixing element speed up to 56 rpm. and above 108 rpm. gave the most efficient mixing with feed rates below three cubic feet per hour and above nine cubic feet per hour. The optimum particle size was in the range from twenty to fifty mesh with feeds of approximately equal percentages by weight of the two materials showing the best mixing efficiency at 108 rpm. At a speed of 56 rpm. the relationship of feed proportions to mixing efficiency was not definitely established. The efficiency of mixing varied inversely with the difference in density at a speed of 108 rpm. and showed the apparent reverse of this at 56 rpm, but dispersion of the materials in each other at this speed was very poor. With materials of 16 mesh size, the size reduction taking place during mixing is negligible, but with materials of 4 mesh size there is an appreciable amount of size reduction taking place. Equations, holding within specified limits and showing the effect of each variable factor on mixing efficiency, were derived. / Master of Science