cup shaped depression having side walls 16 with an angle of about 45 degrees while this angle can be from about 20 degrees to about 60 degrees applicant has found that low cost results are obtained with an angle of about 45 degrees the selection of the side wall angle is dependant on the angle of repose for the back fill or native materials melter depth and the minimum surface area required to process the feed material the soil 12 is then overlain with a quantity of selected back fill 32 to a thickness of about one to three feet the thickness of the material depends on the back fill material selected and its expected service when the back fill is more corrosion resistant less material is required the dimensions of the excavated area will depend upon the desired capacity of the apparatus for example a melter with a capacity to process 50 tons of feed material per day would have an outside diameter of about 32 feet while a capacity of 100 tons day would require a diameter of about 40 feet this outside diameter depends on several variables including the typical melting rate of the feed material the viscosity of the melting material its corrosiveness to the fill material the electrode placement and the depth the electrode penetrates into the molten material the general character of the melter 39 s dimensions are as follows the area inside the circle which contains the electrodes must be sufficient to melt the feed.