Altitude-pressure and Standard Saturation Curves: Chart A (Fig. 55) is used to determine barometric pressure at various altitudes; Chart B is used to determine standard saturation values for various temperatures and pressures. In Chart A, altitudes, in feet, are plotted on the horizontal axis and pressures, in millimeters, on the vertical axis. To find the pressure corresponding to a certain altitude, follow the elevation line downward to its intersection with the curve x, then horizontally to the right, and then read the pressure. For example, if the elevation is 6000 ft., the 6000-ft. line is followed to its intersection o with the curve x; then the corresponding pressure, 607 mm, is obtained from the right-hand side of the chart.
In Chart B, temperatures are plotted on the horizontal axis, and the amount of oxygen, in milligrams per liter of distilled water, is plotted on the vertical axis; various pressure curves also are plotted, as shown. To find the saturation value for a certain temperature and pressure, follow the temperature line upward until the point corresponding to a given pressure is reached, then follow horizontally across to the left-hand side of the chart, and read off the amount, in milligrams, of oxygen per liter of solution. For example, to determine the amount of oxygen in a solution having a temperature of 59°F. at an elevation of 6000 ft.: The pressure is 607 mm; at this elevation, as found in Chart A, therefore, the 607-mm pressure curve must be used; i.e., it is necessary to interpolate between the 600- and the 650-mm curves. The 59°F. line is followed to its intersection R with the 607-mm curve, then from the left-hand side of the chart is read off 8 milligrams of oxygen per liter. The same procedure is used for the various pressures, using the curve corresponding to the particular pressure. For any particular plant, a solubility curve should be plotted based on the altitude at the plant.
Results of experiments conducted in the South Kalgurli dry-crushing, all-roasting, and cyaniding plant at Kalgoorlie on dissolved oxygen in mill solutions were given by C. W. Brown in C.E. and M.R., September, 1934. Pulp agitation was done in vats with ordinary stirrers, but considerable compressed air was also introduced. The plant stands at an altitude of 1271 ft. above sea level. The average value of dissolved oxygen just prior to filter pressing the slime was 25.05 per cent saturation, and the average of many determinations in which the agitators were sampled was 28.0 per cent, with a range of 4.2 to 57.5 per cent.
As discussed in the next section, a number of dissolved and suspended substances in cyanide solutions tend to reduce the oxygen content, among which may be mentioned sulphide sulphur, ferrous iron, metallic iron, and organic material. It is noted that, when the cyanide solution is in contact with ore the oxygen content is lowered, whereas when it leaves the ore at any point it starts at once to dissolve oxygen.