Sodium hydroxide tests were performed to test the digestibility of the completely oxidized ferrotungsten material. Two digestion procedures were used, "low NaOH" and "high NaOH." The "low NaOH" tests were performed by adding the desired amount of NaOH to 0.2 1 of deionized (DI) water. The molar ratio of NaOH/W for each test is given in Table 1. Each "low NaOH" test was roughly sized for approximately 150 ml of 25% (8 to 9N) NaOH per 60 g of WO₃. The caustic solution was cooled to between 10° C. to 15° C. and the completely oxidized ferrotungsten material was added. After slurrying for 1 hour, the slurried material was added to a stainless steel reactor and digested at 120° C. for 4 hours. The slurry was diluted with 0.2 to 0.3 1 of DI water and filtered. The solids were slurried with about 0.3 1 of hot DI water for 1 hour and filtered. Both filtrates were combined and analyzed for tungsten by atomic absorbance (AA). The solids were analyzed by x-ray fluorescence (XRF). The "high NaOH" digestions were performed by adding the desired amount of NaOH to a stainless steel reactor and heating to 120° C. These tests were roughly sized for approximately 150 ml of 50% (.about.19N) NaOH per 60 g of contained WO₃. The completely oxidized ferrotungsten was slowly added to the reactor and digested for 3 hours. The slurry was diluted with 0.5 1 of DI water and filtered. The solids were twice slurried with 0.5 1 of DI water and filtered. A final wash was performed in the filter with 0.3 l of DI water. The filtrates were combined and analyzed by AA. Solids were analyzed by XRF. The results of the digestion tests are given in Table 1. The tungsten recovery efficiency is defined as the total tungsten recovered in solution divided by the sum of the tungsten recovered in solution and the tungsten remaining in the solids.


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The effect of particle size on the oxidation rate of ferrotungsten was determined using a procedure similar to the method used to evaluate the effect of temperature on the oxidation rate. In this set of experiments, the furnace temperature was maintained at a constant 950° C. and the crucibles were removed at approximately 1 hour intervals. The ferrotungsten was separated into five fractions by sieving through a series of screens. The screens sizes were "big," 1/4 mesh, 10 mesh, and 30 mesh. The "big" screen had diamond shaped opening measuring 5/16 by 7/8 inches. The oxidation curves for the different fractions are given in FIG. 2. The finer fractions (-30 mesh, +30 to -10 mesh, and +10 to -1/4) appear to completely oxidize in about 9.5 hours. The two larger fractions require a much longer time, approximately 23 hours. Accordingly, it is desirable to reduce the particle size of the ferrotungsten to -1/4 mesh in order to minimize the time required to completely oxidize the ferrotungsten.

Two additional sets of experiments were conducted in a laboratory tube furnace to determine the effect of the gas flow rate and oxygen content of the gas on the rate of oxidation. Collectively, both parameters dictate the amount of oxygen present in the furnace atmosphere during oxidation.


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In the first set experiments, approximately 100 grams of ferrotungsten was placed into a heated laboratory tube furnace. The oxygen content of the oxidizing gas flowing through the furnace was maintained at 20% by volume (balance nitrogen) while effects of different gas flow rates and furnace temperatures were evaluated. After two hours, the boat was removed and allowed to cool. Once, the weight of the partially oxidized ferrotungsten was recorded, the partially oxidized ferrotungsten was placed in a second tube furnace and completely oxidized (900° C., air (21% O₂) , 6 scfh). The final weight of the completely oxidized material was used to calculate the percent oxidation of the partially oxidized material at 2 hours. In the second set of experiments, the experimental setup was the same as the first except that the oxygen content was varied and the flow rate was fixed at 6 scfh.

FIG. 3 shows that the increase in the flow rate of gas through the furnace had a larger effect on the amount of oxidation after 2 hours than the increase in the furnace temperature. The highest percentage of oxidation after 2 hours was achieved using the combination of high temperature and high flow rate, i.e., 900° C. and 10 scfh. FIG. 4 shows that the increase in the oxygen content of the gas flowing through the furnace yielded a substantially greater percentage of oxidation after 2 hours than did the increase in furnace temperature. Again, the greatest amount of oxidation after 2 hours was obtained using the combination of higher O₂ content and higher furnace temperature, i.e., 30% O₂ and 900° C. Thus, the results of both experiments indicate that increasing the oxygen content of the furnace atmosphere either by increasing the flow rate of the oxygen containing gas or by increasing the oxygen content of the gas itself can significantly improve the oxidation rate.


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FIG. 1 demonstrates the effect of temperature on the oxidation rate of ferrotungsten. In FIG. 1, the percent weight gain of the ferrotungsten (FeW) at different temperatures is plotted against time. The data in FIG. 1 reveal that there is a significant increase in the oxidation rate of ferrotungsten above about 900° C. Unfortunately, the increase rate of oxidation at higher temperatures becomes limited by the ability of the calciner to effectively operate at the higher temperatures while handling production quantities of ferrotungsten. For a typical commercial calciner, the calciner is normally limited to operating at temperatures less than about 1000° C. because the calciner tube must be made out of alloys which have sufficient strength to handle the heavy and abrasive ferrotungsten. This excludes higher temperature tube materials such as alumina and silicon carbide which do not possess the needed strength. Therefore, to increase the rate of oxidation further, it is necessary to either decrease the particle size of the ferrotungsten being calcined or increase the oxygen content of the calciner atmosphere. The effects of these parameters on the oxidation rate of ferrotungsten are described below.

FIG. 1 is a graphical representation of the effect of temperature on the oxidation of ferrotungsten.


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