Table of Contents
The following conclusions are drawn from the results of these investigations:
- The reflux leaching method is an effective means for extracting aluminum from crude Al-Si alloy. Aluminum obtained by this method will contain from 1 to 6 percent silicon, 0.04 to 0.8 percent iron, and 0.01 to 0.9 percent titanium. With only minor changes in composition by dilution of impurities with pure aluminum, the leached aluminum would serve as a base alloy for producing approximately 50 percent of the commercially used sand-casting alloys.
- Dissolution of aluminum and silicon increases with increasing leaching temperature, and the rate of dissolution is affected by alloy particle size, iron content, and the system pressure.
- The rate of aluminum extraction is approximately linear with respect to time between 0- and 40-percent extraction. Complete removal of aluminum can be obtained although the advantage of complete extraction at low leaching temperatures may be lost to the unwarrantable expense of time.
- It is desirable to maintain the system pressure at 100 microns or lower. The rate of aluminum extraction is increased about 20 times by decreasing the pressure from atmospheric pressure to a pressure of 100 microns or less.
- Aluminum extraction can be accomplished with relatively little zinc as the leaching agent by a continuous cycle of operation including evaporation, condensation, leaching, and reevaporation.
- The optimum Al-Si alloy particle size for leaching is one large enough to permit a relatively free flow of zinc through the leaching column and small enough to allow rapid penetration of the zinc into the leach material. Experimental results indicate an optimum particle size of minus ¾- plus ½-inch.
- Aluminum extraction is increased by using Al-Si alloys containing increased amounts of iron. The porosity of the alloy appears to become greater as the iron content increases and allows more rapid penetration of zinc into the alloy particles.
Continuation of the research and development program reported here includes a number of larger scale extraction tests to supplement data obtained in the small-scale work and to reveal difficulties which may be encountered in large-scale operations. Equipment for these tests has been designed for 100-pound charges of the alloy. Studies will be continued on the effect of various Al-Si alloy constituents on leaching rates and efficiency of aluminum recovery. An investigation of the properties of the silicon residues will be made to determine the possibilities of developing processing methods to produce a usable product from this material.
Molten zinc, condensed from zinc vapor, was used by the Federal Bureau of Mines to leach aluminum from a aluminum-silicon alloy made by carbothermic reduction of silicious aluminum ores. Zinc requirements were minimized by refluxing condensed zinc in a low-pressure, Soxhlet-type distillation unit. The minimum ratio of zinc used to aluminum in the crude alloy was approximately 2:1. When the system pressure was reduced from atmospheric pressure to 100 microns or less, the rate of aluminum extraction increased by a factor of 20. Leaching rates were much higher when the zinc vapor was condensed directly on the crude alloy than when molten zinc was passed over the alloy in the absence of appreciable zinc vapors.
Observations indicated higher extraction efficiencies from crude alloys containing greater amounts of iron. Complete extraction was obtained with negligible loss of zinc.
Assuming complete removal of zinc, the aluminum products will contain 1 to 6 percent silicon and about 0.5 percent iron plus titanium. The leached crude-alloy residue is a highly friable spongelike material containing about 80 percent silicon, several percent iron and titanium, with the remainder aluminum.
Although aluminum is the most abundant metal in the earth’s crust, bauxite is the only ore used commercially for producing aluminum. Ordinarily, only ores containing more than 45 percent alumina and less than 15 percent silica are considered commercially valuable.
Because the domestic reserve of high-grade bauxite ore is relatively meager, considerable research has been directed towards developing methods for producing aluminum from low-grade bauxite, clay, anorthosite, and other abundant raw materials that cannot be processed by present commercial methods.
One pertinent project, conducted at the Tennessee Valley Authority laboratories in 1947, was devoted to the production of an aluminum-silicon alloy by electrothermal reduction of clay with coke. More recently, the Bureau of Mines, in cooperation with the Apex Smelting Co. of Chicago, developed significant improvements in this process during electric-smelting experiments on calcined clay, silica, and pyrophyllite.
The crude Al-Si alloy produced in this reduction is a bright, large-grained, brittle material containing 30 to 60 percent aluminum, 30 to 50 per-cent silicon, and a few percent iron and titanium. A similar alloy produced in Europe by this method is reported to be used as a deoxidizing agent in steelmaking and in the production of “silumin” casting alloys. Two commercially important binary alloys of aluminum and silicon, No. 43 and No. 13, each contain 12 percent silicon. Alloy 43 is used in producing sand and permanent mold castings, and alloy 13 is widely used in diecasting. There are no known commercial processes in which the alloy is used as a starting material for producing a marketable grade of aluminum metal.
Methods for separating aluminum from Al-Si alloys have been suggested by other investigators. The most prominent of these proposals is the Loevenstein process in which molten Al-Si alloy is cooled to the eutectic temperature of 577° C. and the aluminum-rich liquid phase, containing 11.7 percent silicon, is filtered off. Further refining is achieved by adding zinc to the filtrate to form a eutectic having a melting point of 382° C. Additional silicon is filtered from the zinc-aluminum at this temperature to form an alloy containing only minor amounts of silicon. Differences in the volatility of aluminum and zinc permit the refining of the aluminum product by evaporating the zinc. The Zn-Al eutectic at 382° C. contains about 95 percent zinc, all of which must be evaporated to obtain refined aluminum.
The aluminum-extraction apparatus and process described in this report were designed to eliminate the need for very large quantities of zinc and to reduce the separate dissolution, filtering, and distillation steps to manipulative phases of one operation. It was proposed to perform the operation at reduced pressure for higher rates of zinc vaporization at low operating temperatures.
The specific objectives of the investigation were:
- To obtain relatively pure aluminum from aluminum-silicon alloys.
- To design, construct, and test gas and electrically-heated extraction units.
- To develop leaching techniques that require a mini¬mum of zinc and retain the advantages of zinc as a leaching agent.
- To determine optimum temperature, time, pressure, particle size, and alloy composition for maximum extraction efficiency.
Aluminum Extraction Process
The fundamentals of the extraction process are illustrated in fig. 1. Zinc is volatilized from the lower vessel and condensed to the liquid state in the upper vessel. The liquid zinc is refluxed into the lower vessel, which contains a charge of crushed Al-Si alloy. Aluminum is selectively leached from the crude alloy by the molten zinc and returns with
the zinc to the lower vessel as an aluminum-saturated zinc alloy. Relatively little zinc is required by this technique because zinc can be vaporized readily from the aluminum and recycled through the Al-Si alloy many times during the extraction operation. After a specified leaching period the upper vessel is cooled below the melting point of zinc by lowering the water-cooled block. The zinc vapors are then condensed to the solid state and retained in the upper vessel. Refined aluminum is left in the lower vessel, and a bed of porous silicon and other impurities remains in the leaching vessel.
Photomicrographs of the various stages of the leaching process are shown in figure 2. View A shows the microstructure of the Al-Si alloy before leaching. The large plates and bars are silicon crystals formed when the molten Al-Si alloy was slowly solidified. The area surrounding these large crystals is a mixture of Al-Si corresponding closely to the eutectic composition, 88.3 percent aluminum and 11.7 percent silicon.
View B shows a section partly leached with zinc. The dark area at the bottom right is where zinc has diffused into the crude alloy. The extent of diffusion is clearly marked by the sharp boundary between the dark and light area. The microstructure of the alloy after a period of complete leaching is shown in view C. The silicon crystals appear to dissolve more slowly than the Al-Si eutectic mixture. The areas about the plates are small channels of Zn- Al alloy. The porosity of the silicon sponge after removal of zinc and aluminum is shown in the photomacrograph, view D.
Application of Extraction Process, Procedure and Results
Crude Alloy Feed
The same crude alloy was used in all tests except those in which the effect of variations in iron content was studied. Analysis of the alloy revealed the following percentages: Al, 47; Si, 42; Fe, 6.5; and Ti, 4.5.
The unit used in the initial phase of the experiment is described in figure 3. Heat is supplied to the unit by a high-frequency induction coil which is inductively coupled to a carbon crucible. A hollow, quartz cylinder provides a vacuum-tight enclosure for the various other carbon vessels shown. Crushed Al-Si alloy is placed in a circular troughlike vessel, which has an axial opening for conducting the zinc vapor from the evaporation crucible. High rates of zinc evaporation were obtained at relatively low distillation temperatures by reducing the system pressure below 100 microns. Samples for analysis were obtained at the end of each time interval by shutting off the furnace and allowing it to cool to room temperature before opening it.
Results of typical extraction experiments with the induction-heated unit are presented in table 1. The composition of the recovered aluminum was calculated on the basis of 100 percent removal of zinc.
Test 1 in the table shows approximately 60 percent of the aluminum extracted after a leaching period of 225 minutes. The aluminum content of the product after removal of zinc was 96.9 percent.
Test 2 shows about 90 percent of the aluminum recovered as a final product having 96.4 percent aluminum and 3.0 percent silicon.
In test 3, a 100-percent aluminum extraction was achieved, but the aluminum contained 5.8 percent silicon. A very low zinc-to-aluminum ratio (2.26:1) was used in this test. Although 100-percent aluminum extraction was achieved, the final product contained a high percentage of impurities due primarily to a higher leaching temperature and a longer leaching period.
Calculated rates of aluminum extraction for these tests are illustrated in figure 4. The rate increased with increasing leaching temperature, but silicon contamination also increased from 2 to 3 percent between tests 1 and 2 and from 3 to 6 percent between tests 2 and 3.
An electrical-resistance furnace was constructed to include the features shown in figure 5. The 24- by 20- by 7-inch, box-shaped evaporator was made large and shallow to provide a maximum evaporation area with a minimum
of zinc. The condenser was an inverted cup-shaped vessel that could be raised or lowered inside of a water-cooled metal jacket. The zinc vapor passed from the evaporator through a heated conduit to the top of the 10-inch-diameter leaching column. Zinc vapor was condensed to the liquid state in the condenser and refluxed into the bed of Al-Si alloy below. Aluminum was leached from the alloy as the zinc passed through the alloy bed. The Al-Zn alloy was collected in the annular recesses of a removable collector plate and transferred through the return line into the evaporator. A liquid-metal trap was provided in the return line to prevent eroded silicon particles from becoming mechanically entrapped in the Zn-Al alloy and carried into the evaporator. The floor of the evaporator was elevated at the entrance so that the molten Zn-Al alloy flowed some distance over the heated floor before it reached the main body of molten metal. The turbulence caused in the metal by this action provided more metal surface for evaporation and reduced the tendency for aluminum to concentrate at the evaporating surface as the alloy became depleted of zinc. Separate heating elements were provided for the various compartments. Cooling elements were not installed about the leaching column, since it was believed that a little heat would be needed in this area to compensate for conduction and radiation losses. The principal advantages of this unit over the induction-heated unit are: (1) Greater evaporation area in proportion to the amount of zinc, (2) greater available volume in the extraction vessel, and (3) easier removal of products.
Several unforeseen performance characteristics were observed when the unit was put in operation. The temperature of the leaching column continued to rise for some time after the heat had been turned off and external cooling had been applied. Also, the condenser cooling capacity was far less than expected because the actual heat conduction from the graphite condenser to the cooling jacket was lower than estimated. However, tests were continued since the work schedule did not permit extensive modification of the unit.
The results for a typical operation follow:
The evaporator was stabilized at 580° C. Under these conditions, the condenser temperature stabilized at 458° C. and the leaching column temperature at 455° C. The test was run for 8 hours; during this period 48.1 percent of the aluminum was extracted. The final product showed 98.4 percent aluminum, 1.5 percent silicon, and less than 0.2 percent iron plus titanium. Although this recovery was lower than that for the induction-heated unit, the quality of the aluminum extracted was greatly improved.
The observed rate of zinc evaporation was considerably less than the theoretical maximum rate corresponding to the temperature and surface area of the evaporator. Factors contributing to the low recovery were: (1) Zinc was not being evaporated from a pure zinc bath, and (2) the failure of the small zinc-vapor conduit leading into the leaching column to permit free escape of zinc vapors from the evaporation crucible.
The Nichrome electrical heating elements showed appreciable deterioration after 50 hours of operation under vacuum.
Neither of the electrically-heated units appeared to offer promising possibilities for practical application because of the high cost per unit of
heat and the relative difficulty in conducting it into a vacuum. A gas-fired unit was constructed to examine the problems in this type of heating. These pertained primarily to furnace design and construction materials. Materials with high thermal conductivity and corrosion resistance were required, and the design would have to provide optimum conditions for the process. Features of the gas-fired extraction apparatus are shown in figure 6. The zinc evaporation chamber is positioned horizontally below the vertical leaching column. The purpose is to expose a maximum surface area of molten zinc for maximum rates of evaporation. The zinc-evaporator section consists of an outside shell of stainless steel, 29 inches long, with a ¼-inch wall thickness and a 4-½-inch inside diameter. A graphite inner liner is used as the crucible to retain 30 pounds of zinc charge and protect the shell from attack by the zinc.
Zinc vapors are conveyed from the evaporation crucible to the condenser through a 4-inch-inside-diameter, graphite-lined stainless steel pipe. The condenser is a water-cooled iron cylinder closed at the bottom and protected on the furnace side by a graphite sleeve. The condenser extends into the leaching section so that the flanged, open end engages a mating flange on the leaching column. A rubber gasket is used to make a vacuum-tight seal. The condensing temperature is controlled by varying the rate of circulation of water or air through the open end of the condenser.
The leaching column comprises a cylindrical, stainless steel outer shell protected by a ¼-inch-thick graphite liner. The column has a charge capacity of 20 pounds of crushed Al-Si alloy. A graphite-lined stainless steel pipe joins the leaching column to the evaporation chamber and serves as a return line for the molten Zn-Al alloy.
This design was intended to permit separate controls for heating each basic section of the unit (condenser, leach column, and evaporator) to within a few degrees of the desired temperature.
Results of tests with the gas-fired extraction unit are shown in table 2. The percentage of aluminum recovered increased approximately linearly within the 448° to 475° C. temperature range. This is in agreement with results of extraction for tests run in the induction-heated unit in which extraction rate is uniform up to about 40 percent aluminum extraction.
Study of Process Conditions
The preceding tests were devoted primarily to an investigation of the effects of various combinations of leaching temperature and time of aluminum extraction efficiency. Additional experiments were run to determine the parameters for other process conditions including system pressure, particle size, alloy composition, and the relative merits of liquid and vaporous zinc as leaching agents.
The beneficial effects of performing the extraction in a vacuum enclosure are shown in table 3. In the a tests, part of an Al-Si alloy was held under a cover of molten zinc for 2 hours at 500° and 600° C., at a pressure of 25 microns. The zinc was prevented from evaporating by a tightly-fitting cover. The tests were then duplicated (tests b) in a system filled with an inert gas at atmospheric pressure.
Results of these experiments show that dissolution of aluminum at low pressure is approximately 20 times greater than at atmospheric pressure. However, the dissolution of silicon also increases at reduced pressure. As expected, the solubilities of silicon and aluminum are greater at higher temperatures. It is believed that the increase in leaching rate at reduced pressure can be attributed in part to the absence of gas films and absorbed gases within the alloy, which would tend to prevent the zinc from wetting the Al-Si surfaces.
A series of tests was conducted to determine the relative effectiveness of liquid zinc and zinc vapors as leaching agents. Table 4 shows the results of tests using liquid zinc only. Leaching was accomplished by passing zinc, at several selected rates, through a vertical evacuated column of crushed Al-Si alloy. Leaching efficiencies were determined by analysis of the amount of aluminum dissolved by the zinc during transit through the column. Temperature within the column was maintained at 475° C. for each test. Results show that the ratio of aluminum extracted per gram of zinc decreased as the flow rate of zinc increased. However, the total weight of aluminum recovered was greatest at the highest zinc flow rate. Aluminum extraction ranged from 1 percent at a rate of 25.4 grams of zinc per minute to 4 percent at a rate of 66.7 grams per minute.
The effect of zinc vapor on leaching was studied with a modified small-scale reflux-leaching unit in which the vapors were condensed directly on the Al-Si alloy during the leaching cycle instead of refluxing liquid zinc through the alloy. This procedure exposed the alloy to high concentrations of zinc vapor for a short period of time until liquid-vapor equilibrium conditions were established in the leaching column. Distillation of zinc into the Al-Si alloy bed was continued until approximately the same amount of zinc was distilled as had been passed through the alloy in the liquid-leaching tests. However, due to the restricted evaporation surface area, it was impossible to achieve distillation rates comparable to the liquid-zinc flow rates.
The results of these tests are shown in table 5. The aluminum extracted ranged from 8 to 28 percent. Although the two groups of tests are not comparable in every respect because of differences in leaching time and the amount of zinc used, they reveal two significant observations. First, zinc in the vapor state penetrates more thoroughly than liquid zinc into the smallest interstices of the aluminum particles where it gradually condenses to the liquid state. Second, the Al-Si particles are more thoroughly wetted by the vapor than by the liquid zinc. These observations indicate that the zinc vapor contributes markedly to the extraction efficiency by saturating the Al-Si particles to a greater degree than liquid zinc. However, after the initial penetration and saturation of the Al-Si alloy, liquid zinc probably leaches as effectively as zinc vapor.
During the regular aluminum extraction tests the extraction rates varied among tests conducted at identical temperature, time, and pressure conditions. An examination of the Al-Si charge material for these tests showed that variations of the average particle size of the charge resulted in differences in the aluminum extracted from the alloy. Therefore, a series of tests was made to compare the aluminum extracted from different-sized pieces of Al-Si alloy. Each size group was tested for 3 hours at leaching temperatures of 450°, 475°, and 500° C. Results of these experiments are shown in table 6.
Within the optimum leaching-temperature range (450°-500° C.) the minus ¾- plus ½-inch charge material yielded the highest aluminum recovery. When small pieces of Al-Si alloy were used, much zinc was retained in the leaching column, usually enough to form a shallow pool of liquid metal on top of the crushed alloy. The solids were too compact to allow the free flow of zinc through the leaching column back to the evaporator section. Tests with minus 1- plus ¾-inch material showed a better aluminum recovery than tests with the smaller material, but recovery was less than in tests with minus ¾- plus ½-inch material treated in the same way. In the case of the large material, penetration of zinc vapor and liquid into the alloy was not as rapid or as complete owing to the smaller total surface area exposed and the larger size of the Al-Si alloy particles.
The amount of aluminum extracted from the minus ¾- plus ½-inch material was 5 to 10 percent greater than that from the other sizes.
Investigation of the effect of iron content on the efficiency of aluminum extraction from Al-Si alloys showed an increase in leaching efficiency with an increase in the iron content of the alloys. Table 7 gives results of tests in which Al-Si alloys containing 5.25, 10.2, and 13.7 percent Iron were leached at 500° C. for 1 to 3 hours.
The only physical differences observed among the alloys were in porosity and brittleness; these were greater in the alloy with the highest iron content. The greater porosity is believed to allow the zinc vapors to penetrate more rapidly and thoroughly into the Al-Si particles to dissolve aluminum at a faster rate. Specimens of the alloys prepared for X-ray diffraction analysis showed no apparent phase differences between the alloys.