Approximately one-half of the weight of the sludge generated in the commercial production of purified TiCl4 from rutile can be recycled with a coincident diminution of waste disposal problems and the more efficient utilization of TiO2 and carbon in the processing operation. The sludge can be separated into soluble and insoluble fractions by water leaching, and the insoluble, consisting largely of carbon and titania, can be recycled in the chlorination feed. Agglomerating the insoluble residue is required. Laboratory chlorination tests showed that addition of the insoluble residue to rutile increased chlorination efficiency and depressed vanadium contamination of the crude TiCl4.
With passage of the 1965 Solid Waste Disposal Act, the Bureau of Mines increased its research on the recovery of valuable constituents from industrial and domestic wastes. The program was aimed at diminishing land pollution and scenic blights, as well as adding to the resource base through recovery of mineral and metal values. One area of industrial waste generation is in the production of titanium tetrachloride (TiCl4), an intermediate chemical used in the production of titanium metal and titania pigments.
All titanium mineral treatment plants constructed in the United States between 1958 and 1966 were based on the chloride process, and in 1967 seven plants were in operation and two more were under construction. The production capacity of the seven active plants was equivalent to 278,000 tons of TiO2 per year. Assuming that 15 tons of byproduct material (referred to as sludge in this report) are produced for every 100 tons of ore processed, and adding an additional plant making captive TiCl4 for titanium metal, 44,000 tons of sludge would be generated annually. Handling this sludge poses the problems of (1) disposal and (2) recovery of valuable materials contained in the sludge.
The sludge contains very fine, unchlorinated mineral, coke, and residues of the TiCl4 purification systems. It is very gassy as removed from the system and cannot be stored in the state in which it is produced. Some plants neutralize the sludge with lime before dumping, but recovering the titanium from neutralized sludge by rechlorination is difficult.
The sludge investigated was residue resulting from the rutile chlorination and TiCl4 purification plant of Titanium Metals Corporation (Timet) at Henderson, Nev. The material was obtained from the still residue “dryers” by taking samples from a number of separate dryer batches in an attempt to obtain a relatively representative sample. All of the work reported in this investigation was performed on residues from the Timet plant.
The major components of the sludge were titanium, as unreacted rutile and unrecovered chloride, carbon added in the chlorination reaction, and chlorine as metal chlorides. The impurity metals-iron, zirconium, vanadium, columbium, etc.-are concentrated in the sludge by removal of most of the titanium as a high-purity TiCl4 product. Although the vanadium and columbium represent the greatest dollar values, titanium and carbon comprise the major bulk of the sludge, and development of methods for recovering and recycling them would not only result in mineral conservation, but also would diminish the disposal problem.
Experimental Procedure and Results
The sludge was separated into soluble and insoluble fractions by leaching. The chlorination characteristics of the insoluble portion, which contained most of the rutile and carbon, were investigated. The soluble fraction was concentrated and stored for future research on development of methods for separating and recovering the valuable constituents.
A typical analysis for rutile used in producing TiCl4 and the sludge generated by the Timet operation is shown in table 1. The sludge analysis is the average of eight determinations made at intervals during a 1-year research period and does not include a host of minor constituents. The major variance in analysis was in the chloride analysis which was about 22 percent at the start and 13 percent at the end of the research period. This indicated a loss of chlorine as HCl, while some of the elements changed from the chloride to the oxychloride and possibly to the oxide.
Leaching the sludge in water with mechanical agitation, with or without the addition of air agitation, produced an exothermic reaction in which hydrolysis occurred. On cooling, the slurry separated into three fractions: (1) A floating slime fraction, (2) a middle liquor fraction, and (3) a bottom residue fraction.
A water leach solubilized about 40 percent of the sludge weight. The manner of leaching did not affect the amount solubilized. The liquor fraction was acidic and had a pH of 0.5 or less. Additions of acid to the leach solution did not increase the proportion of solubles or improve separation of metallic components into the separate fractions. The liquor contained 85 to 90 percent of the vanadium, 75 to 80 percent of the columbium and aluminum, and 80 to 85 percent of the iron contained in the sludge. The liquor also contained chromium and zirconium, plus other minor metallic constituents.
The relative proportion of the insolubles that divided between the slime and residue fractions could be controlled by the leaching procedures and agitation methods. Vigorous mechanical agitation during leaching resulted in 10 percent of the insoluble in the slime fraction; the addition of an air agitator doubled or tripled the weight of slimes. By skimming off the first slime fraction and repeating the stirrer-air agitation, over 50 percent of the solids could be raised as slimes. Washing of the slimes by agitating with water also resulted in a three-layer separation. By combining the residue fraction from this wash with the residues from the leach operation, the total weight of the slime fraction could be held to less than 5 percent of the original sludge weight.
The final method adopted for a bench-scale leach to supply adequate quantities of residue for chlorination tests was to agitate 4 kilograms of sludge in 8 liters of water for 1 hour. After settling, the slimes were skimmed off, repulped twice in water, and stored for later investigation. The residue was separated by filtration and repulped in each of the two slime washes, filtered, and dried. This method yielded 50 percent of the sludge weight as residue.
Table 2 shows the averaged results of triplicate leaches carried out in this manner, and the distribution according to residue, slime, and liquor.
Note.-Residue = 50.4 percent of the original weight.
Slime = 9.7 percent of the original weight.
Liquor = 39.9 percent of the original weight.
Utilization of the residue in a commercial operation would probably be accomplished by recycling it in some combination with the mineral rutile feed. A series of laboratory-scale chlorination tests was made to determine the chlorination characteristics of the residue in combination with various proportions of rutile. The tests were made in a batch-type, vertical tube chlorinator. Figure 1 shows the essential features of the apparatus. The chlorinator column was a 40-mm-diameter Vycor tube. The column was heated by a resistance furnace, and temperatures were measured by a thermocouple immersed in the charge. Chlorine was introduced at the bottom and dispersed through a porous graphite disk on which the charge rested. Offgases were routed through a water-cooled condenser to an ice bath-cooled flask where the crude TiCl4 product was collected. Gases that did not condense in the flask were removed second water-cooled condenser by a water aspirator that maintained a slight negative pressure in the second condenser.
Although it was not possible to duplicate chlorination conditions of a continuous commercial chlorination operation, a standardized evaluation procedure was established for the laboratory chlorinator by operating the equipment using a rutile-carbon feed material similar to that of a commercial operation. These tests showed that optimum chlorine efficiencies and best test reproducibility were obtained by metering to each batch 80 percent of the chlorine theoretically required for chlorination of all the titanium in the charge at a rate of 0.6 gram of chlorine per minute.
The small particle size of the residue material required the use of agglomeration techniques to prevent it from blowing out of the chlorinator. Table 3 compares the screen analyses of rutile supplied by Timet and the residue from the sludge. For the laboratory tests, two methods were used for consolidating the charge, binding with bentonite, and binding with a coal – tar pitch. The charge ingredients-rutile, residue, coke, and binder-were mixed in a laboratory muller. A fine water mist was sprayed into the bentonite mixes until they became plastic. They were then cast in pans and dried at 120° to 130° C. The coal-tar pitch mixes were prepared in a similar manner except that after being placed in the pans they were heated to ignition and allowed to burn themselves out. The mixes were broken from the pans, heated for 3 hours at 700° C under an inert atmosphere, and then crushed to ¼ inch for chlorinator feed. The pellets obtained with the light coal-tar pitch were much stronger than those obtained with the bentonite binder.
The coke used in preparing feed mixes contained 92.6 percent total carbon, 91.8 percent fixed carbon, and 0.8 percent ash. The coal-tar pitch was a refined product with a specific gravity of 1.21 at 15° C and 18.5 percent volatiles at 300° C. Analysis of rutile is given in table 1 and that of the residue is given in table 2.
The operating conditions used for comparative tests were 100 grams of ¼-inch charge, temperature 900° C, 2.3 grams of chlorine per gram of titanium in the charge (80 percent of stoichiometric), and a chlorine flow rate of 0.6 gram per minute. The only variables were charge composition and method of charge preparation.
The mix compositions and results of the comparative chlorination tests are shown in table 4.
The comparative chlorination tests showed that the addition of residue to rutile improved the chlorination characteristics of the charge. The chlorination efficiencies of the mixes that did not contain rutile were also inferior to the rutile-residue mixes. The variations in coke additions showed that the carbon contained in the residue was effective in supplying a substantial portion of the reduction required for the chlorination reaction.
Analysis of the crude TiCl4 product for iron, aluminum, vanadium, zirconium, and silica showed only minor variations in product quality. Zirconium and iron content of all the products was 0.01 percent or lower, and silica ranged from 0.01 to 0.02 percent. The vanadium content was highest in the charges in which all of the TiO2 was supplied by rutile, and lowest in the charges containing no rutile. This indicates the vanadium remaining in the residue is less subject to chlorination than that in the natural rutile mineral or that there is an inhibiting element in the residue that depresses chlorination of vanadium. The distribution of aluminum in the crude TiCl4 products was the opposite of the vanadium, being lowest in the all-rutile charge and highest in the all residue charge.