TREATMENT OF CHLORINATION RESIDUE

Treatment of chlorination residue

The present invention relates to a process for treating a chlorination residue containing an alkali or alkaline earth metal chloride. Residues of this kind result from the chlorination of feed materials containing aluminium, particularly the chlorination of bauxite and clays associated with coal.

Many processes have been proposed for the chlorination of aluminium bearing ores and clays and examples of such processes can be found in U.S. Patent Specifications Nos. 1,605,098,1,600,216, 1,875,105,1,866,731 and 3,244,509. Many of these processes do not address the removal of alkali or alkaline earth metals from any residue which may be produced by the process. U.S. Patent Specifications Nos. 3,244,509 and 3,466,169 both utilize electrolysis to remove alkali chlorides and alkaline earth chlorides from the residues produced in their processes.

U.S. Patent Specification No. 4,237,102 discloses an intricate cyclic process for obtaining very pure alumina by a hydrochloric acid attack of a silicoaluminous material. Afterthe aluminium containing feed material has been leached with hydrochloric acid and the aluminium chloride separated, then the oxide impurities contained in the liquor are extracted by the addition of sulfuric acid, in the presence of hydrochloric acid, to form a sulfohydrochloric leach which precipitates the impurities as their sulfates.

That leach is then degassed to obtain hydrochloric acid and sulfuric acid which are recycled to the process.

U.S. Patent Specification No. 4,239,735 discloses the removal of metal oxide impurities from kaolin clay by the use of a dilute mineral acid, e.g., 2 N-6 Ν hydrochloric, nitric acid or sulfuric acid, as a preleach of kaolin clay prior to subjecting the clay to a 26 percent hydrochloric acid and H₂SiF₆ leach to recover aluminium chloride.

According to the present invention, there is provided a process for treating a chlorination residue containing an alkali or alkaline earth metal chloride, the process comprising adding sulphuric acid to the residue under conditions whereby at least a portion of the alkali or alkaline earth metal chloride is converted to its sulphates and hydrochlorine acid is produced.

As used herein, the term "chlorination residue" refers to the residues remaining after a chlorination treatment of an ore or mineral.

Further according to the present invention, there is provided a process for the chlorination of a metal from a feed material selected from clay associated with coal and bauxite, the process comprising chlorinating the material so that a residue containing an alkali metal chloride or an alkaline earth metal chloride is obtained, adding sulfuric acid to the residue under conditions to cause the conversion of at least a portion of the alkali or alkaline earth metal chlorides to their sulfate forms and to form hydrochloric acid.

Alkali and alkaline earth metal chlorides contained in a residue of a chlorination process of a feed material containing aluminium are rendered environmentally inert by the addition of sulfuric acid which causes the conversion of the metals to their sulfate form and the simultaneous production of hydrochloric acid. The residue can then be disposed of readily, for example, to an ash pond or disposal area for flue gas desulfurization (FGD) sludges. The hydrochloric acid which is produced can be used in the chlorination process. For example, it can be utilized as a binder of the feed material and/orto prechloridize the feed material, e.g. convert a portion of the chlorine consuming constituents to their respective chlorides, or it may be utilized as a portion of the leach solution if the chlorination process utilized is a hydrochloric acid leach.

Processes according to the present invention will now be particularly described by way of example.

The process is applicable to any chlorination processes of feed materials which produce a residue containing alkali or alkaline earth chlorides. It is especially useful in those chlorination processes for the recovery of aluminium from the feed material and is particularly beneficial in the chlorination of bauxite and clays associated with coals wherein the residue produced therefrom contains alkali or alkaline earth metals.

Clays are generally fine-grained earthy material made up of minerals which are essentially hydrous aluminium silicates. The specific mineral content of the clay depends upon the area in which the clay is found. The clays on which the present process is operable are ones found associated with coal, for example, parting clays which are found between seams of coal. Additional examples include top and bottom contact clays, which are found at the top and bottom, respectively, of the coal reserve, clays in the overburden of the coal and clays found in coal refuse, i.e., the washings of coal to remove ash forming minerals from the coal.

The particular chlorination process of the bauxite or clay associated with coal is not critical provided the residue from the process contains alkali or alkaline earth metal chlorides. For example, the chlorination process can be two stages with both chlorination steps being conducted in the presence of a reducing agent and chlorine, for example, that disclosed in U.S. Patent Specifications Nos.

1,605,098 and 1,600,216. The clays can be chlorinated in the presence of carbon monoxide and chlorine at a temperature of 600-900°C to chlorinate the aluminium, iron and titanium. Thereafter, the residue can be treated with carbon, and chlorine at an elevated temperature to chlorinate the silica and aluminium silicates contained in the clay, for example, the process disclosed in U.S. Patent Specification No. 1,875,105. The chlorination process can be comprised of treating the feed material containing aluminium and silica acid with a carbaceous material

This print takes account of replacementlater filed to enablethe application to

comply with the formalof the Patents Rules 1978.

and equal parts of chlorine and silicon tetrachloride in order to chlorinate the aluminium and not the silica contained in the material, for example, as described in U.S. Patent Specification No. 1,866,731.

The chlorination process can utilize a reductive chlorination followed by an oxidative chlorination.

Alternatively, the chlorination process can be a leaching process, for example, leaching with hydrochloric acid.

Essentially, the process is useful in all chlorination processes of bauxite and clay associated with coal which contain aluminium wherein the chlorination process produces a residue containing alkali or alkaline earth metal chlorides. The process is particularly beneficial when the residue contains calcium chloride. The residue istreated with sulfuric acid in an amount which is sufficient to convert the chloride values to their sulfate forms. It is generally preferred that the sulfuric acid be supplied in an amount which is slightly in excess of the stoichiometric amount required for the reaction of the alkali or alkaline earth metals sought to be converted. Generally, the sulfuric acid will be utilized in an amount of from about 250 percent to 350 percent and preferably from 275 percent to 325 percent based on the weight of contained calcium in the residue being treated. To improve gypsum precipitation conditions and extractthe chlorides of alkali metals with high yield, preferably a diluted sulfuric acid, containing approximately 50 weight percent or less sulfuric acid, is utilized. For example, the sulfuric acid can be obtained from a sulfur dioxide scrub-regeneration system utilized on stack gas. The sulfuric acid will cause the precipitation of calcium, if present, as gypsum and will leach out water-soluble chlorides and a small amount of acid soluble chlorides.

Generally, the sulfuric acid leach is conducted for a time of from 10 minutes to 1 hour and preferably from 15 minutes to 30 minutes. Shorter leach times may result in incomplete solubilization of metal chlorides, while longer leach times unnecessarily increase the cost of leach equipment and energy to suspend the leach pulp. Generally, a temperature of from 30°C to 70°C and preferably from 40°C to 60°C produces a rapid filtering residue.

After the leaching, the residue is subjected to a solid-liquid separation and liquor recovered therefrom, which contains dilute hydrochloric acid, sulfuric acid and some small amounts of metal chlorides, is recycled back to the chlorination process preferably for use as a binder for pelletizing the feed material and/or to prechloridize the feed material. The hydrochloric acid is a preferred binder for the feed material as it apparently chemically reacts with the feed material to form hydrates which aid in the binding process. The use of the hydrochloric acid as a binder will also prechloridize the feed material since it will convert at least a portion of the chlorine consuming alkali and alkaline earth metals contained in the feed material to their respective chloride salts. The prechlorination of the feed material is particularly beneficial when it contains high levels of calcium or magnesium.

If the chlorination process utilizes a hydrochloric acid leach, then the hydrochloric acid liquor may form a part of the leach or may be used as a preleach to prechloridize at least a portion of the chlorine consuming alkali and alkaline earth metals contained in the feed material.

The present process is particularly useful in an oxidative, reductive chlorination such as that disclosed in U.S. S.N. 050,549, filed June 20,1979 and incorporated herein by reference. In such a process, generally the clay or bauxite isfirst pelletized with a hydrochloric acid binder solution. The pellets are high-density, high strength pellets. Following pelletizing, the pellets are dried, for example, at about 300°C in a direct fire dryer. Dry pellets are inventoried forfeed to the shaft chlorinator furnace. The clay or bauxite may be ground before pelletizing;

however, this does not affect the recovery of the metal values.

Shaft chlorinations require a high-crush, strong pellet feed which does not lose strength during chlorination. Pelletization of clay or bauxite without any binder produces a weak pellet when sintered at 300°C. Various binders can be utilized, for example, sulfuric acid, hydrochloric acid, sodium chloride and bentonite. When bentonite is utilized the sintering should be done at a temperature of about 1,000°C.

Hydrochloric acid is the preferred binder, particularly hydrochloric acid produced by the reaction of the sulphuric acid with chlorination residue.

The feed material whether or not pelletized, is then subjected to an oxidative chlorination step wherein the iron isfirst removed by selective chlorination. In this step, approximately 90 percent of the iron is converted and volatilized as ferric chloride with substantially no chlorination or volatilization of the other metal values present. The oxidative chlorination is conducted in the presence of chlorine and oxygen gases which are circulated for up to three hours through the charge of feed material to oxidatively chlorinate and volatilize 90-95 percent of the iron content. The oxygen is employed in an amount of from 20 percent to 60 percent and preferably from 30 percent to 50 percent by volume of the total gas composition. The chlorine is employed in an amount which is a small stoichiometric excess of that needed to chlorinate the iron. The oxidative chlorination is conducted at a temperature of from 650 to 900°C and preferably from 750 to 800°C for a time period sufficient to allow for the chlorination of most of the iron present. Generally, the time period is from 0.5 to 2 hours.

Thereafter, the material is subjected to a reductive chlorination. The degree of chlorination if silica in the reductive chlorination step is greatly reduced by using only carbon monoxide as a reducing agent ratherthan a carbonaceous material such as fuel oil or coke. Eliminating solid carbonaceous materials as a reductant has other advantages, such as, permitting initial oxidative chlorination of the pellet charge, increasing the strength of the pellets charged to the chlorinator as there is no loss in pellet strength during the chlorination as there is when coke, pitch or other carbonaceous material is added. Ordinarily, an oxidative chlorination followed by a reductive chlorination would necessitate an intermediate addition of coke to the feed, which would be an expensive process step. Surprisingly, this was found not to be necessary in this process.

The carbon monoxide gas is added to the chlorinator in an amount of from 30 to 70 percent and preferably from 40 to 60 percent by volume of the total gas composition. The chlorine is supplied in slight excess of the stoichiometric amount needed to chlorinate the aluminium present. Chlorine utilization is related to the rate of gas flow or space velocity, with respect to bed volume of the reactor. The reaction rate appears to be proportional to bed temperature with a lesser dependence on chlorinecarbon monoxide ratio in the recation gas.

The injection of silicon tetrachloride into the reaction gas mixture of chlorine and carbon monoxide is effective in reducing the amount of chlorination of siliceous material contained in bauxite, refuse, coal and clays associated with coal. From 3 to 30 percent silicon tetrachloride by volume of the total gascomposition may be injected during the reduction. For example, six percent of silicon tetrachloride combined with carbon monoxide, almost completely rejects silica chlorination with only a small loss in alumina recovery. A preferred method for introducing the silicon chloride is to run the chlorine through the liquid silicon chloride before it enters the reactor. The reaction of carbon monoxide is sufficiently exothermicto be self-heating. Generally, the temperature of the reductive chlorination step is from 600 to 850°C and preferably from 650 to 750°C.

The reductive chlorinator is operated for a time period of from 1 to 3 hours to collect a small amount of residual iron chloride in a first stage condensor and a high purity aluminium chloride in a second stage condensor. A third-stage condensor collects the chlorides of titanium and silicon. The use of frictional distillation to recover volatilized chlorides and noncondensables, e.g., chlorine, carbon monoxide and carbon dioxide, of the process is described in S.N. 050,549, filed June 20,1979.

The cooled, depleted pellets are conveyed to the leach circuit where water soluble chlorides, if present, are removed and calcium chloride is precipitated as gypsum with sulfuric acid. The residue solids are filtered, washed and sent to the disposal, while the hydrochloric acid solution produced is evaporated as required for water balance control and recycled to the pelletization step for reuse as a pellet binder and/or for reuse as a prechloridizer.

The above described process is advantageous in that chlorination residues can be rendered environmentally acceptable which enables them to be readily disposed of. Furthermore, the process is economical because the hydrochlorine acid produced can be recycled to the chlorination process.