US2898188A - Germanium compound recovery process - Google Patents

Description

United States Patent 8 GERMANIUM COMPOUND RECOVERY PROCESS Cortes F. Reed, Anoka, Minn., assignor to Charles L Horn, Minneapolis, Minn.

N Drawing. Application December 29, 1954 Serial No. 478,525

f 1 Claim. c1. 23-19 This invention relates to a new and improved method of recovering germanium fromalkali coal ash, flue dust,

fly ash and the like including lignitic fuel residues. More particularly, this invention relates to a method of recovering germanium in the form of refined metallic compounds, principally sulfides, by the extraction and chemical treatment of finely divided coal ashes and like waste coal products.

This application is related to applicants co-pending application, Serial No. 393,403, for Process and Apparatus for Recovering Germanium, filed November 20, 1953 (now abandoned), and constitutes a continuationin-part thereof, the present invention being an improvement over the invention disclosed and claimed in the co-pend'ing application.

As is pointed out in that application, germanium is a grayish-white, lustrous, brittle metal which has recently achieved great economic importance due to the discovery of its properties in rectifying electrical current and amplifying voltages. Although found in many minerals, no large deposits of germanium have been found anywhere. "It has also been known that germanium was accumulated to an appreciable extent in many coals. The copending application is directed to a method of recovering germanium fromcoal ash and other waste coal products. Thatmethod comprises generally the treatment of finely divided moistened coal residues with a halogen gas to convert the germanium compounds to soluble halides, washing the thus treated coal residues with water to extract the germanium compounds, treating the resulting solution with a Water soluble sulfide and then precipitating the germanium compounds. Because of the nature of the composition of the coal itself, the combustion products of some coals are alkaline. Such coals as mined may contain minor amounts of naturally occuring alkaline substances such as soda ash, potash, limestone and the like intimately distributed through the carbonaceous matter in the coal. The alkaline material is more concentrated after combustion so that the ash, flue dust, fly ash and like combustion products of such alkaline coals are highly alkaline. In treating such alkaline ashes by the method of the co-pending application excessive amounts of the halogen gas. were required to first neutralize the alkalinity of the raw material before converting the germanium compounds to halides. Further investigation led to the discovery that the germanium is present in these alkaline coal residues in the form of water soluble germanium compounds, probably principally as the alkali and alkaline earth metal meta-germanates, which may be extracted by washing with water and thereafter converted to sulfides of germanium and precipitated. At the present time, because of its economic importance, there is great interest in the recovery of germanium, from whatever source. Depending upon its place of origin, coal may also contain minor amounts of other metals, many of which are of great economic importance.

It is the principal object of this invention to provide Patented Aug. 4, 1959 ice an improved process for recovering germanium from alkaline coal residues.

Another object of this invention is to provide a process for treating alkaline coal residues to extract the water soluble compounds of germanium and to convert these soluble compounds to an insoluble form which may be precipitated and recovered;

A further object of this invention is to provide a process of washing finely divided alkaline coal residues with water to extract the soluble germanium compounds and then treating the resulting solution with a soluble sulfide and precipitating and recovering the resulting germanium sulfides.

Other objects of the invention will become apparent as the description proceeds.

To the accomplishment of the foregoing and related ends, this invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

Broadly stated, this invention comprises a process for the recovery of germanium from alkaline coal residues, which includes the steps of washing the finely divided coal residue with water to extract the soluble germanium compounds and treating the resulting, aqueous solution with a water soluble sulfide to precipitate the germanium as sulfides. If desired the precipitate may be burned in the presence of excess oxygen to separate any precipitated sulfur and convert the recovered germanium compounds to oxides which may thereafter be reduced to the metallic state. Preferably an alkali sulfide or polysulfide is added directly to the wash water. The filtrate resulting from the water washing of the coal residue is acidified to precipitate the sulfide-treated compounds when germanium is present in quantity in the coal residue. The resulting precipitate is characteristically chocolate brown in color and consists principally of some free sulfur and both germanium mono-sulfide (GeS) and germanium di-sulfide (GeS Precipitation is assisted and hastened by heating the acidified filtrate to a temperature in about the range from F. to boiling. The precipitate is preferably washed with carbon bisulfide to extract any free sulfur present or the sulfur may be removed by fractional distillation or controlled oxidation.

The raw coal residue starting material is preferably in a finely divided state in order to assure substantially complete extraction of the germanium compounds. .If not originally in a finely divided state the ashes may be pulverized by grinding. The degree of fineness which the ashes should be depends largely upon the physical structure of the ashes themselves. Soft, porous or flaky ashes need not be as finely ground to effect substantially complete extraction as more solid, hard, glassy type ashes. In general, for high yields, the ashes should be fine enough to pass an eight mesh screen and preferably a fourteen mesh screen.

The extraction of the germanium compounds from the coal residues may be carried out in any conventional filtering and extraction apparatus. The ashes are desirably first formed into a suspension or slurry to insure more intimate contact between the ash particles and the water. The extraction may take place on an Oliver drum filter or similar apparatus. The coal residue may also be washed on apparatus such as is described in applicants co-pending application. Alternatively, any other type of apparatus or container may be used to wash the coal residue and recover the extracted soluble compounds. The only requisites. arethorough washing and complete drainage of the wash water. ample, an open railroad gondola car may be used. Rough, incomplete recovery of the soluble compounds maybe accomplished by dumping the coal ashes into a non-draining pond or small lake from which the soluble compounds may be removed when accumulated in sufiicient amount.

Preferably the ash material is washed several times to insure substantially complete extraction of the germanium compounds. Depending upon the number of washes, from about one-half to four or five times the weight of the ashes of water is used in this process. Preferably, the ashes are washed with water in an amount from about one to three times their weight. While great excesses of water may be used, handling of this excess water creates additional and needless expense.

Coal ashes retain about one third of their weight of water when fully saturated. This amount will vary somewhat on the type of ash and its physical condition. If three successive washings are made using a weight of water equivalent to the weight of ashes each time, there will remain about one twenty-seventh of the soluble product in the ashes. If a countercurrent system of washing is employed using a weight of water equivalent to the weight of ashes in each stage, there will be left in the ashes 1.5% of the product in a four stage system, or 0.4% in a five stage system,- and the concentration of the product in the solution will be 1.5 times as great as that of the original ash. At least part of the soluble sulfide may desirably be added to the wash water.

After the germanium compounds are washed from the ashes and the solution is filtered to remove any sedimentary material the spent ashes are discarded and the alkaline solution of germanium compounds is acidified. Suflicient acid is added to bring the pH of the solution lower than and preferably between about pH 2 and 4. The solution is acidified for the purpose of precipitating the germanium compounds from the sulfide treated filtrate and for keeping in solution the other less valuable metals which would otherwise contaminate the final wanted product. For this latter reason, hydrochloric acid is the preferred acid.

Because they will not normally be present in the coal residue material in a form which will permit them to be extracted with water or with an aqueous sulfide solution, there is little likelihood of contamination by iron, aluminum and silicon. However, in most instances, it is desirable to make the solution relatively highly acid to forestall the likelihood of contamination by these compounds. For example, at pH 5 silicon precipitates; between pH 4 and 5 iron and aluminum precipitate. Similarly, if relatively high acidity of the solution is not maintained, gallium and titanium are precipitated if present in the filtrate. If any of these elements is a wanted material it is recovered from the acidified filtrate. Normally, from about 0.5 to 2 or 3 percent by weight of the original starting material of concentrated hydrochloric acid will be required to produce the desired acidity in the filtrate depending upon the alkalinity of the raw coal residue.

To precipitate the germanium compounds from the filtrate, it is preferred to employ water-soluble polysulfides of the alkali or alkaline earth metals, such as, for example, the polysulfides of sodium, potassium, lithium, calcium, barium and strontium. These water-soluble polysulfides are easily and readily prepared by boiling sulfur in a solution of a water-soluble metal hydroxide. For example, sodium polysulfide solution may be prepared by boiling 100 parts by weight of sulfur with 250 parts by weight of caustic soda covered with water until the sulfur dissolves and then diluting to form 2500 parts by weight. The sulfide is preferably added to the wash water or to the filtrate in an amount somewhat greater than the calculated stoichiometric equivalent required For exfor reaction with the germanium compounds in the coal residue. For most ash samples this will range from about 0.5 to 5 percent by weight of concentrated aqueous polysulfide solution based upon the dry weight of the original raw ash starting material. On the basis of sodium polysulfide solution prepared as described, sulfur is added in an amount from about 0.02 to 0.2 percent by weight of the original coal ash. Since any excess polysulfide used may be recovered as free sulfur, larger amounts of polysulfide solution than are necessary may be used without appreciable economic loss.

Coagulation and precipitation of the germanium compounds may be hastened by raising the temperature of the filtrate to the range from about F. to boiling, preferably by the introduction of steam in large scale operations, taking care not to drive off all of the hydrogen sulfide in solution. When polysulfide is added to the original wash water acidification must necessarily follow separation of the ash material from the wash water. When the extraction wash is with water alone the acidification step may precede the addition of the sulfide; it may follow the addition of the sulfide or the two steps may be substantially coincidental. The precipitate may be separated by decantation or filtration or partial decantation followed by filtration.

The precipitate may be collected in a settling tank, a labyrinth or a filter or a combination of these devices. The solution should be settled or filtered before precipitation to minimize contaminants as much as possible.

Because some free sulfur may be precipitated by the action of the soluble sulfide on the acidified solution containing the germanium compounds, the precipitate after removal of the spent liquid is preferably washed with an inert sulfur solvent such as carbon bisulfide to remove the sulfur. If any arsenic is contained in the precipitate it can readily be removed by washing with sodium bicarbonate solution and recovered if desired. The precipitate may be heated out of the presence of air or other oxidizing atmosphere to distill and further concentrate the germanium compounds, after which the precipitate is burned in the presence of free oxygen or an excess of air (in a relatively closed retort to prevent losses by volatilization) to convert all of the precipitate to germanium dioxide. The dioxide may then be heated in a reducing atmosphere to yield metallicgcrmanium. The final products may be purified by methods which are referred to in the co-pending application but do not form part of this invention.

The invention is further illustrated by the following specific examples directed to the recovery of germanium sulfides.

Example I One kilogram of alkaline ashes was covered with one liter of tap water. The mixture was stirred until the ashes were in suspension and then was heated to the boiling point. Thereafter the suspension was filtered and was washed first with a liter of hot water and then with a liter of cold water. Fifty milliliters of 1.20 N sodium sulfide solution was added to the filtrate and admixed. The filtrate was acidified with hydrochloric acid and allowed to stand. The resulting precipitate had a pink color initially and turned dark brown on standing. The precipitate was filtered and dried. The net yield amounted to 1.2970 grams corresponding to 1176.6 grams per short ton of ashes.

Example II One kilogram of alkaline ashes was covered with a liter of water containing 50 ml. of 1.2 N sodium sulfide solution. The mixture was heated to boiling and filtered. The filter was washed twice with one liter of hot water each time. The combined filtrate was acidified with hydrochloric acid and allowed to stand. The precipitate was initially almost white and turned darker upon standing. The filtrate was allowed to stand overnight and then filtered and dried. The precipitate was a characteristic 270.79 grams per ton of ashes.

chocolate brown. The yield amounted to 1.3 grams net, corresponding to 1179.3 grams per short ton of ashes.

Example Ill One kilogram of ashes was washed three times, using one liter of boiling water for each wash. Calcium polysulfide solution was added to the filtrate and the mixture was acidified with sulfuric acid. A voluminous pinkish white precipitate was obtained and separated. An additional amount of calcium sulfide solution was added to the filtrate which was then allowed to stand overnight. The second precipitate was separated and added to the first and the total precipitate was redissolved in sodium hydroxide to purify the product, was filtered and reprecipitated by acidification. The precipitate was extracted with carbon bisulfide to remove free sulfur present. The final product was brown and was present in an amount of 0.3162 gram corresponding to 286.85 grams per ton of ashes.

Example IV One liter of water was added to one kilogram of ashes and the suspension was boiled for 30 minutes and filtered. The ashes were washed twice with one liter of boiling water each time. The filtrate amounted to 2400 ml. and was divided into two 1200 ml. samples. T 0 one portion of the filtrate was added m1. of a sodium sulfide solution consisting of 40 grams of sulfur and 100 grams of caustic soda per liter of solution. To the other portion was added 7.5 ml. of the same sodium sulfide solution. Both portions were acidified with hydrochloric acid and heated to promote coagulation of the product. The first portion produced a yield of 0.2380 gram corresponding to 215.9 grams per ton of ashes. The precipitation of the second portion appeared to be incomplete as indicated by a slightly pink color in the filtrate. The yield here was 0.2365 gram corresponding to 214.5 grams per ton of ashes.

Example V One liter of water containing 40 ml. of saturated calcium polysulfide solution was added to one kilogram of ashes and the suspension was heated to boiling for 30 minutes and filtered. The ashes were washed twice with a liter of water each time. The filtrate was acidified with m1. of concentrated hydrochloric acid and heated to assist coagulation. The brown sulfide product was produced in the amount of 2.9850 grams corresponding to This example was repeated differing only in that the acidified filtrate was heated to boiling for 30 minutes and allowed to stand overnight. The yield was 2.6975 grams corresponding to 244.5 grams per ton.

Example VI One liter of water containing 40 m1. of sodium polysulfide solution was added to one kilogram of ashes and the suspension was heated ot boiling for 30 minutes. The suspension was filtered and the ashes were washed twice,

each time with a liter of water. The last wash was kept Example Vll A 1770 gram sample of ashes was suspended in 1500 m1. of water containing ml. of saturated calcium sulfide solution. The suspension was boiled for 30 minutes, filtered, washed and acidified with 18 m1. concentrated hydrochloric acid. The yield was 5.215 grams corresponding to 2.94 grams per kilogram or 266.7 grams per ton.

Example VIII Ten kilograms of an alkaline fly ash was put in an 8 gallon earthenware vessel. Four liters of water containing 2 mols of sodium polysulfide was added and admixed and left to stand overnight. The following day the thus treated ashes were washed three successive times with 10 liters of water and filtered. The extract was collected in an 8 gallon earthenware vessel and acidified with hydrochloric acid to precipitate the germanium sulfides. Precipitation was assisted by heating the acidified filtrate. The resulting voluminous precipitate was filtered and washed with carbon bisulfide to remove free sulfur leaving a reddish brown sulfide product.

Because of the simplicity and eifectiveness of the recovery process of this invention it would be beneficial and more economical if it could be used for the extraction of germanium compounds from normally nonalkaline coal residues. Since the alkalinity in alkaline ashes is due to naturally occurring alkali materials present in the coal before it is burned, an artificially produced alkaline ash material may be formed by adding alkali to the coal before it is burned. means is available, of course, only where the coal is burned in finely divided form. The alkalinity is introduced by admixing small amounts of soda ash, lime, potash, caustic soda or the like with the coal before it is fed to the furnace. The alkali entraps the germanium present in the coal by fusion and reacts to produce water soluble compounds which may then be extracted and recovered by the method of this invention.

It is apparent that many modifications and variations of this invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of example only and the invention is limited only by the terms of the appended claim.

What is claimed is:

A process for the recovery of germanium compounds from alkaline coal combustion residues which consists essentially of the steps of washing the coal residues with from about /2 to 5 times their weight of water, said water containing a water soluble polysulfide of a metal selected from the group consisting of alkali and alkaline earth metals, said polysulfide being present in an amount sufiicient to add sulfur in an amount from about 0.02 to 0.2 percent by weight of the coal residue starting material, said amount of sulfur being at least the stoichiometric equivalent of the germanium compounds contained therein, separating the solution of extracted germanium compounds from the washed coal residues, acidifying the solution with hydrochloric acid to between about pH 2 and pH 4, heating the acidified solution to about F. to boiling to hasten precipitation and then separating the precipitated germanium compounds.

References Cited in the file of this patent FOREIGN PATENTS Great Britain June 2, 1927 OTHER REFERENCES