US3067113A - Method for producing sodium borates of lowered iron content - Google Patents

Description

United States Patent @fifice 3,067,113 Patented Dec. 4., 1962 3,667,113 lviE'ii-IQD PRQBUCKNG S IBDEUM @F LGWEPED IRON CGNTENT Ray B. Fenland, Spokane, Wash, assignor to Unite States Borax & @Iihemical Corporation, Los Angeles, (lalit, a corporation of Nevada No Drawing. Filed May 1, 1961, Ser. No. 106,484 2 Claims. (Cl. 204112) The present invention relates to a method for reducing the iron content of sodium borate solutions.

Most commercial sodium borates, as for example, sodium tetraborate decahydrate and sodium tetraborate pentahydrate are produced from ores containing the crude borates combined with a gangue comprising clay and small amounts of various other impurities.

The common method for the recovery of the sodium borates is a wet process in which the ore is dissolved in water or mother liquor, treated with carbonate ion and separated from the gangue by subsequent screening, settling and filtration. The finished sodium borate products are then usually obtained from the clarified solution by crystallization. The clarified sodium borate solutions contain on the order of about 20-50 parts per million total iron and the finished products when crystallized from the solutions contain on the order of from about -30 parts per million total iron. For most of the commercial uses of the sodium borates this small quantity of iron is of no importance; however, there are various requirements for sodium borates wherein it is necessary that the iron content be on the order of 8 parts per million and less.

it is, therefore, the principal object of the present invention to provide a wet process method for producing sodium borates having a lowered iron content.

Other objects of the present invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said 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 principle of the invention may be employed.

Broadly stated, the present invention comprises the method of producing sodium borates having a lowered iron content which comprises placing an aqueous solution of sodium borate, containing trace amounts of soluble sulfur compounds and additionally containing iron as a contaminant into an electrolytic cell, passing an electric current through the solution at an applied potential that exceeds the decomposition potential of iron in said solution, removing said iron as a combination of metallic iron and iron sulfide at the cathode, separating said sodium borate solution from any metaliic iron and iron sulfide flakes and crystallizing sodium borate from the purified solution.

From the foregoing broadly stated paragraph it can be seen that the present method for removing iron contaminants from aqueous sodium borate solutions is predicated on the electrolytic deposition of iron from aqueous solutions. Although it is Well known that iron can be removed from aqueous solutions by electrolysis, such a method is normally quite slow and usually requires considerable amounts of electrical power when the concentration of iron. is low, in the range of from to 100 parts per million, and it becomes even more difficult to perform as the concentration of the iron present diminishes.

The present invention, however, prov-ices a method for the electrolytic removal of iron from aqueous sodium bot-ate solutions in such a manner as to increase the iron deposition rate by a factor of from about 5 to 12, and thereby lowering the power requirement substantially while producing a more highly purified sodium borate solution. Although the exact reaction mechanism for this process is not clearly understood, I have found that this can be accomplished by performing the electrolysis in the presence of sulfur compounds, wherein the sulfur is in some form other than sulfate.

When the electrolysis of the contaminated sodium borate solutions is performed in the presence of sulfur compounds the deposits on the cathode are found to be in the form of a combination of metallic iron and iron suifide. These deposits occur even when the sulfur is not present as sulfide per se, but in the form of a com plea or polysultide. The formation of insoluble iron sulfide is quite surprising since this has been found to be a very slow and most difiicult reaction to perform in sodium borate solutions having low iron concentrations.

Since the total iron to be removed from the solution is on the order of from about 2% to 50 parts per million the amount of sulfur required in the present process is small, and since the iron is removed as a combination of metallic iron and iron sulfide, the amount of sulfur required is less than the equivalent amount required to react with the total iron present. Some of the sodium borate ores contain enough soluble sulfur compounds as impurities, so that they themselves supply an adequate quantity of sulfur to the solution for performing the cataiyzed removal of iron by electrolysis. However, when electrolysiug a sodium borate solution which is free of sulfur, the sulfur is added in the form of a watersolu'ole salt other than a sulfate.

The electrolytic cell used in the present process can be of the most simple design and construction, identical to the well known electrolytic cells such as those used in the electrolytic preparation of hydrogen, and oxygen. Since the sodium borate solutions are non-corrosive to iron an inexpensive cell would be comprised of an iron or mild steel tank and since iron is removed at the cathode, the cathode can also be made from iron or mild steel. Many materials were tested as anodes and such materials as graphite, lead, nickel, tungsten, and platinum as well as other materials were found applicable to the present invention.

The prose 3 method for preparing low iron sodium borate soiutions by electrolysis is suitable for both batch type and continuous flow type operations, and in the preferred embodiment of the invention i use a continuous flow system. in a batch type operation, the solution, free from the 'gangue and containing a minimum of insoluble mat rial, is placed in the electrolytic cell and held there until the desired iron concentration, less than about 10 parts per million, is reached. In a continuous flow operation, the substantially solids free solution is continuously passed through an electrolytic cell of such capacity as to provide a proper residence time for the desired electropurification.

In either type of operation the iron is found to be deposited as a combination of metallic iron and iron sulfide on the cathode of the electrolytic cell. However, in both systems a certain amount of the deposits flake off the oathode due to excessive build up and attrition. In the case of the batch type operation the deposits settle to the bottom of the cell and are separated from the solution by decantation, while in a continuous system much of these solids are carried into the flow stream from which they are readily removed by filtration. The purified and clarified solutions containing less than about 10 parts per million iron, are then ready to undergo crystallization, from which a satisfactory product containing less than about 8 parts per million iron can be obtained.

So that the present invention is more clearly understood, the following examples are given for illustrative purposes.

The electrolytic cell used in the following examples comprised an iron box having an inlet at the bottom of one end of the box and an overflow outlet at the other end. The box was constructed so that screen type elec trodes could be fitted into the box, and when in operation the anodes and cathodes were placed alternately in the cell. The electrodes were connected to a variable power source and the flow rate of the solution was controlled by a variable pump. Each of the examples was conducted continuously for 5 to 6 hours with samples of the efiiuent being taken every 15 minutes to make up a large composite sample for analysis.

A 43% sodium tetraborate decahydrate solution containing 19.4 parts per million iron and about 15 parts per million sulfide was prepared. The solution, containmg about 0.015% solids, was pumped through the electrolytic cell at the rate of 1.98 gallons per hour, equivalent to about a 6.5 minute residence time. The power supplied to the electrodes was held constant at 3.0 volts and 4.0 amperes for the entire run. The effiuent was filtered and the composite sample was found to contain 8.2 parts per million iron. Thus after 5 hours at a power requirement of 12 watts, 9.9 gallons of solution containing 8.2 parts iron was obtained. H

A 42.6% sodium tetraborate decahydrate solution containing 34.4 parts per million iron and about 20 parts per million sulfide was prepared. The solution, containing about 0.013% solids, was pumped through the electrolytic cell at the rate of 1 gallon per hour, equivalent to about a 13 minute residence time. The power supplied to the electrodes was held constant at 4.2 volts and 3.2 amperes for the entire run. The efilucnt was filtered and the com posite sample was found to contain 5.6 parts per million iron. Thus after 5 hours at a power requirement of 13.4 watts, 5 gallons of solution containing 5.6 parts iron was obtained.

III

A 43.8% sodium tetraborate decahydrate solution containing 34.3 parts per million iron and about 30 parts per million sulfide was prepared. The solution, containing about 0.018% solids, was pumped through the electrolytic cell at the rate of 1.82 gallons per hour, equivalent to about a 7 minute residence time. The power supplied to the electrodes was held constant at 3.6 volts and 2.5 amperes for the entire run. The effluent was filtered and the composite sample was found to contain 8.0 parts per million iron. Thus after 5 hours at a power requirement of 9 watts, 9.1 gallons of solution containing 8.0 parts iron was obtained.

A 44.1% sodium tetraborate decahydrate solution containing 28.0 parts per million iron and about 18 parts per million sulfide was prepared. The solution, containing about 0.010% solids, was pumped through the electrolytic cell at the rate of 2.54 gallons per hour, equivalent to about a 5 minute residence time. The power supplied to the electrodes was held constant at 3.5 volts and 2.5 amperes for the entire run. The efiluent was filtered and the composite sample was found to contain 10.3 parts per million iron. Thus after 5 hours at a power require This example was done to compare the previous examples with the electrolysis of a sulfur free sodium borate solution. A 43% sodium tetraborate decahydrate solution containing 29.5 parts per million iron and about 0.015% solids was prepared. The solution was passed through the electrolytic cell at the rate of 0.46 gallon per hour, equivalent to about a 28 minute residence time. The power supplied to the electrodes was held constant at 3.2 volts and 6.0 amperes for the entire run. The eifiuent was filtered and the composite sample was found to contain 17.9 parts per million iron. Thus, with the sodium borate solution containing no sulfur, after 5 hours at a power requirement of 19.2 watts, only 2.3 gallons of solution containing 17.9 parts per million iron was obtained.

it will be seen from the foregoing examples that the electrolytic removal of iron from aqueous sodium borate solutions is greatly enhanced by the presence of trace quantities of sulfur compounds. It will also be noted that the present method provides a rapid and economical process for removing iron from such solutions which is readily adaptable to the commonly used wet process for preparing sodium borates.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I, therefore, particularly point out and distinctly claim as my invention:

1. The method of producing sodium borates having a lowered iron content which comprises placing an aqueous solution of sodium borate containing trace amounts of soluble sulfur compounds and additionally containing trace amounts of iron as a contaminant into an electrolytic cell, passing an electric current through the solution at an applied potential that exceeds the decomposition potential of iron in said solution, removing said iron as a combination of metallic iron and iron sulfide at the oathode, separating said sodium borate solution from any metallic iron and iron sulfide flakes and crystallizing sodium borate from the purified solution.

2. The continuous method for producing sodium borates having a lowered iron content which comprises continuously passing an aqueous sodium borate solution containing trace amounts of soluble sulfur compounds and additionally containing trace amounts of iron as a contaminant through an electrolytic cell while passing an electric current through the solution at an applied potential that exceeds the decomposition potential of iron in said solution, removing said iron as a combination of metallic iron and iron sulfide at the cathode, separating said solution from any flakes of metallic iron and iron sulfide by filtration and crystallizing sodium borate from the purified solution.

References Qited in the file of this patent UNITED STATES PATENTS 1,793,906 Christensen Feb. 24, 1931 2,722,480 Kumar NOV. 1, 1955 2,776,184 Kamen Jan. 1, 1957 2,834,727 Gullet May 13, 1958 2,961,294 Taylor ct al. Nov. 22, 1960 FOREIGN PATENTS 113,508 Great Britain Feb. 28, 1918

Claims (1)

1.THE METHOD OF PRODUCING SODIUM BORATES HAVING A LOWERED IRON CONTENT WHICH COMPRISES PLACING AN AQUEOUS SOLUTION OF SODIUM BORATE CONTAINING TRACE AMOUNTS OF SOLUBLE SULFUR COMPOUNDS A ADDITIONALY CONTAINING TRACE AMOUNTS OF IRON AS A CONTAMINANT INTO AN ELECTROLYTIC CELL, PASSING AN ELECTRIC CURRENT THROUGH THE SOLUTION AT AN APPLIED POTENTIAL THAT EXCEDS THE DECOMPOSITION POTENTIAL OF IRON IN SAID SOLUTION, REMOVING SAID IRON AS A COMBINATION OF METALLIC IRON AND IRON SULFIDE AT THE CATHODE, SEPARATING SAID SODIUM BORATE SOLUTION FROM ANY METALLIC IRON AND IRON SULFIDE FLAKES AND CRYSTALLIZING SODIUM BORATE FROM THE PURIFIED SOLUTION.