US1609615A - Production of sodium sulphide - Google Patents

Description

Patented Dec. 7, 1926.

UNITED STATES PATENT OFFICE.

HORACE FREEMAN, OF SHAWINIGAN FALLS, QUEBEC, CANADA, ASSIGNOR OF ONE- HALF TO CANADA CARBIDE COMPANY, LIMITED, 01'' MONTREAL, CANADA.

PRODUCTION OF sulphide.

The principal objects of the invention are the production of a fused sodium sulphide product of much greater purity than heretofore. obtainable and the provisionof a process, such that electric furnaces of the charge resistor type are rendered available and efiicient for this manufacture.

Prcliminaryto a disclosure of the invention and for the purpose of assisting in clearly indicating the novelty of the invention, certain observations may be made regarding existing commercial methods employed in this art and the products resulting therefrom.

The generally employed method of making sodium sulphide is to reduce sodium sulphate by fusing the same, at approximately the temperature of red heat in a fuel fired furnace, with a carbonaceous reducing agent while excluding air. Reduction of the sulphate at first takes place normally and sodium sulphide is produced, the charge being molten, so that good contact is obtained between the sulphate and the reducing agent. As reduction proceeds the charge thickens and, when the sodium sulphide content reaches about 65%, the charge solidifies quite suddenly. This solidification cannot be prevented'and a charge thus solidified cannot be again fused, even by subjection to the most intense heat of an electric furnace. Attempts to fuse the charge cause only excessive volatilization of the sodium sulphide. The solidified product is a hot cokey mass. diflicult to handle, which burns rapidly on exposure to the air.

It has been discovered that this phenomenon-of charge solidificationis due to the infusibility of substantially pure sodium sulphide. In the early stages of the process, the small -present is uxed by the'large amount of sodium suI hate, which accounts for the molten con ition of the charge in these early stages.

roportion of sodium sulphide SODIUM SULPHIDE.

2, 1925. Serial No. 36,747.

To avoid solidification of the charge in the furnace, the charge is usually drawn when the sulphide content is about 60%. The product, therefore, carries 40% of impurities, being mostly unreduced sodium sulphate and unu ed reducing agent with perhaps some silica from the refractory lining of the furnace and, if the temperature of operation has been below 1050 (3., some sodium carbonate. Reasonably pure sodium sulphide can be obtained from this fused product only by lixiviation and crystallization. r

Heretofore, attempts have been made to utilize the electric furnace for this manufacture, because of the high temperatures which could be obtained and which it was thought would permit of the reduction being carried to or much more nearly to completion. Such attempts showed practically no better results than were obtained with fuel fired furnaces and have not led to commercial operations because of the same difficulty as experienced in the existing process, namely, solidification when charge was approximately 65% reduced. That is to say, even in the electric furnace and notwithstanding the application of high electric pressures, the solid fied charge will not carry the current and will not again melt, so that reduction cannot proceedlunless other means are taken to render the charge fluid and electrically conductive.

I have found that the disadvantages of the method just described may be overcome and electric furnaces rendered commercially available if there is added to the furnace charge another substance which will serve to flux the sodium sulphide as it is formed. By the addit on of suitable amounts of such fluxing agents, the charge is maintained freely fluid and satisfactorily electrically conductive throughout the entire reduction period. The freely mobile liquid state of the molten charge secures perfect contact between the sulphate and the reducing agent, so that complete reduction may be realized with little or no more than the theoretical I amount of reducing agent, thus practically eliminating one of the impurities heretoforeblocked by a solidified charge, as happens not infrequently with the previous methods. The completely fluid state of the charge throughout the reduction period produces stable electrical conditions in the furnace,

such that electric furnaces may be economically used to obtain complete reduction, which is not commercially practicable in a fuel fired furnace, even using fluxing agents, because of the great heat absorption during reduction.

I have found that a great number of substances possess the property of fluxing sodium sulphide, but some of these are of little commercial value on account of the relatively large amounts required and because they would'constitute undesirably large proportions of impurity in the product. I therefore do not contemplate, as embraced in this invention, the use of any substance the amount of which, to maintain free fluidity of the charge at temperatures between 1000 and 1500 C., would be 20% ormore of the product. Generally, I prefer to use only those substances which will represent less than 10% impurity in the product and which will maintain the charge liquid at temperatures between 1000 and 1200 C. Certain other substances which have flux-s ing action are undesirable for various reasons.

As indicating the large number of sub stances available, it may be stated that the following will serve 1. Alkali metal sulphides, other than sodium sulphide, such as potassium sulphide, of which less than 5% will sufiice.

2. Carbonates, such as sodium carbonate in large excess or potassium carbonate in excess of 20%.

3. Chlorides in excess of 10%.

4. Heavy metal sulphides in amount usually between 5% and 10%.

5. Alkaline earth metal sulphides in certain proportions, such as calcium sulphide in amount between 20% and 50%.

6. Polysulphides, which are not desirable on account of the fact that they are unstable at the temperatures used.

7. Silica in amount between 5% and 10%.

8. Certain sulphates, such as sodium sulphate in amount over 35%.

Dealing, now, with the sulphides which are for many reasons preferred, it may be said that where these can be produced by reduction or otherwise along with the sodium sulphide and without detrimental effect, the corresponding sulphates or any other compounds may be used. For example, potassium sulphate may be added to a normal sodium sulphate (salt cake) charge and will be reduced to potassium sulphide. Alternatively, potassium chloride may be added to an acid sulphate (nitre cake) charge and will be reacted on by the acid of I have found that less than 5% of potassium sulphide will serve to keep the charge freely liquid at a temperature of approximately 1000 C. throughout theperiod of complete reduction of sodium sulphate and, therefore, potassium sulphide is the preferred fluxing agent. Moreover, it is not an undesirable impurity in the product since it functions exactly as sodium sulphide in most of the uses of the latter. The potassium sulphide may be introduced into the charge as such or in theform of any substance which will yield it during heating of the charge.

The most desirable temperature for reduction of sodium sulphate to sodium sulphide is between 1000 and 1200 C. At slightly over 1000 C., say 1050 C., the gaseous product is carbon monoxide and only sul phide is produced, whereas below l000,C. some carbon dioxide is liberated and sodium carbonate is formed. This represents both a loss of sulphide and an undesirable impurity in the product. An additional reason for the use of temperatures over 1000 C. is that less flux is required than at temperatures below 1000 C. y l

The amount of reducing agent added should be sufficient for complete reduction of the sodium sulphate according to the equationz with perhaps a slight excess to ensure completion of the reduction within a reasonable time and whatever additional amount, if any, may be needed for production of the fluxing agent. Under these conditions (temperature agent) and with the use of flux or liquef ing agent, it is possible to produce fused so ium sulphide of more than purity with and amount of reducing.

great facility and, by use of the electric,

furnace, at remarkably reasonable cost.

The use of fluxing agents, in addition to enabling complete reduction to be effected,

enables the electric furnace to be used commercially in this manufacture; which usev has not heretofore been practicable, and which in turn is materially instrumental in obtain-- ing complete reduction. I have found it best to use the charge as a resistor.

Since all known refractories are fluxed more or less rapidly by sodium sulphideat the temperatures employed, I have found the most suitable type of furnace is a steel shell externally water-cooled. The sodium sulphide, even with the fluxing agent present, has a high melting point and chills in a thick layer on the cool steeLshell, which layer is electrically non-conductive and a good thermal insulator. The product made in a furnace of this type is therefore free from impurities which otherwise would eniaa ter from the furnace walls and the furnace installation is permanent and inexpensive to maintain.

In carrying out the production of sodium sulphide according to this invention, normal sodium sulphate (salt cake) or acid sodium sulphate (nitre cake) with a calculated addition of sodium carbonate to neutralize the acid, is mixed with a properly calcu lated amount of a fluxing agent and a suitable carbonaceous reducing agent, such as coal or coke dust, asphalt base oil Or charcoal, having an available carbon content in amount equal to'or slightly in excess of the theoretical amount required for reduction of the-sulphate, and the mixture heated to and maintained at a temperature between 1000 and 1500 C. and preferably not over 1200 C. until complete reduction of the sulphate is eflected, the charge being a mobile liquid during reduction and after reduction until tapped from the furnace.

Where reduction or reaction is necessary for production of the fiuxing agent in charge, the necessary reducing agent or reacting substance must also be added to the charge.

The product resulting is anhydrous sodium sulphide admixed with the fluxing agent and perhaps a small amount of the reducing agent. When using potassium sulphide as a fluxing agent, the product will contain over and usually over available sodium sulphide. Using other fluxing agents, the proportionof impurities in the product will be approximately as. indicated in the classification of.these agents.

While certain substances and classes of substances'have been referred to,- it will be understood that this is by. way of explanation and illustration and is not to be construed as limiting the invention to the employment' of these substances. The term fiuxing agent may be said to include and is used in the following claims as including any substance which, when added to the charge in amount'not more than 20% of the product, will maintain the sodium sulphide in a completely fluid molten and electrically conductive state at temperatures between 1000 and 1500 C., but excluding substances which have detrimental eflect on the reduction process.

Having thus described my invention, what I claim is 1. A process of making sodium sulphide, which comprises reducing sodium sulphate by melting the same in presence of a carbonaceous reducing agent and a fluxing agent capable of maintaining upwards of by melting the same in presence of a carbonaceous reducing agentand a fluxmg agent capable of maintaining upwards of four times its weight of sodium sulphide fluid at a temperature between 1000 and 1200 C.

3. A process of making sodium sulphide, which comprisesreducing sodium sulphate by melting the same in presence of a carbonaceous reducing agent and a fiuxing agent capable of maintaining upwards of nine times its weight of sodium sulphide fluid at a temperature between 1000 and 1500 C.

4. A process of making sodium sulphide, which comprises reducing sodium sulphate by melting the same in presence of a carbonaceous reducing agent and a fluxing agent capable of maintaining upwards of nine times its weightof sodium sulphide fluid at a temperature between '1000 and 1200 C. I

5. A process of making sodium sulphide, \vhichcomprises' heating at a temperature between 1000and 1500 C. a mixture initially including sodium sulphate, a carbona ceous reducing agent and a substance which will under the operating conditions yield a fiuxing agent capable of maintaining upwards of four times its weight of sodium sulphide fluid at the operating temperature.

6. A process of making sodium sulphide,

-which comprises heating at a temperature between 1000 and 1500 C. a mixture initially including a carbonaceous reducing agent, substances which will under the operating conditions yield sodium sulphate, and a fluxing agent capable of maintaining upwards of four times'its weight of sodium sulphide fluid at the operating temperature.

7. A process of making sodium sulphide, which comprises heating at a temperature between 1000 and 1500 C, a mixture in1- tially including a carbonaceous reducing agent, substances which will under the operating conditions yield sodium sulphate, and potassium sulphide,

8. A process of making sodium sulphide, which comprises heating at a temperature between 1000 and 1500 C. a mixture ini- 3 tially including a carbonaceous reducing agent, sodium sulphate and substances which will under the operating conditions yield potassium sulphide.

9. A process of maln'ng sodium sulphide, which comprises heating at a temperature over 1000. C..a mixture including a carbonaceous reducing agent, a fluxing agent capable of maintaining upwards of four I times its weight of sodium sulphide fluid at the operating temperature and substances which will under the operating conditions yield sodium sulphate. 10. An electri furnace process for the production of sod um sul phide, which COII lprises providing in an electric furnace a mixture of sodium sulphate, a carbonaceous reducing agent and another substance which will cause sodium sulphide to melt to fluidity and become electrically conductive, and pass ing through the mixture an electric current sufficient to maintain a temperature above 1000 C. until reduction of the sulphate is substantially complete, and finally tapping the resulting molten sulphide from the furnace.

11. An electric furnace process for the production of sodium sulphide, which comprises providing in an electric furnace a mixture of sodium sulphate, a carbonaceous reducing agent and another substance which will under the operating conditions yield a lluxing agent which in turn will cause sodium sulphide to melt to fluidity and become electrically conductive, and passing through the mixture an electric current suflicient to maintain a temperature above 1000 C. until reduction of the sulphate is substantially complete, and finally tapping the resulting molten sulphide from the furnace.

12. An electric furnace process for the production of sodium sulphide, which comprises providing in an electric furnace a mixture including a carbonaceous reducing agent and substances which will under the operating conditions yield sodium sulphate and a tluxing agent capable of causing sodium sulphide to melt to fluidity and become electrically conductive, and passing through the mixture an electric current sufficient to maintain a temperature above 1000 C. until fluxing agent which will under the operating conditions cause sodium sulphide to melt to fluidity and become electrically conductive, and substances which will under the operating conditions yield sodium sulphate, passing through the mixture an electric current sufficient to maintain a temperature over 1000 C. until reduction of the sulphate is substantially complete, and finally tapping the resulting molten sulphide from the furnace.

14. An electric furnace process forthe production of sodium sulphide, which comprises providing a mixture including a carbonaceous reducing agent, sodium sulphate and a fluxing agent, in amount not over 20% ofthe sodium sulphide obtainable, the said fluxing agent being capable of maintaining upwards of four times its weight of sodium sulphide molten, fluid and electrically conductive at temperatures over 1000 (1., and passing through the mixture an electric current sufiicient to maintain the same at a temeperature over 1000 C. until reduction is substantially complete.

15. An electric furnace process for the production of sodium sulphide, which comprises providing a mixture including a carbonaceous reducing agent, sodium sulphate and a fiuxing agent in amount not over 10% of the sodium sulphide obtainable, the said fluxing agent being capable of maintaining upwards of nine times its weight of sodium sulphide molten, fluid and electrically conductive at temperatures over 100 0., and passing through the mixture an e1 ctric current sufficient to maintain the same at a temperature over 1000 C. until reduction is substantially complete.

In witness whereof, I have hereunto set my hand.

HORACE FREEMAN.