US1964719A - Method of producing chromates - Google Patents

Description

y 1934- J. B. CARPENTER, JR, ET AL 1,964,719

I METHOD OF PRODUCING CHROMATES Filed July 13, 1932 Zmvemg Emmi j? Sievemsom, Jah a Patented July 3, 1934 UNITED STATES PATENT OFFICE P. Stevenson,

Newton,

Mass., assignors to Arthur D, Little, Incorporated, Cambridge, Mass., a corporation of Massachusetts Application July 13, 1932, Serial No. 622,288 I 6 Claims.

This invention relates to the manufacture of sodium chromate. The commercial procedure in producing sodium chromate at the present time involves mixing chrome ore, sodium carbonate and lime, spreading the mixture on one or more hearths of a reverberatory furnace and roasting it for a prolonged period to effect an oxidation of the chromium constituents of the ore from a trivalent to a hexavalent state. This mode of 10 treatment necessitates laborious and expensive rabbling or turning-over of the mixture in order to obtain suflicient oxidization.

It is the principal object of this invention to provide an economical process for making sodium chromate without the present rabbling operation and which increases the efliciency of the oxidizing reaction involved, and results in the conversion of a high percentage of the chromium in the ore to sodium chromate.

There have been proposed various processes involving the briquetting of the ore with sodium carbonate, and with or without other ingredients, and the subsequent roasting of such briquettes. As far as we are aware, these proposals so to form the reacting constituents have failed to come into commercial use, for the reason apparently that the procedures recommended have entailed greater cost, either by way of increased operating expense or decreased chemical efliciency, than the established practice of rabbling in a reverberatory furnace. Our process contemplates the possibility of the use of low grade chrome ores by virtue of the fact that the type of operation herein described permits the widening of the production range while maintaining efficient operation of the furnace. The principal chemical reaction involved in the process is oxidation. In utilizing briquettes of ore and sodium carbonate with or without the admixture of lime and/or other ingredients, proposals to compensate for the omission of the rabbling operation by excluding furnace gases from the bricks, (presumably to increase the oxygen content of the available air) involve an indi- 45 rect heating of the briquettes, a decreased thermal efllciency of the process and an increased cost of installation and maintenance of the furnace. Proposals to lay the briquettes on the hearths of reverberatory furnaces and there roast them without rabbling involve lack of. adequate and rapid contact of air with the interior of the mass, with much the same detrimental effect as omitting the rabbling operation of ordinary commercial reverberatory furnace roasting. The gas 5: in the zone of the hearth of a reverberatoryfurnace is likely to be comparatively stagnant and not at all disposed to provide a rapid circulation into the interior of a brick placed on the hearth. Proposals to promote oxidation in the absence of rabbling by including an oxidizing chemical in the bricks are prohibitively costly, since the oxidizing chemical used will cost more than the customary rabbling sought to be replaced. The fact that the industry with which this invention is concerned has not adopted the idea of roasting the ore in briquetted form reflects failure to conceive of conditions under which bricks of substantial size can be formed to effect accessibility of oxidizing gases into the interior and means for roasting to insure a uniform diffusion of oxygen throughout the mass of a brick. Ou invention is directed to these ends.

The important reaction that takes place when chrome ore is roasted with sodium carbonate may be expressed by the followingequation:

Accordingly, in the presence of sufficient oxygen, one molecule of ferrous chromite (chrome ironstone or chrome iron ore) reacts with two molecules of sodium carbonate to give two molecules of sodium chromate plus carbon dioxide and ferric oxide. Hence, from a given amount of sodium carbonate only a definite or equivalent amount of sodium chromate can be produced, providing the reaction is complete. We consider the maximum quantity of sodium chromate that can be theoretically produced from a given quantity of sodium carbonate to constitute 100 per cent. conversion, regardless of the amount of chromium present in the ore used, and when we use the term 90% conversion we mean that an amount of sodium chromate has been formed which is equal to 90% of the theoretical equivalent of the sodium carbonate used.

We have discovered a procedure wherein briquetting of the roast may be practiced consistently with lowering the cost of sodium chromate production. Ourimproved process involves as an initial step the formation of self-sustaining masses of the material, (for example masses in the shape of oblong bricks or briquettes), and hereinafter for convenience referred to as bricks. We have discovered that for the eflicient and economical practice of the process the factors involved in the formation of the bricks should be so regulated as to render the bricks quite permeable to oxidizing gases at temperatures up no to 2000 F. and for this purpose to provide in the bricks a certain minimum proportion of voids, as more fully described below. These voidcontaining bricks are then subjected to a roasting process in a furnace wherein the oxygen in the furnace gases comes into direct contact with the bricks.

A preferred form of furnace is 9. cont uous tunnel kiln which permits piling of the bricks on cars or other conveying means in such manner as to occupy in the furnace the major part of the cross-sectional area accessible to the oxygen in the furnace gases. The object is thus to provide a body of piled-up bricks of porous character through, between and close to which, the draft of the furnace carries the oxygen in the furnace gases, with the consequent attainment and maintenance of velocitiesand local pressures of this oxygen suflicient to cause enhanced diffusion of the oxygen through the interior of the porous bricks. Sufficient air can be admitted to the furnace of the kiln to maintain'an adequate oxidizing condition. The passing of the furnace gases containing suitable oxygen contents through a porous obstruction constituted of a pile of the porous bricks is considered to be a very advantageous feature. The heat of the gas is imparted to the bricks directly, as distinguished from the less efficient and prohibitively costly'indirect heating action of a muille furnace; the acquisition of heat by the bricks is accompanied by an abundant forced penetration of oxygen through the voids of the bricks, as distinguished from the heating of bricks by radiation in the more or less stagnant zone of the hearth of a reverberatory furnace and/or the usual type of muflle furnace.

A specific instance of our improved process will now be described by way of example.

In the drawing:

Fig. 1 is a diagrammatic plan view, partly broken away, of a direct-fired tunnel kiln adapted to be employed in our improved process; and

Fig. 2 is a vertical transverse sectional view of the kiln of Fig. 1 showing particularly the manner in which the bricks including chrome ore and sodium carbonate are piled so as to be subjected to the direct flow of gas in the tunnel of the kiln.

We have discovered that the efliciency of conversion or percentage of yield, may be'considerably increased if the bricks are carefully formed so as to have a suflicient porosity to be quite permeable to the hot oxidizing gases to which they are subjected during roasting. This may be explained by the fact that the principal chemical reaction is one of oxidation and that the ability of the hot oxidizing gases to enter the innermost parts of the brick determines to a large extent the completeness of the reaction. We have further discovered that if a certain minimum porosity is not attained, ineflicient conversion will result. We prefer to form the bricks with at least 30% voids as measured by the volumenometer described on page 169, Vol. I, 1929, of the Journal of Industrial and Engineering Chemistry, this percentage of voids relating to the condition of the bricks when they have been air dried prior to roasting; a percentage of voids of 40% is even more preferable.

Conversions of over 95% have been consistently obtained by forming bricks measuring "x2"x3" from a uniform mixture of the following ingredients, no particle of which will fail to pass through an mesh U. S. standard Tyler sieve:

- Per cent on Grams dry basis Chrome ore (48.98% C1'2O3) 608 40.8 Soda ash 380 25.5 Lime (CaO) 500 33.? Water for mixing 300 Percent Chrome ore (48.98% C12O3) 43.5 Soda ash 30.0 Lime, CaO 26.5

when formed in the above manner and roasted for 6 hours at a temperature of 1900 F. give a conversion of chromite to chromate in excess of even though the percentage of lime used is considerably below that found necessary for present commercial roasting of chrome ore. We have found that bricks of this composition show a lower volatilization loss of chromium than those made with the initial mixture.

The bricks may, if desired, be formed by an ordinary pressing or molding operation in which, in order to obtain the desired porosity, only sufficient pressure is applied to cause the bricks to maintain their form when subjected to the usual shocks of handling. As illustrative of the advantage of carefully forming the bricks with sufficient voids, we have found that, with a given mixture of ingredients, a pressure of 350 lbs. per square inch will make a brick approximately %"x2x3 .sufliciently capable of standing shock provided the molding or forming pressure is released as soon as it reaches 350 lbs. per square inch. If all of the other conditions-are maintained identical and the pressure of 350 lbs. per square inch is retained for thirty seconds, the bricks thus formed show when roasted a decrease in conversion of as much as 8% (based upon the sodium carbonate content) below the bricks from which pressure is released immediately upon reaching 350 lbs.

Bricks for use in our improved process may also be made by extrusion. Bricks formed in this manner are in some respects preferable to bricks formed by molding in that the extruded bricks can carry more water and require less pressure to hold their shape and hence can be made in larger sizes and still retain a satisfactory porosity. Bricks have been extruded in the standard fire brick size (2}5"x4'/2"x8 /2") and roasted with the resulting conversion of over 90% (based on the sodium carbonate content).

The exact pressures to be employed in forming the bricks either by molding or extrusion, and the amounts of water to be mixed with the ingredients in forming the bricks will, of course, de-

1,964,719 pend upon the size of the bricks formed. The

conditions of pressure and water content can best be regulated to control the porosity of the bricks by measuring the percentage of voids in a dry brick by the volumenometer mentioned above and then decreasing the pressure or increasing the water content, if necessary, to provide the necessary percentage of voids.

Referring to Figs. 1 and 2, the formed bricks are piled upon cars 11 such as are used in continuous tunnel kilns, and the cars then passed continuously through a dryer (not shown) at a rate of travel such that the moisture is removed sufliciently to prevent undue cracking of the bricks in the subsequent roasting operation. Thereupon the loaded cars 11 pass continuously through the kiln 12 from its entrance end 12 to its exit end 12'. The tunnel kiln may, for example, have adjacent to its entrance end a preheating zone, may have its hottest zone in the middle of its length in which are located furnace units 13 discharging their products of combustion into the tunnel of the kiln, andmay have an unheated cooling zone located adjacent to the exit end of the tunnel, as indicated in Fig. 1. The tunnel kiln is of the direct-fired type in which the flow of the products of combustion of the furnace is in a direction opposite to that of the travel of the bricks, which action is due to the arrangement of the exhauster 17 which draws the spent gases through the preheating zone and discharges the same adjacent to the entrance 12 and the arrangement of the blower 16 which continually discharges a current of air into the cooling zone where it is preheated by being forced around the bricks discharged from the firing zone. The air thus admitted not only serves to coolthe bricks, but also insures the continued maintenance of a positive oxidizing atmosphere within the firing zone of the furnace. Such kilns are well known and need not be further described.

Referring to Fig. 2, the bricks are piled in a more or less open stack 15 on the cars 11 in such a way that the stack of bricks thus formed occupies as much as possible of the cross-sectional area of the tunnel, occupying 'as shown considerably more than a major portion of such cross section( The effect of this is to confine the stream of furnace gases, including the necessary air for oxidation, largely to flow through the stack of bricks.

The temperature of the hottest zone of the kiln is preferably adjusted to from 1700 F. to 2000 F., although higher or lower temperatures may be employed if desired. The gas stream flowing through the tunnel of the kiln is adjusted by the admission of sufiicient air to the furnaces of the kiln, and if desired, at other points (e. g., the discharge end of the furnace), to cause the stream to have an adequate oxidizing action. The

Y within the kiln is to cause an abundant absorption of oxygen into the voids of the bricks accompanied by a rapid and eflicient transfer of heat to the bricks. The draft of the tunnel kiln insures rapid circulation of the hot oxidizing and heating gases, preventing stagnation and providing uniformity of action on all of the bricks passed through the kiln.

The increase in temperature and amount of hot oxidizing gases caused to penetrate the bricks by our improved process renders possible a very eflicient chemical reaction between the chrome ore and sodium carbonate ingredients. The conditions of heat transfer are materially improved over those prevailing in the usual practice of rabbling the mixture in a reverberatory furnace, and over those which would prevail were any of the proposed briquette-roasting processes to be attempted. The general overall efficiency of our process is such that it can replace the customary procedure, with resulting economy in operating cost. It will be evident that the working conditions prevailing in the practice of our process are greatly improved over all processes involving the rabbling of mixtures in furnaces, since the only handling to which the bricks are subjected is done wholly outside of the kiln, in stacking and in removing the bricks from the kiln cars. Whereas in reverberatory furnaces, the mixture has had to be laboriously rabbled, and manually transferred from one section of the furnace to another, the cars carrying the bricks in our process can be propelled through the kiln with a minimum of effort, any suitable mechanical car pusher serving to advance the whole line of cars at desired intervals.

The bricks after having been roasted are then treated by any suitable extraction process, to obtain a solution containing the desired sodium chromate, and if desired, the residue can be treated as described in our copending application Serial No. 622,289, filed concurrently.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

We claim:

1. Process of making chromates by forming bricks including chrome ore and a suitable carbonate and roasting the bricks, characterized in that the bricks are sufficiently porous to be permeable to hot gases and are arranged in stacks, and a stream of hot oxidizing combustion gases from a furnace is caused to flow through the stack of bricks, thereby to promote a uniform penetration of heated oxidizing gas into the interior of the bricks.

2. Process of making chromates by forming bricks including chrome ore and a suitable carbonate and roasting the bricks, characterized in that the bricks are stacked and a stream of hot oxidizing combustion gases from a furnace is caused to flow through the stack of bricks, the bricks having at least approximately 30% of voids, by volume, thereby to promote penetration of heated oxidizing gas into the interior of the bricks.

3. Process of making chromates by forming bricks including chrome ore and a suitable carbonate and roasting the bricks, characterized in that the bricks are stacked and a stream of hot oxidizing combustion gases from a furnace is caused to flow through the stack of bricks, the bricks having at least approximately 40% of voids, by volume, thereby to promote penetration of heated oxidizing gas into the interior of the bricks.

4. Process of making chromates by forming bricks including chrome ore and an alkali-metal carbonate and roasting the bricks, including moving the bricks through the preheating, hottest and cooling zones of a direct-fired continuous kiln having a flow of hot oxidizing combustion gases, the bricks being sumciently porous to be permeable to hot gases and being in the form of a stack occupying the major part of the cross-section of the kiln, whereby to constrain the stream of hot gases in the kiln to flow through the stack and penetrate the bricks.

5. Process of making sodium chromate which comprises forming bricks which include chrome ore and sodium carbonate and contain at least 30% voids, by volume, and subjecting the bricks to a roasting operation, including moving the bricks through the preheating, hottest and cooling zones of a direct-fired continuous kiln having

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