Prof. V. I. Minaev.

Adjunct in Chemical Technology.

(Jointly with students B. M. Fromberg and P. I. Goretovskii).

On the Issue of Methodical Leaching of Wood-Fuel Ash for the Purpose of Extracting Potash

(Experimental study; 1917–18).

“How many valuables are lost each year to the state in the form of unused wastes, residues, or by-products, dirt, etc.? How many losses does the country suffer from imperfect methods of manufacture and incomplete chemical reactions? Millions and millions.”

“It may be stated directly that, in the chemical industry, every production process is worth the more, the fewer wastes and residues it has.”

“We are in great need of potassium salts; in part they go for fertilizing fields, in part for making explosives, etc. In our cities, for heating residential premises, at factories, etc., birch firewood is now used, leaving in the form of ‘waste’ colossal quantities of ash; and this ash contains, among other things, potassium salts.”

“With more careful handling of our ‘wastes,’ we could produce annually from unnecessary ash potash worth tens of millions of rubles, while thereby reducing foreign imports. The economy of our cities would expand and receive new sources of revenue; our cities would transfer this potash to factories and to the countryside to increase the fertility of fields, and the unused waste—ash—would turn into such valuable daily bread!”

(Acad. P. I. Valden).

Introduction. Wood ash, as a source for obtaining potash, is not a novelty in the potash industry. On the contrary, quite recently this source was the chief, if not the only, raw material for extracting potash. As late as the beginning of the seventies, up to 200 potash works operated in Russia, processing ash, and the potash obtained at these works was a significant item of export abroad. But in the form in which this method had existed earlier—when, in order to obtain ash, entire forest areas were burned—it could not exist for long. Being a highly characteristic example of predatory economy, when to obtain potash worth 50–100 kopecks it was necessary to burn an entire cubic sazhen of firewood, this source of potash naturally had to give way to another, as soon as one was found—the Leblanc method of obtaining potash from Stassfurt potassium chloride.

Returning here to the old source of potash, wood ash, we see its merit precisely in what earlier was its chief drawback. The process of extracting potash, formerly predatory, is now transformed into its direct opposite, into a process of rational economy, for in the present case we have in view ash that is ordinarily a waste, the ash of wood fuel.

For the purpose of obtaining heat, Siberia burns enormous quantities of timber; the ash, in the best case, sometimes finds some exceedingly unsuitable application in domestic use. Meanwhile, a rationally organized collection of ash would make it possible to create an industry yielding a valuable product.

What has been said is sufficient to understand the motives that prompted us to attempt an investigation of the technical extraction of potash from wood-fuel ash.

The extraction of potash from wood-fuel ash reduces to the following operations:

1) Leaching;
2) Boiling down the lye with fractional crystallization;
3) Calcination;
4) Purification of the potash.

Therefore, consideration of the question of extraction reduces to studying these processes from the standpoint of the greatest yield and purity of the product.

Leaching. With regard to leaching, it is first of all necessary to choose the temperature of the water with which the leaching is carried out. Since the solubility of potash, as well as of the impurities ($\mathrm{K_2SO_4}$, $\mathrm{KCl}$, $\mathrm{Na_2CO_3}$), increases with increasing temperature, it is obvious that leaching with hot water will proceed more rapidly, will give an economy in the amount of water consumed, in the duration of the process, and, finally, in the number of apparatuses. But, on the other hand, practice has established that with cold leaching a product of better quality is obtained.

By comparing the additional expenditure for heating the water and, if possible, for purifying the potash obtained, with the saving in water consumption, etc., one may establish the comparative advantage of cold and hot leaching. In this respect it is of interest to dwell on three temperatures: room temperature—15°C, then 50°C, and, finally, a temperature close to the boiling point of water, i.e., about 100°C.

The next factor characterizing leaching is concentration. In Russia, wooden vats with a double bottom are usually used, one of which, namely the upper one, is made perforated (false bottom). Leaching is begun with a weak lye obtained from previous operations, at 10–12° Bé, and finished with pure water. For further operations, lyes of 25° Bé are used.

Meanwhile, there are more perfect methods of leaching, based on the principle of countercurrent and systematic operation, making it possible to obtain strong lyes and, with low water consumption, to achieve complete leaching. This is achieved by the fact that, in a series of successive operations, fresh material is leached with strong lyes, while material that has already been almost leached out is treated with fresh water. The products of intermediate qualities, in turn, are leached with corresponding intermediate lyes.

Borrowing these methods of systematic leaching from the chapter on soda manufacture¹), we may divide them into two types, differing from one another in that in one case both the material being leached and the liquid circulate, while in the other case the material remains at rest and only the liquid circulates. In the latter case, the basic principle of systematic leaching (strong lyes—to fresh material, fresh water—to material already almost leached out) is achieved by means of purposeful communication between pipes. Without dwelling on a description of these methods, we shall merely note that the first method is carried out in the Clements—Desormes apparatus, and the second in the Buff—Dunlop or Shanks apparatus, the latter often being called a “chenks.”

¹) G. Lunge. Handbuch der Soda-Industrie, (1909), vol. II, p. 524 ff.

And if, in the leaching of soda, preference must undoubtedly be given to Shanks’s apparatus, both because in leaching in the latter no oxidation of the product occurs, the melt (unripened, in pieces) retaining its form and character to the end, and also because of the saving in labor, then in the leaching of ash only the latter circumstance is of importance.

On the contrary, the dust-like character of the ash, which causes caking of the product, and also the comparatively considerable content of alumina (from the furnaces), owing to which the permeability of the ash to water is diminished, compels one to assume the necessity of special measures for stirring the mass; this again increases the expenditure of labor and is easier for the Clements—Desormes apparatus than for the tank. On the other hand, the small content of potash in the ash (about 12% for birch ash), soluble in water, and consequently the considerable insoluble residue, presents certain difficulties, as we shall see below, for the methodical and continuous character of the leaching; to avoid this, the consumption of water has to be increased. Nevertheless, it is first of all necessary to study the process of leaching in Shanks’s apparatus, which has the greatest productivity with the minimum number of workers.

In the present case it is first necessary to determine the number of compartments in the tank required and sufficient so that the issuing lye should have a strength of approximately 25° Bé, and the lye flowing out of the box before its new charging should show that the leaching has been carried to the limit beyond which further leaching is no longer advantageous.

This limit can be found from the condition that the additional consumption of steam (in hot leaching) and the increase in the number of boxes in the tank associated with the possibly complete extraction would be covered by the value of the potash that can be extracted by means of the extra compartment in Shanks’s apparatus.

In addition, the composition of the last leached salts is also of importance. It may be supposed that, owing to the more difficult solubility of potassium chloride and potassium sulfate compared with the solubility of potassium carbonate, the final lyes will be richer in these salts, and therefore they should either be collected separately or left in the bottom ash, since the latter goes for fertilizer.

At least in the leaching of soda this circumstance is sometimes taken into account, and lyes of strength \(1/3^\circ\) Bé are collected separately, for these lyes prove rich in sodium sulfide \((\mathrm{Na_2S})\). However, even there different opinions exist on this question. As the limit of leaching for soda, lye at \(0.5^\circ\) Bé is taken. In our work the criterion of the completeness of leaching was chemical analysis—the testing of the lye with barium chloride and silver nitrate:

\[ \mathrm{K_2SO_4 + BaCl_2 = 2KCl + BaSO_4} \]

\[ \mathrm{K_2CO_3 + BaCl_2 = 2KCl + BaCO_3} \]

\[ \mathrm{KCl + AgNO_3 = KNO_3 + AgCl} \]

It goes without saying that the conditions indicated above do not yet determine the number of compartments in the tank, since it is possible to obtain in the last compartment a sufficiently strong lye while, in the first, the leaching will not yet be finished; conversely, it is possible to leach the mass of the first compartment completely, while the lye going to evaporation may prove too weak. Obviously, in order to obtain a fully definite solution, additional data are also necessary.

These data are the quantity of water relative to the mass of ash charged, as well as the time of leaching. The latter, evidently, is connected with the rate of outflow of pure water onto ash that has almost already been leached, for it also determines the rate of outflow of the finished lye, as follows from the principle on which Shanks’s apparatus is constructed.

Summarizing all that has been said above about leaching, the following order of investigation may be outlined:

1) Determine what quantity of water is used to moisten the ash, since, obviously, this water is lost, being discarded to the dump together with the under-ash. It must be borne in mind, moreover, that there are comparatively few soluble substances in wood ash (for birch ash, as stated, about 12%).

2) Taking an arbitrary quantity of ash (and, accordingly, of course, the capacity of the vessel in which the leaching is performed), determine the volume of water sufficient for moistening and for covering the entire surface of the ash.

3) Obtain a steady process, i.e., prepare a series of products differing from one another in the quantity of lyes that have passed through them, beginning with the product that has been completely leached and ending with fresh product, as occurs in the steady process in the “chenk.”

4) Choose a definite rate of flow, for example, from the calculation that within 2—3—4 hours an amount of liquid equal to the volume of that compartment should circulate through each compartment of the “chenk.” It is obvious that the slower the flow of liquid, the longer the ash is in contact with the lye and the more concentrated will be the finished lye issuing from the last compartment of the “chenk.” At the same time, the number of boxes of the “chenk” is reduced.

Thus, having a series of processes that differ as a result from one another in the concentration of the final lye and in the number of compartments of the “chenk,” and selecting from among them the one that gives a final lye of 25° Bé, we simultaneously obtain both the rate of flow and the number of boxes in Shanks’s apparatus.

5) Carry out all these operations at temperature: a) room temperature, b) 50° C, c) ∼ 100° C.

6) For a complete characterization of the process.

a) Make a quantitative chemical analysis of the lyes in each compartment of the “chenk” and of the final lye. Compare the results of the analyses with one another and with the content of the lye obtained by simple leaching.

As we know, the soluble part of ash contains, in addition to \(K_2CO_3\), also \(K_2SO_4\), \(KCl\), and \(Na_2CO_3\). The solubility of these salts is different, and in this respect they may be arranged in the following series: \(K_2SO_4\), \(KCl\), \(Na_2CO_3\), \(KNaCO_3\), and \(K_2CO_3\); of these, the first is the most sparingly soluble, while the last, potash, has almost infinite solubility, so that it deliquesces already in air. Therefore it may be supposed that, as the lyes pass through a series of “chenk” boxes with ash, the carbonate salt (\(K_2CO_3\)) will displace the sparingly soluble salts from solution and, in this way, the lye will become enriched in carbonate salt. Analysis of the lyes in each compartment of the “chenk” will indicate whether this supposition is correct or not. In the event of a positive answer, it may be possible to use this circumstance to obtain a purer and, consequently, more valuable potash.

b) By measuring with a Baumé hydrometer the concentration of the lyes in each compartment of the “chenk,” establish the law according to which this concentration increases.

Boiling down the lye and fractional crystallization. The next operation necessary in the process of obtaining potash is the boiling down of the lye obtained, for its subsequent calcination. In Russia we usually confine ourselves to evaporating the lye and calcining the crude potash in a reverberatory furnace. But potash obtained in this way obviously also contains all the impurities of salts present in the ash and soluble in water. Thus, according to the analyses of Prof. D. Mendeleev, Russian potash from wood ash contains:

\[ \begin{array}{llll} \mathrm{K_2CO_3} & \text{from } 60.84\% & \text{to} & 71.04\%,\\ \mathrm{Na_2CO_3} & \text{”} & 1.81\% & \text{” } 8.09\%,\\ \mathrm{K_2SO_4} & \text{”} & 16.50\% & \text{” } 24.87\%,\\ \mathrm{KCl} & \text{”} & 1.48\% & \text{” } 4.74\%. \end{array} \]

We thus see that the impurities sometimes reach considerable amounts, up to 24.87% for \(\mathrm{K_2SO_4}\). In many cases these impurities are needless ballast, for example in soapmaking; and in the potash treatment of Chile saltpeter they complicate the process of separating pure conversion saltpeter and soda. Therefore the production of pure potash is a question of no small importance. As has already been said, the solubility of the salts contained in the lyes is different, and therefore there is every possibility of separating them by fractional crystallization.

In this process, according to Mendeleev, when evaporating, the \(\mathrm{K_2SO_4}\), which is sparingly soluble in a potash solution, crystallizes out first; then, upon cooling, \(\mathrm{KCl}\) and \(\mathrm{Na_2CO_3}\). Finally there begins an abundant crystallization of the double salt \(\mathrm{KNaCO_3 \cdot 6H_2O}\). The lye separated from these crystals contains chiefly \(\mathrm{K_2CO_3}\).

In the present case the task of the investigation is reduced to determining those limits at which the separation of one or another component ends. These limits may be characterized by the concentration of the solution, using the Baumé hydrometer. In concrete terms, the question is resolved by analysis of the lye and of the crystals that have separated out at definite intervals of degrees Baumé.

The operation of calcination does not require special study. In the present case it is of interest only to make a chemical analysis of the product after calcination and to determine the degree of its purity and the yield, in percent, relative to the entire amount of potash contained in the ash.

As for the question of purification of the potash obtained after calcination, the necessity of this operation, as well as the method of purification, depends on the data that will be obtained as a result of the experiments on fractional crystallization and calcination. Therefore we shall not dwell on this question.

Such are the scopes of the experiments that must be set up in order to characterize the entire process of extracting potash from wood-fuel ash.

Under the conditions of our work we could carry out only part of the experiments indicated above, and we stopped at the study of the leaching process in Shanks’s apparatus—the process most important among the others. But even here, not having a battery at hand, we strove only to approximate as accurately as possible the process in the form in which it occurs in Shanks’s apparatus. Considering our experiments as preliminary, for obtaining certain basic data, we conducted the process only at room temperature. On the other hand, for the same reasons, we were able to carry out the leaching only at one interval of time of contact of the ash with the lyes, and

namely, we selected this interval as equal to one hour. Having ordinary laboratory glassware at our disposal, we made use of beakers with a capacity of 1.5 liters, considering them as compartments of a leaching vat. The flow of liquid, caused in the Shanks apparatus by the excess of the liquid level in one of the compartments as compared with the others—which, taken together, constitute a system of communicating vessels—and the separation of the liquors owing to the difference in their specific gravities, in turn determined by the difference in concentration, we accomplished by placing the ash in a series of beakers, the liquid being decanted through a filter from one beaker into another at definite intervals of time (1 hour).

To characterize the leaching process in the Shanks apparatus, as has already been said above, it was necessary to have an established process, i.e., on the one hand it was necessary to have a series of masses being leached, from completely fresh ash to almost fully leached ash, and on the other hand a series of liquors, from pure water to liquor of limiting concentration. At the same time it was necessary to establish the number of elements of the cycle, i.e., the number of compartments in the vat.

In order to solve this problem, we proceed from the following considerations: let us suppose that the number of compartments has already been found by us. Let this number be 6. We take six tall chemical beakers and charge them with a definite and equal quantity of ash.

Scheme No. 1.

Beakers Nos.: 1 2 3 4 5 6

Let us take into account that one compartment of the vat must always be free for unloading spent ash and loading fresh ash. We pour water over the ash in the 1st beaker. After an hour the liquor from the first beaker is transferred to the 2nd, and the first beaker is filled with pure water. After another hour, the liquor from the 2nd beaker passes into the 3rd, the liquor from the 1st beaker passes into the 2nd, and the 1st beaker is filled with pure water, and so on. When the liquor from the 4th beaker passes into the 5th with fresh ash, and the intermediate liquors pass into the corresponding subsequent beakers, then the ash in the 1st must already be almost leached; thus, subsequently, when the liquor from the 5th beaker goes to evaporation, the 1st beaker will be filled with pure water for the last time. At the next moment in time the 6th beaker is included in the system, and the liquor from the 5th passes into the 6th, from the 4th into the 5th, and so on. When the liquor from the 1st beaker goes into the 2nd, the 1st beaker is freed of underdrainage and is recharged with fresh ash. If one counts the number of liquors that have passed successively through the different beakers, the following results are obtained: pure water has passed through the ash in the 1st beaker 6 times; in the second beaker the liquor has passed 5 times; in the third 4 times; in the fourth—3 times, and so on; in the sixth, correspondingly, the liquor has passed only 1 time. We see, therefore, that by this method we have the possibility of preparing the ash in such a series as approximately corresponds to the leaching process in the vat. But only approximately, for it turns out that the mass of the first beaker was leached all the time with pure water, and that the concentrated liquor passes through fresh ash only in the 6th beaker. In order to regard the process as established, it is necessary to continue the operation further, placing the 1st beaker with newly charged ash after the 6th and beginning to pour fresh water into the 2nd beaker. Further, after the first beaker the place must be occupied by the second, after the second—the third, and so on, until we return to the initial position.

All this is clearly seen from the following scheme No. 2:

Scheme No. 2

(Arrangement of the beakers with the ash being leached, according to their numbering).

Beakers Nos.: 2 3 4 5 6 1
            ⟶
3 4 5 6 1 2
            ⟶
4 5 6 1 2 3
            ⟶
5 6 1 2 3 4
            ⟶
6 1 2 3 4 5
            ⟶

This scheme shows that, by the indicated method, the ash can be prepared in the required sequence. In fact, as a result, each of the beakers in the last row will be filled both with strong lyes and with pure water, as well as with all lyes of an intermediate character. Consequently, if the ash thus prepared is now run through again, in the order indicated above, with water and lyes, then they may be used for analyzing and characterizing the established leaching process in the Shanks apparatus. The number of sections of the vat is thereby determined of itself, if one sets as a condition the satisfaction of only one of the requirements stated above, namely, the complete leaching of the ash in the 1st section—in our case, in the first beaker. Indeed, conducting the leaching in precisely the manner indicated and ceasing to add new beakers at the moment when pure water, after passing through the 1st beaker, shows the absence of soluble salts in the ash, we obviously determine, by the number of beakers in operation at that moment, the number of sections.

It was already said earlier that, in order to satisfy the second requirement set forth—the concentration of the finished lye at 25° Bé—it is necessary to vary the interval of leaching time and the temperature.

Already at the beginning of the experiment thus arranged, serious difficulties were encountered, the result of which was the absence of continuity in the leaching process. The reason for this was the excessively large quantity of liquid spent on wetting the ash, namely: for every 500 g of ash, approximately 50 cubic cm of liquid are consumed.

It is clear that, under such conditions, the lye must sooner or later become stuck in one of the beakers, whatever quantity of water may be taken for leaching. With a double quantity of water, relative to the liquid spent on wetting, the lye will remain in every 2nd beaker; with a triple quantity—in every 3rd, and so on. In practice this entails an interruption of the process until new lyes arrive from the rear rows. Taking into account the capacity of the beakers and allowing a reserve of 50 cubic cm for wetting, we loaded 500 g of ash into the beakers and began the leaching of each new portion of fresh ash with 800 cubic cm of water or lye, subsequently pouring into the beakers with already moistened ash only 550 cubic cm of liquid. Owing to the above-mentioned wetting, after leaching a fresh portion of ash we could draw off only 800 − 550 = 250 cubic cm of lye, which we kept in a reserve flask until the next operation gave us another 550 cubic cm, which, together with the 250 cubic cm, made 800 cubic cm.

The following scheme No. 3 of the first experiment in preparing ash for the established process will explain what has been said. From it one sees that the quantities of lyes that passed through each beaker separately are expressed by the figures

2 : 4 : 6 : 8 : 10 : 12 : 14 : 16 : 18 : 20;

in other words, while through the tenth

...2 lyes passed through the glass; through the ninth, 4 passed; through the eighth, 6, etc. This same scheme indicates that we found the number of chenk sections to be equal to ten.

Scheme No. 3 and Table I¹).

Glasses: 1 2 3 4 5 6 7 8 9 10

Thus, the effect of wetting was manifested in the fact that, with ten sections of chenk, the number of leaching operations falling to each section is not 10, but more—in our case, twice as many.

From this one must draw the first conclusion: when applying Shanks’s apparatus to the leaching of ash, measures must be taken so that there is no interruption caused by the reasons indicated above. We shall not dwell on the results of this preliminary experiment. It is sufficient to note,

¹) The figures in the columns denote the quantity, in cubic centimeters, of solvent entering the glasses: for the 1st glass, pure water is taken; for all subsequent glasses, lyes flowing out of the preceding glasses.

The figures “250,” standing to the right of the figures “800” beyond the broken line, indicate the place and quantity (in cubic cm) of the first lyes taken off.

The italic figures in parentheses denote the density of the first outflowing lyes in degrees of the Baumé hydrometer.

that the finished lye obtained by us had a strength of 10.75° Bé, which indicates that the adopted interval of time for the contact of the lye with the ash is too short. This is also confirmed by the excessively large number of beakers (ten). The usual number of compartments in a Shanks system is not more than six.

Let us also present a graph of the changes in the concentrations of the lyes obtained when leaching ash with pure water (beaker No. 1). In diagram No. 1, the coordinates of the number of leachings on the abscissa may be regarded as time coordinates, since each new leaching corresponds to a time interval of one hour. We then have the possibility of noting that, as the ash becomes depleted in soluble salts, leaching slows down. As a supplement to scheme No. 3, in the following table we give experimental data characterizing the fall in the concentrations of the draining lyes in each individual beaker and the general increase in concentration in the direction from the first beaker to the tenth:

Table II.

We do not yet have a steady-state process. The given moment is characterized by the beginning of the first row of scheme No. 2.

The further preparation of the ash, as we know, must consist in the continuation of leaching, while from time to time a beaker with fresh ash should be placed on the right, removing from the left a beaker with ash already leached. In so doing, the lyes obtained in the preceding operation must take part in the process.

In order to arrive at the last row of scheme No. 2, it must be set up in the following manner:¹

¹ In view of certain changes in the order of leaching indicated below, we began the new leaching with 800 cu. cm of pure water.

Scheme No. 4 and Table III.

As is not difficult to calculate from the scheme, the quantity of liquid that has passed through the individual beakers is expressed by the ratio:

$$ 18 : 16 : 14 : 12 : 10 : 8 : 6 : 4 : 2 = 9 : 8 : 7 : 6 : 5 : 4 : 3 : 2 : 1 $$

On the other hand, the concentrations of the lyes obtained are evidently characterized by the same ratio, but in the reverse direction, i.e. the lye that has passed through the beaker subjected to the smallest number of leachings is the most concentrated, and vice versa.—If we turn to Scheme No. 2, we shall note that its last row corresponds precisely to the end of Scheme No. 4; consequently, by the indicated method we obtain a series of ash masses fully prepared in the required order.

In order to obtain lyes corresponding to the lyes of the Chenkov installations under a steady-state process, it remains only for us to pass water and the lyes already at our disposal, in a definite order, through the ash thus prepared. But bearing in mind that the time interval adopted by us does not make it possible to obtain a final lye of the required concentration, and that therefore, as well as for other reasons, our experiment as a whole can be regarded only as preliminary, we can, in what follows, set ourselves the task of using the lyes obtained only to resolve the question of their comparative chemical composition; for, as stated above, there is reason to suppose that, as a result of such leaching, enrichment of the lye with the carbonate salt $K_2CO_3$ should occur; this means that the stronger the lye, the greater, in percentage terms, should be its $K_2CO_3$ content. From this point of view, we may be interested not in the absolute chemical content of the lyes, but only in the relative content; and this latter circumstance makes it possible to shorten considerably the leaching work, which, generally speaking, requires a very great deal of time, as is evident from the scheme. It is possible, in our opinion—without detriment to the matter—to discard the series of operations located in Scheme No. 4 to the right of the broken line. In practice this means that the lyes we have obtained at the present moment must be set aside, and the operation must be begun with the leaching of beaker No. 2 with pure water. The lye thereby obtained is directed into beaker No. 3, and beaker No. 2 is set aside. At the next moment the lyes then proceed in order from No. 3 to No. 4, while No. 3 is filled with pure water, and so on. As we shall see below, the required ratio between the lyes is thereby preserved.

The order of the leaching operations actually carried out by us is shown in Scheme No. 5.

Diagram No. 5 and Table IV¹.

2   3   4   5   6   7   8   9   10   1′

Pure water.

Pure water.

Pure water

Pure water

Pure water

800

800 / 0.5

800        800 / 0.75

800 / 0.75        800 / 1.0

800  —  800 / 0.75        800 / 1.5

800 / 0.75        800 / 1.25        800 / 2.5

800        800 / 1.0        800 / 2.25        800 / 3.75

800 / 0.75        800 / 2.5        800 / 3.5        800 / 5.5

800        800 / 1.0        800 / 2.0        800 / 4.25        800 / 7.25

800 / 0.5        800 / 1.75        800 / 3.25        800 / 6.0        800 / 10.0

\[ \frac{250\ \text{cubic cm.}}{12.0^\circ\ \mathrm{B\acute e}} \]

We see that, as a result of this operation, we actually have only five glasses. At the end of the leaching, as will be seen from the following diagrams, this number of glasses will continuously decrease; but, by virtue of the considerations stated above, this circumstance is of no significance, for our task consists only in obtaining lyes whose concentration changes according to a definite law—which, as will be seen below, has been achieved. Nevertheless, it is important to us that these lyes pass through the ash in a manner sufficiently close to the real, established process taking place in the vat; and therefore we continue the leaching until the first ten glasses have been completely leached out.

The next experiment is therefore set up according to Diagram No. 6.

Scheme No. 6 and Table V.

7     8     9     10     1′     2′     3′     4′     5′

Pure water

      800      800      800      800      800
              ———      ———      ———      ———
              1.0      2.25     4.25     8.0

      800      800      800      800      800
      ———      ———      ———      ———      ———
      0.75     1.75     3.5      6.0      10.0

Pure water

      800      800      800      800      800
               ———      ———      ———      ———
               1.25     2.5      4.25     7.75

      800      800      800      800      800
      ———      ———      ———      ———      ———
      0.75     1.75     3.5      5.75     10.0

Pure water

      800      800      800      800      800
               ———      ———      ———      ———
               1.0      2.0      5.0      7.75

      800      800      800      800      800
      ———      ———      ———      ———      ———
      0.5      1.0      3.0      6.25     10.0

Pure water

      800      800      800      800      800
               ———      ———      ———      ———
               1.0      2.75     4.75     7.75

      800      800      800      800      800
      ———      ———      ———      ———      ———
      0.75     1.75     3.5      6.0      10.0

\[ \Rightarrow \frac{250}{12.5} \]

\[ \Rightarrow \frac{250}{12.0} \]

\[ \Rightarrow \frac{250}{11.75} \]

\[ \Rightarrow \frac{250\ \text{cc}}{11.50^\circ Bé} \tag{10} \]

We have here the moment when the first ten glasses have already been completely leached. Now the lye may already be taken for analysis. The 250 cc of lye obtained at the end of this operation is the strongest lye, which must go to evaporation. From Schemes No. 4 and No. 5 we see that this lye was obtained from pure water, which began leaching almost exhausted ash in glass No. 6, and ended with glass No. 5′ containing fresh ash, passing through a series of glasses with ash of intermediate character. On the other hand, it is not difficult to convince oneself, by following the arrows, that it took part in ten leachings.

It remains for us now to obtain another nine lyes, the potash content of which decreases continuously and whose limit is pure water. On the other hand, they must satisfy the stated requirement both with respect to the number of leachings in which they took part and with respect to the character of the ash through which they passed. These lyes were obtained according to the following Scheme No. 7.

Scheme No. 7 and Table VI.

The leaching process is completed.

It is not difficult to verify that the lyes obtained satisfy the requirements set forth above. Let us take, for example, lye No. 5. Tracing it by the arrows, we see that it took part in leaching five times, starting from pure water. The ash through which it passed the last time had, before this leaching, also been subjected five times to lyes whose concentration varied according to the law we established. And if we trace all the lyes in the same way, we shall observe that, with respect to the number of leachings in which they participated, they may be characterized by the figures 10:9:8:7:6:5:4:3:2:1, i.e., exactly as we established on p. 33. It is likewise not difficult to verify that the ash, in all cases, with respect to the number of lyes that have passed through it, is characterized by the very same ratio, but in the reverse order, i.e. 1:2:3:4:5:6:7:8:9:10.

All that has been said shows that the lyes we obtained correspond sufficiently to the established process of leaching in Shanks’s apparatus.

¹ This numbering of the lyes corresponds to the number of leachings in which the lye took part.

Our further task consists in, first, establishing the law of change in the concentrations of the lyes; second, determining their content by chemical analysis; and, third, comparing the data obtained both with one another and with the results of the analysis of ash leached in the ordinary way. This will give us the possibility of clarifying the influence of methodical leaching, on the one hand, on the speed of the process, and, on the other, on the quality of the finished product, potash, in the sense of its enrichment or impoverishment in potassium carbonate ($\mathrm{K_2CO_3}$).

The density of each lye obtained by us was measured with a Bé hydrometer; all data relating to this were indicated above (see schemes Nos. 5, 6, and 7—the italic figures below the line). We are interested only in the change in the concentrations of the lyes successively obtained during leaching in the last five beakers, since the ash in them, being leached to completion, constituted a series of products of an established process.

The concentration of the lyes in all five cases changed quite regularly, and the existing deviations are explained partly by the inaccuracy of the readings of the Bé hydrometer, and partly by the fact that our work, of necessity, was interrupted for the night. And although during the interruption we did not leave the lyes in the beakers with the ash, nevertheless, evidently, it was impossible to extract all the liquid from them; and therefore the concentration of the lye obtained in the morning showed a certain upward jump. If, however, one takes the figures averaged from the five, then by this we come closer to the true picture of the changes in concentration. The data relating to this, as well as the average concentration of the lyes, are shown in Table VII.

Table VII.

Taking as abscissas the numbers of the lyes (= the number of leachings) in which the lye took part, and as ordinates the concentrations of the lyes, expressed in degrees Bé, we graphically obtain the law of change in concentration, presented in diagram No. 2.

This diagram is a characteristic of methodical leaching. First of all, we see here a definite regularity, expressed in the smoothness of the curve. Its character (concavity) remains unchanged. The small deviation observed at the beginning of the curve (or, what is the same, at the end of the leaching of the ash) is probably an accidental phenomenon, perhaps the result of the overnight interruption or of inaccuracies in the readings of the Bé hydrometer. We are convinced of this, on the one hand, by diagram No. 3 of the concentrations of the lyes taken for analysis (cf. Table VI), in which the absence of an averaging principle makes itself felt. In it, as we shall see, the deviation is more

sharp character. On the other hand, the diagram constructed on the basis of chemical analysis—obviously giving more exact data—shows no deviations. Below we present these diagrams, as well as the numerical data relating to them.

Passing to the results of the chemical analysis, we must first of all say that the qualitative analysis gave us no indication, despite the concentration of the solution, of the presence of sodium, which is at variance with Prof. Dement'ev’s analytical data.

Of the cations, besides \(K^{\cdot}\), we found only \(Si^{\cdots\cdots}\) and \(Al^{\cdots}\). The presence of the latter in furnace ash is quite natural, and in the alkaline solution it forms a soluble aluminate. Of the anions we found \(SO_4^{\prime\prime}\), \(Cl^{\prime}\), and \(CO_3^{\prime\prime}\).

The content of Si and Al in the lyes was so insignificant that we considered it possible, without detriment to the value of the results, to confine ourselves to the quantitative determination of \(SO_4^{\prime\prime}\), \(Cl^{\prime}\), and \(CO_3^{\prime\prime}\), combining them in the calculation with \(K^{\cdot}\). An exception was made only for the final lye No. 10 and for the analysis of the ash.

The determination of \(K_2CO_3\) was carried out with a titrated solution of hydrochloric acid \(\left(\sim \frac{1}{10}\ \text{normal}\right)\). The determination of KCl was carried out by Volhard’s method, by back-titrating the excess of \(AgNO_3\) \(\left(\frac{1}{10}\ \text{normal}\right)\) with ammonium thiocyanate, using ferric alum as indicator and in the presence of \(HNO_3\). The determination of \(K_2SO_4\) was carried out gravimetrically with the aid of \(BaCl_2\). In all cases, the mean numerical results of two concordant analytical determinations were taken. The results of the quantitative analysis of lyes Nos. 1–10 (cf. scheme No. 7 and Table VI) are brought together in the following Table VIII.

Table VIII.

In this table the quantities of the salts \(K_2CO_3 + KCl - K_2SO_4\) are summed; using these figures and taking as the abscissa axis the number of leachings (= the numbers of the lyes), and as the ordinate axis the content of salts in the lyes, we shall construct the curve shown in Diagram No. 4.

Diagrams Nos. 2, 3, and 4 make it possible to draw a conclusion about the rate of methodical leaching. Here two factors are of importance, the action of which is mutually opposite. On the one hand, as saturation increases more and more, the dissolving capacity of the lye decreases. On the other hand, passing from compartment to compartment, the lye encounters ash that is less and less exhausted, owing to which the rate of its saturation increases. All three of the last diagrams show that the second factor is the predominant one, i.e., despite the increasing saturation of the lye with salts, the index of change in concentration increases.

We have already indicated that we made a complete analysis of lye No. 10 and of the soluble part of the ash. By this we intended, on the one hand, to ascertain the influence of methodical leaching on the quality of the product obtained; on the other hand, since for the analysis the ash was leached with hot water, we wanted to find out wherein lies the advantage of cold leaching over hot leaching, for practice shows, as K. G. Dement'ev says, that in the first case a product of better quality is obtained than in the second.

Analysis of the soluble part of the ash. 100 g of the investigated ash from birch firewood were leached with hot water; the solutions were concentrated to 1000 cc. In all, from 100 g of ash, 15.5730 g of soluble substances were extracted, or, what is the same, 15.57%; of this amount 13.6146 g (13.61%) falls to \(K_2CO_3\).

The results of the ash analysis, in comparison with the data of the complete analysis of lye No. 10, are brought together in Table IX:^1)

Table IX.

Table IX gives no indication of the advantage of one or the other, hot or cold, leaching; although with respect to \(Al_2O_3\) and \(SiO_2\) we observe that their total under cold leaching is greater, evidently the high concentration of lye No. 10 is of significance here. In any case, the content of these impurities in lye No. 10 and in the soluble part of the ash is so insignificant that this circumstance can hardly serve as a serious obstacle to the use of the more advantageous hot leaching.

It is true that here we also observe that the potash from lye No. 10 is considerably poorer in carbonate salt (\(K_2CO_3\)) and, conversely, richer in impurities than corresponds to their content in the ash; but, as we shall see below, this is not connected with the temperature of the leaching water, but is the result of methodi-

^1) In all cases the average numerical results from two concordant analytical determinations were taken.—An attempt to determine the solid residues did not give exact results because of the hygroscopicity of potash. Therefore all numerical results are referred, in percentages, to the sum of salts found by us in the analysis.

…methodical leaching. This is shown to us by the comparative analysis of the ten above-mentioned lyes, obtained by the cold method and differing only in concentration.

Below are given the results of these analyses in percentages (Table X), and diagrams Nos. 5, 6, and 7 are also given, whose coordinate axes are, on the one hand, the content of salts in %%, and, on the other hand, the numbers of the leachings in which the lyes took part.

Table X.

From consideration of these diagrams it is evident that the results of the analysis lead us to conclusions completely opposite to what might have been expected. We supposed, as was indicated earlier, that as the lyes passed through a series of beakers with ash, and therefore as their concentrations increased, the content of K₂CO₃ ought relatively to increase, while, on the contrary, the percentage content of KCl and K₂SO₄ ought to decrease; this follows from the solubility of these salts. We hoped thereby to obtain, as a result, a lye going to evaporation enriched with potassium carbonate, K₂CO₃; in other words, we thought that the Shanks apparatus would give us yet one more advantage: a purer and, consequently, more valuable potash. Reality showed otherwise. The strongest lye, which should give the finished potash, proved to be the poorest with respect to K₂CO₃ and the richest in the impurities KCl and K₂SO₄. Conversely, the pure water leaching the residues gives a lye which, in percentage terms, is richest in potassium carbonate and has the lowest content of impurities. All the intermediate lyes show that this is not accidental, and reveal in this respect a definite regularity. As a result, as we have seen, the final product proves to be of worse quality than corresponds to the content of salts in the ash.

Conclusion. Summing up all that has been said above, we may draw the following conclusions:

1) The interval of time chosen by us for the contact of the lyes with the ash, equal to one hour (in the case of working with a Shanks apparatus this corresponds to the rate of flow of the liquid), is too short (the rate of flow is high), for the concentration of the final lye is far from the necessary concentration of 25° Bé.

Minaev, V. I.

On Leaching of Ash.

Diagram No. 1.

Decrease in the density of solutions as a function of the number of leachings with pure water.

Vertical axis: Density of alkalis in degrees Baumé.

Horizontal axis: Number of leachings.

Diagram No. 2.

Increase in the concentration of the ash lye from 0° Bé (pure water) as the lye, distributed over the course of the production process, passes toward the outlet through fresh ash.

Vertical axis: Density by Baumé (according to average figures from Table III).

Horizontal axis: Numbers of the lyes (= number of leachings).

Mikhaev, V. N.

On leaching of ash.

Diagram No. 3
(According to Table VI.)

Vertical axis: Amount of potash by weight

Horizontal axis: Number of leachings

Curve point labels: 1.0°, 1.75°, 2.75°, 3.0°, 4.0°, 5.25°, 6.75°, 8.5°, 9.0°, 11.5°.

Diagram No. 4.
(According to Table VII.)

Vertical axis: Potash content in grams

Horizontal axis: Number of leachings

Minaev, V. N.
On the leaching of ash.

Diagrams No. 5 — for \(K_2CO_3\)
No. 6 — for \(KCl\)
No. 7 — for \(K_2SO_4\)
(Cf. Table X.)

Legend:

• No. 5 — \(K_2CO_3\): solid heavy line.
• No. 6 — \(KCl\): dotted line with small circles.
• No. 7 — \(K_2SO_4\): dashed line with plus signs.

Axes and scales:

• Horizontal axis: Number of leachings — 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
• Left vertical axis: Content of \(K_2CO_3\), in percent of the salts — 83.0, 85.0, 87.0, 89.0, 91.0, 93.0, 95.0.
• Inner right vertical axis: Content of \(KCl\), in percent — 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70.
• Outer right vertical axis: Content of \(K_2SO_4\), in percent — 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.

2) In the process of systematic leaching, the lye becomes enriched with impurities, which must be counted among the shortcomings of this method of leaching.

3) The investigation gave no indication of the advantages of cold leaching over hot leaching that are noted in the technical literature.

4) Owing to the large consumption of water for wetting the ash, an interruption occurs in the process, and this must be eliminated.

In view of the importance of the last question, we shall dwell on it in greater detail and try to find a way of avoiding an interruption in the work. For this purpose let us consider the process as it occurs in the leaching of soda. Suppose we have a battery of six compartments. One of them must be excluded from the scheme, since it is being prepared for the next operation. Let this compartment be the 6th. We next take the moment when the 1st compartment is to be leached for the last time, and the 5th has only just been charged with a fresh melt of soda. Pure water entering the 1st compartment displaces from it the lye into the 2nd; the lye from the 2nd compartment enters the 3rd, and so on. As a result, the lye from the 4th compartment enters the 5th, onto the fresh melt. At the next moment the lye from the 5th compartment, being finished, must go to evaporation; at the same time the lye from the 1st compartment must free it, so that the 1st compartment can be charged with fresh melt. This is achieved by pumping the lye into the 2nd compartment by means of a pump. Then, by the time compartment 6 is included in the scheme, the 1st is free, and pure water enters the 2nd compartment. The process is then repeated. In this case we have no obstacles to the pumping operation, since the mass of the 1st compartment, already leached, gives a small residue; consequently almost the entire compartment is occupied by liquid.

It is a different matter in the leaching of ash. Soluble salts in the ash (birch ash) amount to only 15.5%. Consequently 84.5% of the entire mass remains in the vat, retaining by wetting a large part of the water. Obviously, by pumping one can extract only a small quantity of liquid, quite insufficient for displacing the final lye from the last compartment of the vat. As a result there must occur the very interruption that is observed in our work.

This difficulty, in our opinion, however, is easy to overcome. One need only replace the pumping of the weakest lye by displacing it with a new portion of pure water. Obviously, then all the lye stuck in the 1st compartment will pass into the 2nd compartment, ultimately displacing the finished lye from the last compartment. The pure water, which in this case replaces the pump, can then be drained through the lower cock, whereby the 1st compartment will be free of water. Thus the difference in the process of leaching ash and soda melt consists in the fact that in the first case pure water is passed through the 1st compartment twice, with the greater part of the second portion of water being unable to be used, remaining in the wetted ash. In other words, in the leaching of ash the consumption of water will be considerably greater than in the leaching of soda.

In conclusion, let us add a few words concerning the arrangement of the experiment. Having no vat, we replaced it with a row of beakers and tried, with their aid, to set up a process approximating the process of leaching in the Shanks apparatus. We succeeded in this. But the operations of draining and preparing the ash and lyes require so much time that it will be much more economical in the work to use a vat, even one of small volume. Its dimensions, however, must be sufficient so that the withdrawal of small portions of lye for investigation does not affect the leaching process.

1. The italic figures below the line denote the densities of the lyes in degrees Baumé. ↩