EFFECT OF ULTRASOUND ON THE YIELD OF HUMIC ACIDS FROM PEAT

K. K. Stramkovskaya, V. M. Meshcheryakova, V. A. Utkin,
L. F. Manenkova

(Recommended by the Scientific-Methodological Seminar of the Department of Chemical Technology)

Weak aqueous solutions of caustic and carbonate alkalis, as well as ammonia, extract from peat and brown coals a group of substances of humic nature known as humic acids. These acids are currently widely used in the national economy. However, under the existing technology, humic acids are extracted with very low yields—from 17 to 20% on a dry basis, whereas their content in the fuel reaches 30 and even 58% (1).

The extraction of humic acids from solid fuel by aqueous alkali solutions is an extraction process complicated by a chemical reaction. Intensification of this process may be achieved by dispersing the solid phase, for example with the aid of ultrasound, by increasing the temperature, since humic acids interact with caustic alkali, and by the action of other factors.

In the present work, the influence on the yield of humic acids from peat was studied for the duration of exposure to ultrasonic waves, the temperature during sonication, the moisture content of the peat, the ratio of fuel to alkaline reagent, the concentration of alkali in the initial solution, and the heating time of the peat-alkali suspension after sonication.

Lowland peat from the Tagan bog in Tomsk Oblast was studied. The source of ultrasound was a magnetostrictive transducer powered by a UZM-1.5 vacuum-tube generator with a frequency of 22 kHz.

The peat was placed in a conical flask with a capacity of 500 ml and covered with a 1% aqueous solution of caustic soda. The flask was then placed in a water bath, the bottom of which served as the magnetostrictive transducer, and sonication was carried out.

Humic acids were determined by a procedure developed by the Kalinin Peat Institute, based on the general principles of the gravimetric method (2, 3).

The influence of sonication time from 15 to 60 minutes was investigated. A peat sample with moisture corresponding to the air-dry state was tested. The ratio of dry peat to alkali solution was taken as 1:50. The temperature in the ultrasonic bath was maintained at 47–48°C. The results obtained are shown in Fig. 1.

These data show that, with the use of ultrasound, the yield of humic acids increases, but the duration of ultrasonic action—

more than 30 min gives an insignificant increase in the yield of these products. Thus, with sonication for 30 min, the yield of humic acids increases, in comparison with that from the unsonicated sample, by 11.3%; increasing the duration of ultrasonic action from 30 min to one hour led to an increase in their yield of only 1.4%.

The effect of temperature during sonication on the yield of humic acids was determined both with an air-dry analytical sample and with a moist sample passed once through a meat grinder. The ratio of dry peat to alkaline solution was taken as 1:50. The results of the experiments are summarized in Table 1.

Fig. 1. Dependence of the yield of humic acids on sonication time

From the data in this table it follows that, as the temperature during sonication is increased from 27 to 57°C, the yield of humic acids also increases. However, the increment in this yield slows when the temperature is increased from 47 to 57°C.

The effect of peat moisture on the yield of humic acids was studied at a sonication time of 30 min, a temperature of 47–48°C, and a dry peat : alkaline solution ratio of 1:50.

Peat with a moisture content of 84.0% was passed through a meat grinder once, then dried to different moisture contents and was not subjected to additional grinding.

Table 1

Effect of temperature during sonication on the yield of humic acids

The yield of humic acids was also determined for all samples with different moisture contents without sonication.

The experimental results presented in Fig. 2 show that the yield of humic acids increases significantly with increasing fuel moisture. This phenomenon is evidently explained by the fact that moist peat is a colloid in a state of maximum swelling. The reaction of humic acids with alkali in this state proceeds much faster and more completely. With drying,

the peat, it loses its colloidal moisture. G. L. Stadnikov [5] indicates that a moisture content of 38% is the boundary of the complete loss, by the peat colloid, of all swelling water. The data we obtained show a sharp decrease in the yield of humic acids after drying the peat to a moisture content of less than 38%.

In subsequent experiments, peat with a moisture content of 83.7% was sonicated at a temperature of 37°C for 30 min, with a ratio of dry matter to alkaline solution of 1 : 50. As a result, the following was obtained, calculated on the dry mass of peat:

humic acids — 43.80%;
unreacted residue — 27.0%;
fulvic acids — 29.2%.

For the unreacted residue, the group composition was determined by the usual method adopted for determining the group composition of peat. The results of the determination are given in Table 2, from which it is clear that the group composition of the residue obtained after extraction of the humic acids differs from that of the initial peat. It contains significantly fewer readily hydrolyzable substances, 10% as against 33, as well as a noticeable decrease in lignin and cellulose.

Thus, the action of the ultrasonic-wave field on peat evidently leads to its fine dispersion and to a more complete extraction of the humic acids already present in the fuel. It is also known that water, under the action of ultrasound, decomposes into hydrogen and hydroxyl radicals (4), which can, apparently, interact with lignin and other readily oxidizable substances of the peat, giving an additional amount of substances soluble in weak alkali.

Fig. 2. Dependence of the yield of humic acids on peat moisture. Yield of humic acids from peat: x — unsonicated, o — sonicated at a temperature of 37°C, △ — sonicated at a temperature of 47°C.

The humic acids obtained with the use of ultrasound differ somewhat from those isolated from peat without irradiation in elemental composition and in the increased content of reactive groups (Table 3).

In all the preceding experiments, humic acids were determined by a method providing for three-hour heating of the peat–alkali suspension with three portions of fresh alkali. Such prolonged heating of the suspension and the large consumption of alkali are impractical in the technology of this process. Therefore, we determined the optimal alkali consumption and duration of sonication by the following method. A weighed portion of an analytical peat sample was mixed with 1% alkali in a ratio of 1 : 10, sonicated, and boiled for 30 minutes. Then, to stop the reaction and improve filtration, the suspension was diluted with a tenfold amount of cold water. Humic acids were determined from the filtrate.

The yield of humic acids (Table 4) after 30-minute irradiation increased 1.7-fold in comparison with unirradiated peat and differed little from the results presented in Fig. 1, although the alkali consumption and heating time were reduced many times.

Table 2

Group Composition of Substances

Table 3

Characteristics of Humic Acids

Table 4

Effect of Sonication Time on the Yield of Humic Acids

Table 5

Effect of Boiling Time on the Yield of Humic Acids

The optimal heating time for the peat–alkali suspension after irradiation, as shown by the data in Table 5, should be considered 30 minutes, and the ratio of peat to alkaline reagent should be taken as 1:10 (Table 6).

As for the concentration of the alkali solution, according to the data in Table 7, a 1% solution should be accepted as optimal, since at a solid-to-liquid phase ratio of 1:10, with an increase in concentration from 0.25%

Table 6

Effect of the ratio of reagents on the yield of humic acids

Table 7

Effect of the concentration of the alkaline reagent on the yield of humic acids from peat

to 1% a sharp increase in the yield of humic acids is obtained, while the use of 2% alkali gives only an insignificant increase in yield (4%), whereas the consumption of alkali is doubled.

Conclusions

1. It has been shown that, with the use of ultrasound, the yield of humic acids from peat can be increased by a factor of 1.5–1.8.

2. The optimal conditions for extracting humic acids with the use of ultrasound have been determined.

References

1. S. G. Aronov, L. L. Nesterenko. Chemistry of Solid Fossil Fuels. Kharkov University Press, 171, 1960.

2. G. A. Kukharenko. Methods for determining the content of humic acids in solid fossil fuels. Chemistry and Technology of Fuels, No. 5, 32, 1956.

3. G. L. Stadnikov. Analysis and Study of Coals. USSR Academy of Sciences Press, 1936.

4. I. E. Elpiner. Ultrasound, Physicochemical and Biological Effects. Fizmatgiz, 1963.

5. G. L. Stadnikov. Origin of Coals and Petroleum. USSR Academy of Sciences Press, 1937.