On the Use of Alkaline Humates in Internal Boiler-Water Treatment

I. V. Gebler

I.

As is known, in the absence of preliminary water softening carried out in special stationary installations, antiscale agents serve as the means of preventing the formation of scale in boilers; with proper dosing of these agents and regular blowdowns, good results are achieved in preventing damage to boilers and increasing the interval between washings of locomotives.

The essence of the action of an antiscale agent is that, when it reacts with scale-forming substances in boilers, instead of hard and dense scale, loose precipitates are formed (sludge, silt), which are more or less easily removed during boiler blowdown.

On our railways, trisodium phosphate, caustic soda (caustic alkali, sodium hydroxide), and oak extract are used as antiscale agents.

Trisodium phosphate reacts with scale-forming salts, producing phosphoric-acid salts of the corresponding cations, which precipitate in the form of sludge.

Caustic soda is added to create an alkaline regime in the boiler water, which promotes the proper action of sodium phosphate and oak extract, and also prevents the deposition of gypsum and siliceous scale; in addition, alkali protects the boiler walls from corrosion under the influence of corrosive agents, which include carbon dioxide and, in some cases, hydrochloric acid, which may be present in the water, chiefly when magnesium chloride is present.

The action of oak extract is as follows. It contains so-called tannic acids; these substances are high-molecular phenol-carboxylic acids of aromatic structure, also known as tannides. With caustic soda in the boiler they form sodium salts, or sodium tannates. Alkaline tannates in aqueous solution, being readily oxidizable, have the ability to absorb oxygen, which is always present (in dissolved form) in the feed water and, entering the boiler, causes corrosion of its walls. Thus, oak extract is above all an anticorrosive agent with respect to oxygen, to which the action of caustic soda alone does not extend.

In addition, alkaline tannates have the property of imparting so-called lyophilicity to concentrated aqueous suspensions of mineral substances, owing to the fact that these colloids exert a peptizing action on precipitates, enveloping the precipitate particles with a colloidal film that prevents the enlargement of these particles by coalescence into larger aggregates, as well as the growth of crystals.

As a result, instead of coarse sludges, fine, flowing slimes are obtained, whose mobility and fluidity approach the mobility and fluidity of a colloidal solution. This circumstance is of great importance in the operation of a locomotive boiler when water is treated with the above-mentioned mineral antiscalants; the precipitates formed, under the influence of the tannins of oak extract, are converted into finer and more mobile slimes, which are removed more easily and more completely during blowdowns.

At the same time, the high dispersity of the precipitates formed in the course of their formation leads, owing to their large surface area, to a more intense and complete deposition of scale-forming substances in suspension, but not on the boiler walls.

The alkaline reaction of boiler water and the presence in it of a mineral precipitate cause unsteady boiling, violent ebullition, foaming, and carryover of water into the steam space of the boiler; this is promoted by high alkalinity, abundance, and coarse dispersity of the precipitate. At one and the same alkalinity and with the same weight quantity of precipitate, the phenomenon indicated depends only on the lyophilicity of the sludge suspension, and the greater the latter is, the less the foaming.

This occurs because steam formation and the release of steam bubbles will be distributed over a larger surface, with a simultaneous intensification of convection owing to the presence of finely dispersed particles in the liquid.

From this the role of oak extract as an agent that reduces foaming becomes clear, as is always stated in the relevant practical literature.

Acids extracted in the form of alkaline salts from peat and from brown humic coals possess properties analogous to the tannins of oak extract. In their chemical constitution they likewise constitute high-molecular phenol-carboxylic acids, whose alkaline salts, humates, are readily soluble in water; moreover, the properties and action of such solutions are in many respects similar to the properties of solutions of alkaline tannates. Thus, humate solutions have the ability to absorb oxygen and have the same peptizing action with respect to suspensions as do tannate solutions (oak extract).

In this connection, it appears entirely possible to replace (as one of the antiscalants) a solution of oak extract with a solution of alkaline humate obtained by treating peat or brown coal with a solution of caustic soda.

The expediency of such a replacement is dictated, first of all, by economic considerations. Oak extract, obtained from oak wood, is an expensive product. An equivalent quantity of humic acid, in terms of its action, should cost approximately ten times less. Suitable materials for this purpose may be humic brown coals containing a large quantity of humic acids.

There should be no special additional expenses in obtaining a humate solution from brown coal in a locomotive depot, since the same equipment can be used for this purpose as for dissolving oak extract.

However, the expediency of replacing oak extract is not limited to economic considerations alone. Oak extract is not only an expensive product but also one in short supply, and it is absolutely necessary for tanning heavy leathers (soles, half-soles). Hence the replacement of oak extract as an antiscalant is even more urgent.

For obtaining humic acid, brown coal from the Yar deposit was taken (40 km from the city of Tomsk). This coal belongs to humus coals of Tertiary age. It is characterized by the following composition:

\[ \mathrm{C^r}: 61.6—65.5\%;\quad \mathrm{H^r}: 6.8\%;\quad \mathrm{O^r}: 26.6—29.9\%; \]

\[ \mathrm{N^r}: 0.9—1.0\%;\quad \mathrm{S^r_{ob}}: 0.56—0.58\%;\quad \mathrm{W^r}: 59.7—61.2\%. \]

The bed moisture is very considerable and reaches 55–57%. The ash content is very variable; for the samples investigated it ranges from 13 to 30% (on dry substance).

When coal is extracted with a dilute solution of caustic soda, the acids contained in the coal are extracted. The product precipitated with mineral acid, washed and dried, consists of slightly lustrous pieces with a dense fracture, dark-brown in color, practically insoluble in water.

The product contains humic acid, as well as resinous and waxy acids, which can be extracted with alcohol-benzene. The composition of the product found in this way is as follows (on an anhydrous and ash-free basis):

\[ \begin{array}{lr} \text{Humic acid} \ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots & 81.52\% \\ \text{Resinous-waxy acids} \ldots\ldots\ldots\ldots\ldots & 17.48\% \end{array} \]

This product, containing approximately 80 percent humic acid, is hereinafter called “conditional humic acid.”

The yield of this acid, when extracted with a solution of caustic soda, in relation to dry coal, depends on the amount of mineral substances in the coal and ranges from 30 to 35%.

The acid equivalent of the conditional humic acid was 30.04 g NaOH per 100 g of anhydrous and ash-free acid.

For oak extract, the following was found:

\[ \begin{array}{lr} \text{Moisture} \ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots & 11.25\% \\ \text{Ash} \ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots & 2.87\% \end{array} \]

The acid equivalent, on an anhydrous and ash-free substance basis, was 6.11 g NaOH per 100 g.

Comparing both products with respect to acidity, we find that the acidity of humic acid exceeds that of oak extract by 4.9 times. This means that humic acid precipitates, in a correspondingly larger quantity, Ca and Mg ions in the form of calcium and magnesium humates, completely insoluble in water, when water-soluble sodium humate interacts with any salt of calcium or magnesium.

Calcium and magnesium humates are deposited in the form of very loose flocculent precipitates and subsequently form readily mobile sludges.

The fact that humic acid is capable of precipitating a larger quantity of Ca and Mg ions (i.e., scale-formers), compared with oak extract under identical conditions, is favorable, since when these substances are used as antiscale agents, at the same dosage, a greater amount of scale-formers will be converted into sludge in the case of humic acid, or, to achieve the same effect in the indicated sense, the addition of humic acid may be smaller.

To obtain the corresponding data, experiments in the laboratory were carried out under the following conditions. One liter of water with an addition of antina-

boiling was carried out in a beaker, with evaporation brought to 0.25 l. Each time the experiment was conducted in two identical beakers; in one beaker a solution of oak extract was added, and in the other—a solution of sodium humate in equal amounts, for which purpose their solutions were prepared at a concentration of 1 g per liter, calculated as an anhydrous and ash-free substance for both substances.

After boiling, with a fourfold reduction in volume, the beakers were placed in an autoclave and kept in it at a pressure of about 8 atmospheres for one hour, in order to establish the composition of the residual water at the corresponding temperature, i.e., about 170°. This residual water was taken as boiler water, and its total hardness was determined. In addition, the amount of scale deposited on the bottom and walls of the beaker was determined by weighing.

Of course, the indicated conditions for carrying out the experiments differ greatly from what occurs in locomotive boilers, but it is very difficult to create laboratory conditions close to those in steam boilers; under the circumstances available to us, it was impossible.

Nevertheless, the experiments, carried out under strictly identical conditions for comparing the antiscale action of oak extract and humic acid, give comparable figures, quite sufficient for a preliminary conclusion.

For the experiments, water from the Ushaika River was taken, of the following composition.

Consequently, the water had excess alkalinity, and the addition of caustic was unnecessary.

The results of the experiments are given in the following table:

The total hardness in the residual water depends on the temperature and the degree of reduction of the initial volume of water. Under the stated conditions, for all experiments the total hardness of the residual water was 0.5–0.6°.

A very characteristic indicator is the amount of scale on the bottom and walls of the beaker. In this case scale is formed, consisting mainly of calcium carbonate, and partly of magnesium carbonate, according to the reactions:

\[ \mathrm{Ca(HCO_3)_2 = CaCO_3 + H_2O + CO_2} \]

\[ \mathrm{Mg(HCO_3)_2 = MgCO_3 + H_2O + CO_2} \]

Without the addition of antiscalants, the amount of scale is greatest and amounts to 278 mg/liter (experiment 1).

The addition of phosphate alone in an amount of 5 g/liter (experiment 2) only slightly reduces the amount of carbonate scale, of which 254 mg/liter was obtained.

The conversion of scale-forming substances into sludge is based mainly not on the formation of loose precipitates of calcium and magnesium phosphate (and also calcium and magnesium tannates or humates when oak extract or sodium humate is introduced into the water), owing to their insufficient quantity at ordinary dosages, but on the deposition of the same carbonate scale on the total surface of the above-mentioned loose precipitates, instead of its deposition on the much smaller surface of the walls and bottom of the vessel. The greater the dispersity of the precipitates and, consequently, the greater the total surface of their grains, the greater the amount of carbonate scale that will pass into the sludge and the less will be deposited on the vessel walls.

In the case of introducing phosphate alone into the water (experiment 2), for complete conversion into phosphate salts it would require 152.7 mg of CaO and 11.5 mg of MgO (see above the analysis of the water taken)—329.5 mg per liter, as follows from the stoichiometric ratios in the reactions:

\[ \mathrm{3Ca(HCO_3)_2 + 2Na_3PO_4 = Ca_3(PO_4)_2 + 6NaHCO_3} \]

\[ \mathrm{3Mg(HCO_3)_2 + 2Na_3PO_4 = Mg_3(PO_4)_2 + 6NaHCO_3} \]

With the introduction of 5 mg/liter of phosphate, only 2.56 mg of calcium oxide or 1.85 mg of magnesium oxide can be precipitated in the form of phosphate salts, i.e., an entirely negligible amount in relation to their content in the water.

The precipitate of calcium and magnesium phosphates, being crystalline in itself, does not possess sufficient dispersity; moreover, the grains of this precipitate coarsen during boiling, forming larger crystalline aggregates—as a result, the surface obtained is insufficiently developed, and a considerable amount of carbonate scale is deposited on the walls of the vessel.

When sodium humate was introduced into the water in the same amount, 5 mg/liter (experiment 3), the amount of carbonate scale on the bottom and walls of the beaker proved to be 102 mg/liter, i.e., compared with phosphate, almost 2.5 times less under otherwise completely identical conditions. This circumstance is very favorable and is explained by the fact that sodium humate, as indicated above, forms with calcium and magnesium salts (in the present case with their bicarbonates) amorphous, very loose precipitates, the dispersity of which, in comparison with the phosphate salts of calcium and magnesium, proves to be much greater.

Increasing the humate dosage to 10 mg/liter reduced the amount of scale to 72 mg/liter (experiment 5b).

As for oak extract, we see that even at 10 mg per liter (without phosphate) the amount of carbonate scale is 234 mg/liter (experiment 5a), i.e., roughly three times greater than occurs for 10 mg/liter of humate. This circumstance is due to the considerably greater acidity of humic acid, as a result of which on

per unit of humate, considerably more precipitate of calcium and magnesium humates is obtained than of the corresponding tannates when oak extract is used.

When oak extract or humate is used with the simultaneous addition of phosphate at 5 mg/l for each of the indicated substances (experiment 4a and b), the amount of carbonate scale on the bottom and walls of the beaker is 140 mg/l with oak extract and 76 mg/l with humate, i.e., in the latter case it is 1.8 times less. This is explained, first, again by the greater acidity of humic acid and, second, evidently by the greater peptizing action of humate on the precipitates of phosphoric-acid salts of calcium and magnesium, which gives humate an advantage over oak extract.

It should also be noted that, in the experiments described, no difference was observed between them with respect to the frothiness of the water during boiling with the addition of oak extract or humate.

Oak extract in an alkaline solution is capable of binding oxygen dissolved in water, being oxidized in the process, and therefore has an anticorrosive action in a steam boiler.

It is known that humic acids and humates also possess the capacity for oxidation, and it may be expected that in this respect their action will be analogous to that of oak extract.

Conclusion

Summing up the results of the experiments carried out, we arrive at the following conclusions.

1. The use of humic acid obtained from brown humic coals, in the form of its sodium salt, as an anti-scale agent instead of oak extract should, on the basis of laboratory experiments, be considered entirely possible and theoretically justified.

2. The indicated use of sodium humate should lead to considerable savings as a result of replacing oak extract, whose cost is much higher.

At the same time, oak extract, not being consumed for internal boiler-water treatment, is preserved in the corresponding quantity as a necessary product for the leather industry.

1. On the basis of laboratory experiments it may be concluded that the use of humate instead of oak extract is not only economically advantageous, but also expedient by virtue of its greater capacity to reduce the amount of carbonate scale in comparison with oak extract under otherwise equal conditions.

2. The possibility is not excluded, in some cases, of using humate alone to convert scale-forming substances into sludge.