Water softening process

Description

Precipitating agents, such as soda or trisodium phosphate, are used. However, this excess has the disadvantage of making the softened water too alkaline. Furthermore, these precipitates become trapped in the pores of fabrics, making them stiff and difficult to handle. The third, most common method is based on base exchange and consists of passing hard water through a system in which the sodium of a zeolite is exchanged for the calcium and / or magnesium of the water. This method requires not only special equipment but also careful monitoring. It has now been found that soft water, i.e., water no longer containing free calcium or magnesium ions, can be obtained by using water-soluble, complexing alkali metaphosphates. Although it has been suggested that alkali metaphosphates be used to treat boiler feedwater, the metaphosphates were only used in such quantities that they could initially only form normal calcium or magnesium metaphosphate, which then precipitate the residual hardness of the water in the boiler by being converted back into calcium or magnesium orthophosphate. In contrast, according to the invention, alkaline metaphosphates are to be added to the water in such quantities that the calcium and magnesium ions present in the water are converted into soluble complex compounds which do not cause limescale precipitation upon the addition of soap. Sodium metaphosphates, potassium metaphosphates, lithium metaphosphates, and ammonium metaphosphates are suitable for this purpose, but the former are preferable due to their low price. For the sake of textual simplification, only sodium metaphosphate will be discussed below. A major advantage of the process according to the invention is that effective softening, or rather, softening—i.e., the removal of free calcium and magnesium ions—is achieved without a harmful increase or any increase at all in the alkalinity of the water. In this way, one can obtain soft water that is neutral, very weakly acidic, or arbitrarily weakly alkaline.Furthermore, the softening process is achieved without any precipitate formation, meaning the softened water retains its crystal-clear quality. No special equipment is required, as the metaphosphates or their solution are simply added to the water. This makes the process particularly suitable for domestic use, since the water remains perfectly clear, is non-alkaline or only slightly alkaline, and is therefore perfectly suitable for toilet use, washing laundry, delicate fabrics, etc. In many cases, water treated with metaphosphates is even superior to naturally soft water, or water softened by conventional softening processes, because the alkaline metaphosphate gives the water certain purifying properties, in particular, for example, it is able to dissolve lime soap or other lime deposits. The preferably used soluble metaphosphate, also called Graham's salt, is conveniently mixed with a small amount of sodium pyrophosphate to neutralize the weakly acidic reaction of the metaphosphate. Graham's salt is a sodium hexametaphosphate to which The following complex formula is attributed to Na2 (Na4 P6 O18). However, some authors also believe in the existence of salts with the formulas Na5 (Na P8 O18) and Na4 (Na2 P0 O18). The readily soluble sodium hexametaphosphate can be prepared, although this is not the subject of the present invention, by strongly heating the primary sodium orthophosphate (NaH₂PO₄ + 2 aq.) and rapidly cooling the molten mass. Rapid cooling is essential for the formation of the readily soluble salt, while slow cooling promotes the degradation of other sodium metaphosphates, such as the less readily soluble sodium tri- and tetrametaphosphate. The readily soluble sodium hexametaphosphate is preferable for the present purposes; however, the other complexing metaphosphates, such as octo- or dodecametaphosphate, are also suitable. Sodium hexametaphosphate is slightly acidic and therefore requires a small addition of weakly alkaline sodium pyrophosphate if a neutral water softener is desired. By adding small amounts of sodium carbonate to primary orthophosphate during melting, an equivalent amount of sodium pyrophosphate can be generated in the melt during the metaphosphate production process, with the amount of sodium carbonate simultaneously determining the neutral or weakly alkaline consistency of the melt. This neutralizes the acidic reaction of sodium metaphosphate. Small amounts of caustic soda, calcined soda, sodium sesquicarbonate, borax, tertiary phosphate, sodium metasilicate, and similar chemicals may also be added to aqueous solution. The amounts of these neutralizing agents are relatively small, so that the plasticizer produced consists essentially of sodium metaphosphate. However, Under this designation, not only pure sodium metaphosphate is understood, but also one which contains small additions of the aforementioned acid-neutralizing agents. The mode of action of a sodium phosphate additive according to the invention differs significantly from that of conventional water softeners, because the calcium metaphosphate that precipitates first, as is known, is soluble in excess of the added sodium metaphosphate, forming weakly dissociated, complex compounds; the free calcium and magnesium ions disappear as such, but not by precipitation as in the known process, but by being incorporated into the complex ion of the soluble, weakly dissociated compound. The following experiments indicate this mode of action. If a suitable amount of soluble, complexing sodium metaphosphate is added to hard water, e.g., water containing calcium carbonate, and the water is then brought to a boil, carbon dioxide is driven off, while no precipitate of calcium carbonate forms. This proves that the calcium has formed a very weakly dissociated compound, which means that the concentration of free calcium ions necessary to reach the saturation equilibrium of calcium carbonate is no longer achieved. If such sodium metaphosphate is added to a suspension of solid CaO in water, it is observed that the lime turbidity disappears and a clear liquid is formed. The same phenomenon is observed when sodium metaphosphate is added to a suspension of tricalcium phosphate or calcium soap in water.It must therefore be required that the resulting soluble compound is extremely little dissociated, and that the concentration of calcium ions is lower than that of a saturated solution of the calcium salt in question. It has been found that sodium pyrophosphate, although it also reduces the calcium ion concentration to some extent, is less effective at softening calcium-containing water compared to sodium metaphosphate. Consequently, significantly larger quantities of this salt are required to achieve a comparable degree of softening when using pyrophosphate to effectively soften calcium-containing water. However, sodium pyrophosphate has a very noticeable softening effect with regard to magnesium. On the other hand, pyrophosphate is strongly alkaline. Therefore, it is advisable to use a relatively small amount (for example, about 10 ml) of sodium pyrophosphate as an additive to the metaphosphate. In this way, the pyrophosphate not only neutralizes the acidic reaction of the metaphosphate but also exerts a certain influence on the softening effect with respect to magnesium.The amount of pyrophosphate or another acid-neutralizing agent, such as caustic soda or sodium carbonate, can be varied according to the desired neutrality or alkalinity of the softened water. For most purposes, however, only enough acid-neutralizing agent is added to the metaphosphate to ensure neutrality or slight alkalinity of the water, as only such water is suitable for most applications. It was further found that as the alkalinity of the water is increased, the amount of metaphosphate required for softening must also be increased. For example, if a particular water requires 200 g of sodium metaphosphate per 1000 liters at a pK value of 8.5, the same water at a pK value of 10 will require 340 g of the salt for the same degree of softening. Consequently, it is preferable to avoid excessive alkalinity, not only because neutral or slightly alkaline water is usually required, but also because neutral or only slightly alkaline water can be softened more effectively and economically with metaphosphate than strongly alkaline water.In contrast to known softening methods, the treatment of water according to the present invention, to emphasize once again, eliminates the content of Ca ions not by the formation of insoluble precipitates, but by the formation of soluble compounds in which the Ca atom is complexly bound and cannot appear externally and thus cannot give the appearance of hardness. Once the amount of sodium hexametaphosphate Na2 [Na4 (PO3)6] required to form the soluble compound is added, a compound is likely to form,

Claims

whose formula can be expressed as Na₂(Ca₂P₆O₆). This salt is now dissociated to a certain degree into sodium ions and (Ca₂P₆O₆)₂ ions. This latter residue, in turn, also exhibits a certain degree of dissociation and yields free calcium ions. The extent to which this latter dissociation occurs appears to be controlled by the excess of sodium metaphosphate used. An addition of such an amount of sodium metaphosphate that there are 3 equivalent weights of Na₂PO₃ for every 1 equivalent weight of calcium softens the water to a very significant degree, specifically against sodium carbonate, but not against soap. This is only achieved by adding another equivalent weight of sodium metaphosphate, so that a total of 4 equivalent weights of it are present for every 1 equivalent weight of calcium.The magnesium probably forms a similar complex residue with sodium metaphosphate, which is structured similarly to the aforementioned calcium residue, but has a lower degree of dissociation, which can also be reduced by an excess of sodium metaphosphate. As can be seen from these theoretical considerations, only metaphosphates with complex-forming properties are suitable for the present process. These include, primarily, hexa-, octo-, dodeca-, and other metaphosphates. Whether a metaphosphate is suitable can be easily determined with the .35 soap test. If the metaphosphate in question is added to some water containing n / iocalcium chloride, the solution must remain crystal clear after initial turbidity and produce a persistent foam upon addition of soap solution. The water softened according to the described process can practically be described as a new product, because despite complete softening, it simultaneously contains such an amount of calcium that it would make the water hard if it were not a complex. The ion would be bound. In practice, an addition of 15 g of metaphosphate per degree of hardness and 100 liters of water has proven sufficient for complete softening. The water treated according to this process can be used in households and businesses for all washing, cleaning, and rinsing processes. The treated objects are freed from insoluble limescale deposits, such as lime soaps or lime carbonates, and acquire a completely pure, clear, white, and shiny appearance. Patent claims: r. Method for softening water, characterized in that a soluble complex-forming alkali metaphosphate, e.g. hexametaphosphate, is added to the water in such quantities that the calcium or magnesium ions present in the water are converted into a soluble complex compound which does not cause limescale precipitation upon the addition of soap.

2. Method according to claim 1, characterized in that, in addition to the alkali metaphosphate, an additive of alkali prophosphate is used to soften the water.