Water for ice making
Ivan Tikhonov
Annotation
The article discusses how impurities in water can affect the quality of ice. Of course, not ice for drinks is meant, but ice for sporting events.
The analysis of existing schemes of water preparation for ice making leads to the following conclusion: the lower the salinity of water is and the concentration of gases contained in it, the higher the quality of the ice obtained. For these purposes, the technology of reverse osmotic desalination of water is widely used. It is permissible to use fresh water for ice making, i.e. water with a salinity of up to 1000 mg/l, but it should have necessarily passed preliminary clarification and softening. Degassing systems must be used in any case. Let’s figure out why the above mentioned conditions must be met to obtain high-quality ice.
Water is a collection of identical molecules (H2O) connected in a certain way. The bond between water molecules is determined by the magnetic moment of each water molecule. Oxygen has a small surface negative charge, and two hydrogens have a small positive charge. This can be explained by the fact that despite the fact that divalent oxygen joins two monovalent hydrogen, and the strength of the covalent relationship is fully compensated, the electronegativity of these elements still remains uncompensated. Electronegativity is the ability (strength) of elements to attract electrons to themselves. The electronegativity of oxygen is 3.44, it is more than hydrogen which is 2.2. Therefore, oxygen tries to attract electrons shared with hydrogen with greater force than hydrogen. It is obvious that for the oxygen in a water molecule it does not matter which electrons to attract, “its” hydrogen or “someone else’s”. As a result, one water molecule is attracted to another. But the attraction between water molecules is much less than the attraction force of the covalent bond inside the water molecule. Therefore, there is a continuous water known to everyone for its liquid form.
As a result, the water molecule H2O exhibits a physical and chemical interaction. The physical interaction is due to the strong dipole (magnetic) properties of the water molecule. The chemical interaction is due to the fact that the water molecule, strictly speaking, is an inorganic chemical compound. This suggests that physical and chemical processes occur in the water.
Figure 1 shows the process of dissolving table salt (NaCl) in water. In the “dry” state, table salt has a solid crystalline form. But if you add water, the process of dissociation (dissolution) of salt in water will begin. The following happens. Having a small surface charge, water molecules begin to interact with the NaCl molecule. It is necessary to understand that not only nearby water molecules interact with salt, but in general all water molecules of a given volume in which dissolution occurs. It’s like a tug of war with an elephant. If one person or several people are against the elephant, then the elephant will win. But if there are a hundred of people against an elephant, then the elephant will most likely give in. So it is in the water. All water molecules of a given volume will interact with the NaCl molecule simultaneously. Despite the fact that the covalent bond in the NaCl molecule is much stronger than the surface bond of water molecules, nevertheless, by common efforts, water molecules can compensate for part of the covalent bond of the salt molecule. As a result, the salt dissociates into two ions in the volume of water molecules. Sodium cation, positively charged, and chlorine anion, negatively charged. Ions constantly retain the force of attraction between themselves, but at the same time, part of this force is compensated by water molecules. Therefore, the ions acquire the ability to move separately (not in pairs) throughout the entire volume of water while ensuring the equilibrium state of the entire volume of water. NaCl ions bring structure to water molecules around themselves in a certain way.
Figure 1
Figure 1 schematically shows how the orientation of water molecules around the Na cation and Cl anion occurs. It is obvious that nearby water molecules will be in the greatest engagement with ions compared to other water molecules. The water molecules adjacent to the ion form the so-called primary hydration shell. The water molecules in this shell are rather tightly bound to the ion and have a certain ice-like structure. The further away from the ion, the less the force of attraction of water molecules to the ion is. Therefore, the following hydration shells are less and less connected to the ion. But nevertheless they are connected. All water molecules of the volume in which table salt is dissolved participate in the interaction. Clearly, a secondary hydration shell is also isolated. Nevertheless, all water molecules are involved in dissolution. Therefore, we can talk about the third, fourth, etc. of hydration shells, as a way of forming water molecules around an ion. Or rather, we should not talk about tertiary and subsequent hydration shells, but about a certain gradient of the force of the action of water molecules on a specific ion. The further away a water molecule is from an ion, the less it is bound to it.
Figure 1 shows how the hydration shells of the cation can be superimposed on the hydration shells of the anion. Table salt is in a dissolved state. If you measure the electrical conductivity of a given volume of water, it will be the same at all points of a given volume. This means that the cation and anion are located simultaneously at each point of a given volume of water. This, in turn, indicates the desire of water molecules to provide a certain equilibrium state, an attempt to fully compensate for the charge of ions, ensuring its uniform distribution in its volume. If the volume (number of molecules) of water is not enough, then the dissociation of the salt crystal into ions does not occur. This ability is characterized by such a characteristic of the salt as solubility. For example, it is impossible to dissolve more than 0.35 kg of table salt in 1 kg of water at 25 0C.
Now back to the water for ice making. Obviously, the more homogeneous the structure of ice crystals, the stronger the ice. Various foreign inclusions destroy the uniformity of the ice and worsen its performance characteristics. Inclusions can be solid mechanical particles or suspended substances, organic acids and colloidal impurities, inorganic chemical compounds and gases dissolved in water. To remove suspended solids, filtration can be applied starting from bulk sand filters and ending with polypropylene filters with a rating of up to 1 micron. To remove colloidal impurities and organic acids, water coagulation with subsequent filtration can be used. Let’s focus on inorganic compounds in more detail.
Inorganic compounds in water are contained in the form of ions. When water freezes, the number of active “liquid” water molecules becomes smaller and the solubility limit for a particular salt present in the water is gradually reached. As a result, there are no ions in ice, there are solid crystalline salt compounds that existed in liquid water in the form of cations and anions. The unevenness of freezing will lead to the fact that clumps of salt crystals will concentrate specifically in the place where the state of liquid water remained the longest. Therefore, such ice will be uneven and, accordingly, relatively fragile. If the water for ice making was originally fresh, then the salt content in it is relatively small and for many purposes such water is quite acceptable. Never the less there is one significant “but”. When frozen ice under the action of forces applied to it begins to melt in the form of a thin film, sodium salts such as NaCl, Na2SO4, NaHCO3 immediately pass into a dissolved state, without causing solid formations on the ice surface. Salts Ca(HCO3)2 and CaSO4, having passed into a solid state during ice freezing, will remain in a solid state during reverse thawing, while significantly impairing the characteristics of the ice surface. Therefore, the water for ice making must be subjected to Na – cation. As a result, all divalent and trivalent cations in water will be replaced by monovalent easily soluble sodium cations. To obtain high-quality ice, reverse osmotic water desalination systems are used. These systems reduce water mineralization by up to 99%, thereby allowing to obtain homogeneous ice without any inclusions.
One more question remains. How to remove all gases present in the water from the water. When freezing, gas bubbles will significantly increase the fragility of the ice, destroying its uniformity. If we talk about a surface uncontaminated water supply source that has constant contact with the atmosphere, then the water of such a source contains three gases: oxygen, carbon dioxide and nitrogen. At normal atmospheric pressure and a temperature of +20 0C, distilled water contains 9 mg/l of oxygen, 14.8 mg/l of nitrogen and 0.5 mg/l of carbon dioxide. Although for natural surface water supply sources, the oxygen content will be somewhat less, and carbon dioxide somewhat more due to the ongoing processes of oxidation of organic matter. For underground sources that do not have contact with the atmosphere, hydrogen sulfide or ammonia may be present in the water, which will require preliminary specific water treatment.
Oxygen and carbon dioxide can be removed from water by chemical methods. To do this, caustic and sodium bisulfite are dosed into the prepared water (which has undergone a minimum of softening and osmosis as an addition). Caustic will bind carbon dioxide to sodium bicarbonate, and sodium bisulfite will bind oxygen to sodium sulfate. But what to do with nitrogen? It should be noted that chemical methods of binding dissolved gases significantly increase the final salinity of water. For example, to remove 1 mg/l of oxygen, 7 mg/l of sodium bisulfite is required. As a result, if the water contains 8 mg/l of oxygen, the total salinity of water after chemical deaeration will increase by 7 *8 = 56 mg/l. This circumstance devalues the use of reverse osmosis. Therefore, for the purposes of complete degassing, it is better to use two methods. Thermal method and absorption of gases through a membrane contactor. In the first case, heating the water in the form of a thin film up to 100 0C will ensure complete degassing of the water. Obviously, this method is quite complex and expensive for the purposes of ice pouring. Degassed water should be stored without contact with the atmosphere, while reducing its temperature to the technological parameters of ice making. The use of a gas-liquid membrane contactor operating in vacuum mode will remove all dissolved gases from the water. The lower the vacuum is, the lower the residual concentration of gases in the treated water. I would like to draw your attention to the fact that if you degas not softened water, then calcium carbonate will actively settle in the process of degassing water in the form of a thin solid insoluble film in water.
Modern water treatment technologies make it possible to obtain water of a very high quality on compact fully automated installations that do not require permanent maintenance personnel and ensure the greatest efficiency from the operation of both the water treatment plant itself and the entire production complex as a whole.
