4?eservation that crystallization is a method of purification. The reason natural ice is clear is that after the first inch or two of ice has been frozen, the freezing or crystallizing takes place so slowly that thte impurities are rejected from the crystal ntass. Since natural ice is frozen from the siarface down, gravity aids in the elimination of impurities as long as free water esrists below the freezing surface.
Figure 1 .
More agitation is needed CHEMICAL
Artificial ice produced from fresh water is frozen so quickly that unless special precautions are taken the impurities present will be trapped in the ice and opaque blocks formed (Figures 1, 2, and 3). By agitating the water with air while it is being frozen such impurities may be swefà; free of the freezing surface and concentrated in the "core". When the core water becomes so concen trated with impurities that precipitation begins (Table I), the core is pumped out and replaced with fresh water and the freeze carried to completion. Such a procedure will give a transparent block which compares favorably with either natural or distilled water ice. The size of the core at the time of pulling will depend on the nature and amount of dissolved solids in the water used—generally the volume sucked will be from 3 to 5 gallons or approximately 10% of the original volume of water taken. The most important problem with which the ice industry is concerned is the elimination of opaque ice. Practice has shown that opacity formation occurs at two different stages of the freeze. The first stage takes place during the initial minutes of freezing when the rate of crystallization is so high that ordinary air agitation fails to bring about sufficient diffusion of dissolved salts from the fluid film at the crystal interface. Although such a condition ordinarily exists only until an insulating shell of ice is formed inside the can, it
Figure 2. A shattered block of ice A N - D . E < N 6 J N E g B I M , < 5 u N E>WyS
may cause unsightly white *'shelis" or white''butts". The degree of formation of such opacities is directly dependent on the concentration of total dissolved soHds but is not greatly affected by the nature of these solids. Elimination of white shells and butts can often be accomplished by increasing the flowing rate of the agitating air to 3 cubic feet per minute or by using special cans or auxiliary air feed devices which give more thorough agitation at the sides of the cans. Developments along these lines are the results of the classical researches of Burks (£). Quite logically, the second method of eliminating white shells and white butts is to reduce the total solids content of the vrater to be frozen. The methods used for such condi tioning will be considered in the discussion of water-treatment methods. The second type of opacity is that which results from concentration o>f dissolved solids during the freeze. Concentration alone may lead to the formation of white cakes, but just as often, concentration in duces the precipitation of.vaxious salts, such as calcium carbonate, which are very readily entrapped in the ice to form stains and "flowers". The reaction in the case of calcium carbonate formation is: Ca(HC0 3 )s ·
• CaC03 4- H 2 0 -I- C0 2
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carbon dioxide. By ensuring the presence of free carbon dioxide in the water to be frozen, a softer ice can be produced which is comparatively resistant to stress and mechanical shock.
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Figure 4. A graph showing free carbon d i o x i d e in core water. The final increase indicates the breakdown of soluble bicar bonates, resuitins in carbonate precipitates
1,200 to 1,400 p.p.m. Figure 4 shows the analytical indication of the bicarbonate breakdown during low temperature con centration. The initial decrease in natu rally occurring carbon dioxide results from aeration (agitation), while the final increase in carbon dioxide is the result of the conversion of bicarbonates to carbon ates. Prevention of opacities of the second type depends on reducing the ini tial total solid concentration or converting the bicarbonates to the corresponding sul fates or chlorides by means of acid treat ment or alum coagulation. The second major problem of the ice industry is the tendency of many manu factured ices to crack or shatter when frozen at low temperatures (Figures 2 and 3). There are probably many causes for crack ing difficulties, the direct cause being strains initiated by the expansion of the block ice when removed from the low tempera ture of the freezing brine to the higher temperature of the storage room or de livery platform. In many instances, ini tial strains are put in the ice during the freeze. Such strains may be caused by temperature gradients within the brine tank, dented or warped cans, and by in clusions within the ice itself. In some cases, therefore, cracking can be elimi nated by removing faulty cans or by im proving the brine circulation. Where in clusions are sources of trouble, chemical conditioning is usually required. The ma jority of inclusions are due to precipitation of calcium carbonate from concentrated core waters; the heavy particles of pre cipitate are caught in the ice and give rise to areas of differential expansion when changes in temperature occur. The gen eral method of eliminating such inclusions is to soften the water or add anticrack com pounds—mainly ammonium chloride— which function by increasing the solubility of calcium carbonate. A new method of attacking cracking difficulties, which has- been pointed out by the author (δ), is based on the use of
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1944
Water Requirements of the Ice Industry The choice of water supply for the pro duction of manufactured ice is based al most entirely on the mineral characteristics of the waters available (Table II). Since clarity is an absolute requisite where ice must be sold for domestic use on an open market, every effort must be made to ob tain waters which give clear ice, either directly or with a minimum of treatment. Because the capacity of any given plant is greatly increased by maintaining the low est freezing temperature consistent with the production of solid ice, it is also im portant to select waters which would be least likely to produce cracking difficulties when frozen at low temperatures. The importance of cracking is emphasized by consideration of the relationship between temperature an&freezing time (Figure 5), since cracking tendencies usually dictate the lowest practical freezing temperature. Since the physical characteristics of any manufactured ice are controlled almost entirely by the nature and extent of the mineralization of the water used, obviously the total dissolved solids value constitutes a valuable index for determining the suit ability of any water for ice manufacture. So far as is known, carbon dioxide is the only common constituent of water which is actually beneficial to ice production. Since dissolved material, other than carbon di oxide, is detrimental to the formation of clear, solid ice, a general rule is that the higher the total solids of any water, the less suitable it will be for ice manufacture. The upper limit of total solids concentra-
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fective method of treatment was available and waters of these types which were high in total solids could not be considered for ice production. This condition often forced the use of imported ice or, in some cases, the importation of satisfactory water by means of tank car so that acceptable ice could be produced. Organic Exchange Filters. In recent years, the development of organic exchange materials has provided new possibilities to the ice industry for the treatment of waters having high total solids, particularly those with high sodium bicarbonate content (Figure 8). For example, a special hydrogen zeolite (ZeoKarb H) has been used to convert a previously unsatisfactory water to one yielding excellent ice (I). Originally, this water had a total solids content of 899 p.p.m., most of which was due to sodium bicarbonate. Through treatment with the organic exchange filter, the total solids have been reduced to about 150 p.p.m., the cost being only a small fraction of that formerly incurred through importation of water from nearby towns.
imam Table I. Concentration of Dissolved Solids in Waters Frozen by the Can M e t h o d '•ïà-^'^w^. -\iiPK*?fi^p&iT ^
9 » » » » MAY
iitly higher total solids can be compensated for by increasing tbe pressure of the air used for agitation. Excess solids due to bicarbonates of calcium are readily removed by lime softening. Solids due t o other constituents, as well a s those due to calcium and magnesium, can b e removed b y organic exchange filters. Effectively reduced by lime softening or organic exchange niters. Calcium bicarbonate is about 1.5 fames more troublesome than the magnesium salt. Conversion to chlorides or. sulfates by treatment with acid or alum is often helpful. Removed by lime softening or organic exchange filters.' If only objectionable impurity, i t can b e kept from staining ice by adding calgon or by removing it through aeration, sedimentation, end nitration. Removed by organic exchange filters. Slight opacities due t o sodium bicarbonate can be eliminated by acid or alum treatment with the formation of tne corresponding chlorides or -sulfates. Removed by exchange filters Removed by exchange filters. Least harmful of all major constituents. Carbonation i s often advisable where free CO* is not naturally present or has been removed v b y limeeofteningBest controlled by aeration Best controlled, through lime softening and coagulation» Filtration or coagulation, fc A by nitration suffices where suspended matter is only impurity- ' Usually removed b y iron o r aluminum coagulation at controlled pH.
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on the use of such coagulants as Ferrisul, alum, or ferrous sulfate. CarbonatioiL. A s mentioned previously, recent researches (£) have shown that carbon dioxide has an important role in determining the physical characteristics of manufactured ice. By ensuring a residual o f free carbon dioxide for the first few hours of the freeze, waters can be frozen into ice which is physically softer than ice frozen from carbon dioxide-free water (Figure 7). Elimination of brittleness through carbonation is o f considerable importance, especially in time of war, since it permits an increase i n capacity in many plants without requiring enlargement of existing plant facilities. Such increases in pro duction result from lowering freezing temperatures, and are made possible by the elimination of cracking and shattering tendencies. Commercial application of t h e carbonation process i s carried out under selected conditions. A s a rule» free carbon dioxide is passed into the waters to be treated by means of glass frits or bubblers, a conveni ent source of the gas being liquid carbonic.
Summary The ice industry, because o f the close correlation between the quality of the raw water and the quality of the frozen block, has a number of w a t e r problems peculiar t o itself. Those o f almost universal con cern are t h e formation of opaque ices and the tendency of many water* t o yield ice which shatters or cracks. Opaque ice results from high concentra tions of total dissolved solids in the waters being frozen, the bicarbonates of calcium, magnesium, and iron being particularly significant. Elimination of opacities is best accomplished through reduction in total solids of the waters concerned. Con siderable benefit m a y result from conver sion of bicarbonates t o less harmful chlo rides or sulfates.
Cracking difficulties are of importance because they increase with lowering freez ing temperatures and thus often prevent efficient plant operation. Most cracking troubles occur with waters which are ex-
Ice, Manufactured Products, b y Band, Quantity, a n d Value, for the United States: 1937 1. Ice, manufactured, industry, all SI 3β,541.Θ82 products, total value 2. Ice m a d e for sale (platform value) 131, 423.468 3. Receipts for cold storage 4, 481.802 4. Other products (not classified 636,712 in this industry), value 5. Ice m a d e for sale as a secondary p r o d u c t in other industries 3,,565.262 Manufactured ice made for sale: T o t a l tone (2,000 pounds) 34 ,069,027 988,730 T o t a l value (sum of 2 and 5) 134, Ice m a d e for sale, by process—ice industry only Can ice Tons 33,195,170 Value 131,108,310 Plate ice Tons 63,623 Value 315,158 Ice m a d e for sale as a secondary p r o d u c t in other industries, n o t reported by process Tons 810,234 Value 3,565,262 Ice m a d e and consumed in speci fied industries, t o t a l tons 299,438 Ice cream 145,806 Butter 91,109 Alcoholic beverages 49,777 Condensed a n d evaporated milk 6,352 Cheese 6,394
Relative Importance of the Ice Industry to Leading Industries, for the U. S.
encies can be carried out by softening the water or by carbonation. Recent advances in water treatment methods for the ice industry which are of particular significance are the introduction of demineralization filters of the organic exchange type, the development of watertreatment units of the Accelator and Spauiding precipitator types, and the dis covery that many cracking difficulties can be obviated b y carbonating the water to be frozen by means of liquid carbonic. The great importance of organic exchange filters lies in the fact that they can be used to demineralize waters which could not be treated by the older, conventional methods. Filters of this type now make available t o the ice industry hundreds of waters which previously could not be considered. Chemical conditioning units of the pre cipitator type are of interest because they are simple to operate and are capable of supplying a better effluent than the older designs of softening and coagulating tanks. T h e widespread interest in the carbona tion process arises from t h e reports that it often provides a method of preventing cracking, even in cases when softening and anticrack compounds fail. The process is very economical and requires practically no special training or equipment t o oper ate. In some cases, carbonation has en abled as high as 3 5 % increase in pro duction without addition t o existing plant facilities.
Source, U. S. Bureau of Census, 1940 Ice, manufactured Wage earners Average for the year Rank Cost of materials Amount Rank Value of products Amount Rank Value added b y manufacturers Amount Rank
Literature Cited 16,009 119 $ 26,010,218 194 S130.166.312 101 9104,156,094 56
cessively hard or deficient in free carbon dioxide. Elimination of cracking tend
(1)
A p p l e b a u m , S . B . , a n d R i l e y , R a y , Ind. Eng. Chem., 3 0 , 8 0 - 8 2 ( 1 9 3 8 ) . (2) B u r k s , D a n a , J r . , 111. E n g . E x p t . S t a . , Bull. 2 1 9 ( 1 9 3 0 ) . (3) B u r k s , D a n a , J r . , Ind. Eng. Chem., 2A, 605-10(1932). (4) I n t e r n a t i o n a l F i l t e r C o . , Bull. 1950-A a n d 1970. (5) W e s t , P h i l i p W . , Ind. Eng. Chem., 3 4 , 1515-18 (1942). P R E S E N T E D before t h e Division of Agricultural and Food Chemistry at t h e P i t t s b u r g h meeting of the
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A M E R I C A N C H E M I C A L SOCIETV, April β
to
10, 1943.
AND
•^^^•^II^M'
ENGINEERING
NEWS
