Enhancing Ice-Melting Action of Rock Salt by Prewetting with Calcium Chloride

D. R. Larrimore, E. N. Mossner, J. C. Nixon, Dow Chemical U.S.A.

This investigation was prompted by the interest extend its effective application temperature, and in liquid calcium chloride (Cad2) for pre- increase ice melting action (14). Reductions in wetting rock salt and abrasives used for snow salt use per mile of 25-50%, resulting in more and ice control. The findings demonstrate that miles spread per load of salt, have been claimed by prewetting can increase the effectiveness of various agencies (2,3,4,5,6,7,8) due to prewetting rock salt and abrasives and thus reduce appli- which decreases both costs and adverse environ- cation rates and costs and may result in mental effects of deicing chemical use (Fig. 2). decreased damage to nearby salt sensitive vege- Reàpreading and overspreading are also reported to tation and reduced infiltration into ground be reduced by prewetting rock salt. water. Both field and laboratory information are included, and both substantiate the increased efficiency of rock salt and abra- sives; data indicates that salt use may be COMPARISON OF KALKASKA reduced by 25-50% and that rock salt may be WINTER MAINTENANCE DATAt effectively used at temperatures as low as 1973-74 (Dry Salt) vs 1974-75 (Wet Salt) -17.8°C (0°F).

Almost every maintenance engineer in the 130 - 70 1400 DRY snow-belt states has been faced with the problem of —12% removing snow or ice at temperatures lower than 120 26% 60 35% 1300 WEl -6.7°C (20°F), where the effectiveness of rock salt is greatly reduced. According to literature data, at temperatures below -3.9°C (25°F), salt will melt Bus 50 - 1200 less than 30% of the amount of ice it would melt at

-1.1°C (30°F) or above (1). 100 - 40 1100 A method recently introduced to combat this low temperature effectiveness problem is the concept of 1000 prewetting rock salt with liquid calcium chloride 95 30 brine (8-12 gallons 30-32% CaC12 per ton salt) to reduce bounce-off (Fig. 1) and loss due to traffic, 80 - - 20 - - 900_ ___ 73-74 74.75 73-74 74.75 73-74 74.75 * Slot. of NShlg.n (

SALT RETRIEVED FROM FIGURE 2 PAVEMENT SURFACEt

CENTER 1/3 OF OUTSIDE ~ OF NOT RETRIEVED 24 FT. PAV'T. 24 FT. PAV'T. FROM PAV'T. Field Experience 100%

78% Background

The first field test of rock salt prewet with liquid calcium chloride at the rate of 10 gal. 32% 46% 50% CaC12/ton salt was conducted in Clayton County, 30% Iowa during the 1968-1969 winter and successfully 2 4% demonstrated that prewetting substantially increased the melting action of rock salt and reduced loss due to salt bouncing off the pavement. F-i In the years since that date, many other PRE-WET DRY PRE-WET DRY PRE-WET DRY cities, counties, and states have experimented with Stole of NLcl,ig,n () liquid calcium chloride, proven its effectiveness FIGURE 1 with field tests, and outfitted their maintenance

282 283

yards with a variety of equipment to accomplish the salt from the road (14) (Figure 1). prewetting. The most notable experimenters are the The faster penetration and undercutting action states of Iowa, Michigan, and Ohio (2,4,5,6,9) who will also reduce salt use, since operators can see have conducted closely monitored field trials and melting action earlier and are less inclined to presented their findings. overspread or respread treated areas. Users of the prewet salt have reported savings Calcium Chloride Properties of 25% (and more) of their total salt usage in comparison with' dry rock salt (Figure 2). Calcium chloride, chemical formula CaCl2, is This reduction in the amount of salt used, in the salt of a strong acid and a strong base. addition to decreasing the cost of a maintenance Commercially, calcium chloride is obtained either program, reduces the potential effect of such from natural brines or as a by-product of the deicers upon the environment. reaction between lime and ammonium chloride. It is extremely soluble in water and' forms many hydrates. APPLICATION PACTS Although calcium chloride is highly soluble in water at ordinary temperatures, solid phase sepa- PREWET DRY Holds in close pattern. No doss. Some segregation. Large particles spread wide. ration will occur under certain temperature- Dust euident. concentration conditions as shown in the phase diagram in Figure 3. Although this figure is com- Stays in place, little sliding on thick Stays in place on loose slush, slides wide on ice, thick ice. prised of data for pure CaC12, brines made from Inheds in ice cooer immediately. Oormant when applied. Inbeds in 3-5 minutes commercially available calcium chloride closely @30°F, 19 minutes @25°F, much greater time approximate the heavy black line in the diagram. below 20°F. The eutectic point is 30.2% CaCl2 and -50°C Minimal early loss caused by Loss nariable: (-58°F). whipping of traffic. High temperatures: minimal Low temperatures: considerable

MELTING ACTION PHASE DIAGRAM FOR THE CaCl2-H2O SYSTEM Starts immediately 28°-32°F Minor delay Starts immediately 25°-28°F 10.20 minute delay 180 356 I I I I Minor delay Below 20°F 30 minutes or greater delay CaCl2 H20 + Solution 160 L 320

1401- I - I 1 284 Melting Action Comparison

120 248 Winter testing at eight separate district CaCl2 2HO maintenance unit headquarters in the State of + Solution 100 I 1212 Michigan (3) was conducted to determine comparative advantages of prewet salt vs. dry salt. The follow- o ing section is a composite summary of observation 80 Solution 176 °. cc cr reports received from these headquarters. 60 140 '— CaCl2 4H2 0 Melting Action 0. + Solution 40 ° 104 wI- uJI- Starts immediately 28°-32°F Minor delay Starts immediately 25°-28°F 10-20 minute delay 68 20b Ice U Minor delay Below 20°F 30 minutes or + Ca012 6H2 0 greater delay 0IF_ 32 Limitations -4 .201- It must be remembered that prewetting with I Solution liquid calcium chloride is not the total solution -40 + Ice -40 for snow and ice control. For the most accelerated deicing action at lower temperatures, dry calcium chloride alone or in mixtures with salt, must be 0 10 20 30 40 50 60 70 80 applied. At lower temperatures (as illustrated in WEIGHT PERCENT CALCIUM CHLORIDE Figure 4), dry calcium chloride is much more effective than salt or salt prewetted with liquid FIGURE 3 calcium chloride.

Calcium chloride is both strongly hygroscopic TIME REQUIRED TO BREAK ICE BOND (attracts water) and deliquescent (the solid can BY 'PENETRATION OF CHEMICAL dissolve by absorbing atmospheric water). Calcium TO SURFACE OF ROAD chloride also has a highly exothermic heat of Rate: 500 Ibslfane mile on solid, glare ice solution. These properties, among others, make it 75 effective in dust control, deicing, concrete

acceleration, road stabilization, and oil well By - drilling applications. It is commercially avail- s.n able in anhydrous and dihydrate solid forms or in 45 liquid form in various concentrations.

0 — Benefits Cited : Prewetting with liquid calcium chloride extends the effective temperature range of salt to 0°F, and —Ia 5 +10 +15 +20 +25 reduces the total amount of deicer required through TEMPERATURE, F faster action per unit of chemical applied and reduction of loss due to scattering and bouncing of FIGURE 4 284

If speed of action is critical under low The system devised by the Iowa State Highway temperature conditions, then dry calcium chloride Commission, and depicted in Figure 9, is also more is the answer to the problem. The use of pre- complex. It involves a truck mounted set of wetting or dry calcium chloride must be tailored to components. Liquid calcium chloride is placed in a the user's operations, concerns, and weather fiberglass tank mounted along the side of the truck conditions. bed. A pump which is belt-driven from the spreader drive shaft applies the liquid to two 50-degree fan nozzles drilled at a given spacing in a spray bar. Application Systems This is inserted into the spreader discharge chute immediately above the spinner assembly. Field application systems have ranged from equipment as simple as a bucket and sprinkler can INSTALLED APPLICATOR KIT ON SALT SPREADER TRUCK for trials to a sophisticated, electronic device IOWA STATE HIGHWAY COMMISSION with logic circuitry. The basic components for all permanent systems are a tank, a pump, and the associated piping and electrical equipment (Figure 5). Figure 6 shows a simple system which consists FILTER of a garden hose water nozzle or a gasoline pump- SPRAY oA R_\ ,\\ \—POWER TAKEOFF type nozzle which can be attached through a rubber SPREADER hose to a valve, meter, and pump and used to spray ' LJ,,,,a SPREA DER \-k \ ',I-' DRIVE.SHAFT liquid calcium chloride over rock salt already DISCHARGE ,/' CHLORIDE LIOUOR' loaded into a truck. Nozzle material may be either CHUTE SUPPLY TANK brass or steel. The City of Indianapolis, Indiana, 3-WAY FLUSHING VALVE among others, uses this system. POSITIVE Figure 7 is a simple hand wand which has been DISPLACEMENT used in place of the nozzles in the above system. PUMP This is made of standard steel pipe.

FIGURE 9

The Michigan Department of State Highways and Transportation also uses a truck-mounted system similar to that used by Iowa (Figure 10). Liquid V STRAINER (Option, I) VENT calcium chloride is pumped from a fiberglass tank

STORAGE on the side of the truck to a nozzle set just below TANK the discharge chute. Prewet salt can then be applied through the spinner assembly or to an auger feed which applies it just above the roadbed.

PUMP DRAIN 1.2-3-4 are all parr of a 4 riuume to wmpie bysiem wir, cable tram Control be. SAMPLE VALVE

FIGURE 5 Basic Components FIGURE 7 Hand Wand

District No. 3 of the State of Ohio Department of Transportation is using a more sophisticated tight In 'NO. Disconnect Red wire from lormiral and Splice to White wire from system which is shown in Figure 8. A truck with a hydraulic pro,suro Switch, Connect Rtack load of salt drives into the stall, the truck oire. tram hydraulic pros,,,, Switch, to tnrnrinct the Rod wire was remoced from. driver hits the timer control, and the liquid calcium chloride is sprayed over the rock salt in FUUUHE 1U WIrIng Ulagrarn tar "Michigan Wet Satt" Centrol Ban the truck. The most complex system currently on the market OntO DOT PREWET SYSTEM DISTRICT Of RACK AND SPRAY BAR LAYOLIT today, and used in Evanston, Illinois and Kettering, Ohio, Consists of a radio-controlled wetting arm which applies liquid calcium chloride to salt in the end loader bucket before it is placed in the truck (Fig. 11).

FIGURE

FIGURE 11 285

Laboratory Experimentation freeze unit held at the desired temperature (see Figure 12b). A small fan in the freezer Although field data demonstrated an increase in circulated the cold air to eliminate tempera- effectiveness, there were still questions which ture gradients. Weighed particles of chemical needed to be answered by controlled experimenta- were lightly dusted with a dye, the sodium salt tion. The principal question was whether the 1:57 of fluorescein, and loaded into a dispenser in (by weight) ratio of CaCl2 to rock salt used in which the particles were isolated in individual prewetting was actually producing increased compartments. The dispenser was placed over effectiveness since most dry premixes are 1:3 to the ice covered concrete block and the system 1:5, calcium chloride:rock salt, vol:vol (10), or allowed to come to a uniform temperature as if the trial results were due to other variables measured by thermocouples or thermometers. present in field experimentation. The effective temperature range of prewet salt PREWET STUDY EQUIPMENT was also being questioned since some users were claiming effectiveness down to -28.9°C (-20°F) Camera although the eutectic temperature for rock salt, which comprises approximately 98% of the mixture, .-Plexiglas Window is -21.1°C (-6°F) at best (4). The answers to these questions could be obtained by well controlled laboratory studies of the penetration Black Light and undercutting action of untreated rock salt compared with salt prewet with various agents which depress the freezing point of water. In this study the method used and reported by Fan Sinke and Mossner (IL) in their comparison of Timer Salt Ice calcium chloride and rock salt was modified and used as the basis for comparison of prewet with untreated rock salt. The prewetting agents chosen to be studied were 30% calcium chloride, 59% Concrete propylene glycol, 56% ethylene glycol, and water (all concentrations on a weight basis). Each prewetting agent was applied, as commonly practiced with calcium chloride in the field, at the rate of FIGURE 12b 0.04 liters per kilogram of rock salt (10 gallons/ton). Two effects were studied: time to undercut a sheet of ice and time to penetrate a In our experiment, salt was treated with the block of ice. various "freeze-proof" wetting agents first and the system was then allowed to return to the chosen temperature. Prewetting with water was done just Undercutting Studies prior to application of the rock salt to the ice surface. Continuing with Sinke's and Mossner's To study the effects of prewet rock salt on (11) technique: undercutting, an adaptation of the experimental The dispenser was then actuated by pulling method developed by C. C. Sinke and E. H. Mossner cords passing through the freezer lid. The (11) was used. The following is an excerpt from particles were thus dropped on the ice and the their report: dispenser drawn out of the way. As melting In this study a laboratory method for con- action began, the dye dissolved and fluoresced tinuous observation of ice melting action in a due to an interior ultraviolet light source. situation simulating an ice covered concrete The fluorescent brine clearly outlined the highway has been used. A particle of chemical extent to which the bond between the ice and when dropped on the ice will melt down through the concrete was dissolved. Color photographs the ice and the brine will then spread Out of the melting action were taken through a underneath the ice as illustrated in plastic window in the freezer lid at appro- Figure 12a. This bond breaking action is the priate intervals. most significant effect of deicing chemicals, Selected photographs from an experiment at since calculations show that it would be -12.2°C (10°F) are shown in black and white in prohibitively expensive to melt all the ice. Figures 13a & b. Continuing to quote from Sinke In these experiments, a 0.3175 cm (1/8") layer of clear ice was built up on a concrete block SELECTED PHOTOGRAPHS SHOWING TECHNIQUE USED IN UNDERCUTtING STUDY and the block was placed in acornmercial deep

ICE BOND-BREAKING AT INTERFACE WITH PAVEMENT (11)

ICE

PAVEMENT

FIGURE 12a FIGURE 13 286

Two small fans circulated the air in the reezer to minimize temperature gradients and Improve freezer efficiency. The salt particles ere lightly dusted with the sodium salt of luorescein and loaded into the dispenser. After ihe system reached the desired temperature, the salt was treated with the various "freeze-proof" setting agents and the system returned to steady state. Prewetting with water was performed just prior to application of the salt to the ice sur- ace. The dispenser was then actuated and drawn out of the way. As penetration proceeded, the sodium fluorescein dissolved and fluoresced underneath internally-mounted black lights. Color photographs of the progressing experiment were taken at appropriate intervals through the plexiglass window mounted in the freezer lid. Photographs from an experiment at -12.2°C (10°F) are shown in black and white in Figure 15.

FIGURE 13 b

and Mossner (11): Unfortunately, the distinctiveness in melting and undercutting action evidenced in the colored photographs is not as clearly shown in the black and white copies. The initial hole melted through the ice becomes surrounded by a lighter colored area which is the thin layer of brine between the ice and concrete. The area "undercut" by each chemical particle is deter- mined by comparison with the 2 cm standard length in each picture. The rate of ice melting as well as the ultimate area of under- cutting can be determined in one experiment at a given temperature.

Penetration Studies

In this study a laboratory method for con- tinuous observation of ice penetration was used. block of ice approximately 2 cm x 2 cm x 8 cm was polished and the top surface branded with a linear grid design of four 2 cm x 2 cm squares. This effectively formed gutters and isolated each salt particle from its neighbors. The block was then placed into a commercial deep freeze. A mirror mounted at an angle of 45° was placed in front of the ice so that penetration could be viewed from above. The dispenser was then placed over the ice and the system described above allowed to attain steady-state at the desired temperature. The equipment was arranged as depicted in Figure 14

SKETCH OF PENETRATION STUDY EQUIPMENT

Camera

FIGURE 15

Preparation

Pure fused sodium chloride was used to reduce the number of random variables to a minimum (CaSO4 concentration variance in particular, a comon alt impurity present in Michigan rock salt (11)). All salt particles were cut to 60 mg ± 1 mg, a weight found by Sinke and Mossner (11) to represent the Mirro Ice weight of approximately 60% of Michigan rock salt. Care was taken to produce cubic salt particles since variations in salt shape can adversely affect FIGURE 14 the reproducibility of the results. All prewet particles were treated with 2.5 microliter (p1) of wetting agent using a 10 pl syringe. The appli- 287

cation rate of 2.5 p1/60 mg is equivalent to PENETRATION OF PREWET AND UNTREATED NaCl 10 gal/ton of rock salt. AS A FUNCTION OF TIME AND TEMPERATURE Prewetting agent concentrations were adjusted so that each solution had a freezing point of -47.8°C (-54°F). Deionized water was used when -12.2°C / -15.0°C (10* F) water prewetting was studied. (5°F) II Il Results Undercutting Studies. A graph of the area / undercut as a function of melting time and tempera- turê is shown in Figure 16. An average of the / results of all of the prewet salts is shown for convenience since all prewet salt results were very similar. In all cases, prewet salts were better than the dry salts with respect to the onset and extent of undercutting. The prewet curves are com- ,, prised of data from salts treated with 30% Cad2, II ii ,'.7.178°C the glycols, and water for the -6.7°C (20°F) and (0°F) -12.2°C (10 °F) runs, and only 30% CaCl2 and the glycols for the -15.0°C (5°F) and -17.8°C (0°F) 000, runs. Water was not used at -15°C (5°F) and -17.8°C (0°F) due to freezing problems. Prewet Salt AREA UNDERCUT BY PREWET AND UNTREATED NaCI AS A FUNCTION OF TIME AND TEMPERATURE Untreated Salt

- - Prewet Salt -6.7°C 30 (20°F) 0 10 20 30 40 50 60 70 Untreated Salt .01 Time, Minutes FIGURE 17 50 / / / Discussion to / / From this research, it is shown that prewetting / of pure fused NaCl increases the effectiveness of -12.2°C NaCl in both undercutting of a sheet of ice bonded (0 (10° F) to pavement and penetration of a block of ice. The prewet data was generally in very good agreement for each run except at -15.0°C (5°F). Scatter was increased for both undercutting and penetration. It would appear that -15.0°C (5°F) is near the transition point from relatively fast .15.0°C (5° F) reaction rates at -6.7°C (20°F) and -12.2°C (10°F) to the slow rates experienced at -17.8°C (0°F). 0 Results shown here may represent the minimum -17.8°C advantage that may be realized by prewetting rock (0° F) salt since the pure fused NaCl contained none of the CaSO4 found in regular highway rock salt. CaSO4 was found by Sinke and Mossner (11) to reduce 10 20 30 40 50 60 70 the melting action of regular rock salt. The Time, Minutes increased ability of the prewet rock salt to form brine may overcome to some degree the slower reac- FIGURE 16 tivity of rock salt containing C5SO4 because once.a brine is formed, the NaC1 around the CaSO4 particle can be dissolved. Penetration Studies, A graph of the depth of Also of great advantage is the reduction in the penetration as a function of melting time and amount of prewet salt necessary to treat a given temperature is shown in Figure 17. Due to the high portion of roadway. This advantage is reflected rate of penetration shown by the salt at -12.2°C not only in dollar savings due to reduced salt use, (10°F), the decision was made not to study the but also in a decreased burden of NaCl on the effects of prewetting at higher temperatures. An environment. Environmental damage could be reduced average of the results of all prewet salts is shown due to the lower rate of salt application and the for convenience since all prewet salt results were significant reduction in salt bounce off and very similar. The prewet curve at -12.2°C (10°F) scatter (8). Figure 16 should be helpful in is comprised of data from salts treated with 30% optimizing deicing operations. Water as a pre- CaC1, 59% propylene glycol, and water. Water was wetting agent was considered for the sake of not used at -15.0°C (5°F) and -17.8°C (0°F) due to completeness. Its use in the field, however, is freezing problems. The prewet salts have a higher impractical, because rock salt prewet with water initial penetration rate than the untreated salts. will freeze and clump. This initial rate soon decays to the rate of untreated salt since the prewetting agent is diluted and flushed away with the salt brine formed during penetration. 288

Economic Analysis References

Since all prewetting agents examined in this The Snowfighter's Handbook, A Practical Guide study were effective in increasing melting action for Snow and Ice Control. The Salt Institute, of rock salt, an economic analysis of market prices copyright 1967, revised 1973, p. 15. of the various agents was conducted. The results F. M. Bozarth. Implementation Package for Use of this analysis are shown in Figure 18. Calcium of Liquid Calcium Chloride to Improve Deicing chloride prewetting is about one-fourteenth the and Snow Removal Operations. U.S. DOT, FHWA cost of either glycol. 73-2, 1973, l6pp. H. Lemon. 1974-1975 Prewetted Salt Report. COMPARISON OF Michigan Dept. of State Highways and PREWETTING AGENT COSTS Transportation, 1975. The Wetted Salt Process for Improved Snow and Ice Control. Public Technology, Inc., 1977, 20 31pp. 1$15.10-16.50/Ton 32% Calcium Chloride (12, 13) H. Lemon. Liquid Calcium Chloride Improves $021 -0.24/Lb. Industrial Grade E.G. (12. 13) Salt Patterns. BETTER ROADS, July. 1974, 18 pp. 20-21. 3$0.24-0.27/Lb. Industrial Grade P.G. (12. 13) Iowa's deicing mix is fast, with less salt. RURAL AND URBAN ROADS, August, 1975, pp. 38-39. J. Sprang. County's calcium 'trigger' brings 16 down deicer costs. RURAL AND URBAN ROADS, uJ 0 August, 1975. Save Salt: Save the Environment. BETTER a- 14 ROADS, March, 1973, pp. 28-29. 0 T. L. Copas and H. A. Pennock. Minimizing Deicing Chemical Use. TRB, NCRRP, Synthesis of Highway Practice 24, 1974, pp. 9-11, 15, 17. J. Hode Keyser. Deicing Chemicals and Abrasives: State of the art. HRB, Highway Research Record 425, 1973, pp. 36-51. •G. C. Sinke and E. H. Mossner. Laboratory Comparison of Calcium Chloride and Rock Salt as Ice Removal Agents. TRB, Transportation 0. ;i :: Research Record 598, 1976, pp. 54-57.

0 <2 coLO U, Chemical Marketing Reporter, Vol. 212, No. 19, 8 C,) : Nov. 7, 1977, pp.44. 46, 49. 0Q, 6 co Dow Chemical Co., Midland, MI, Functional 0 o .- C', -J Products and Systems and Organic Chemicals -j - Departments, November, 1977. -J 0 L. D. Minsk , et al. Workshop Proceedings, 0 Snow and Ice Control: Road Salt Use In o-J I >- J Minnesota. Workshop Proceedings, House o -j -(J 4 Committee on Transportation and Science & w w 2 LU Technology Project, Sept. 13, 1977, Chapter XI. 2 w -J (5 -J -j >- a.>- I 0 2 C.) ICr -w a. C., r1 tn 0 - - - FIGURE 18

Conc lus ions

On the basis of the data obtained, the following conclusions may be drawn:

The use of rock salt with prewetting agents offers several advantages over untreated rock salt. Among these are increased melting action and decreased salt usage per mile which results in lower costs, greater productivity, and a probable reduction in NaCl damage to the surrounding environment. Liquid calcium chloride is the most economical agent to use in prewetting of rock salt as shown by the cost data in Figure 18. Liquid calcium chloride prewet salt provides a more efficient use of deicing chemicals, equipment and winter road maintenance budget when both performance and economics are considered.