Process for the production of road deicers from water plant residuals

An improved process for the production of road deicer products preferably containing calcium magnesium acetate or calcium magnesium acetate/calcium magnesium propionate is provided. The process involves first providing a residual water plant sludge starting material containing calcium and magnesium compounds, and reacting this sludge with stoichiometric quantities of at least acetic acid and more preferably both acetic and propionic acids. The resultant reaction product can then be dried, ground and granulated to provide a dry, free flowing road deicer product.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is broadly concerned with an improved, low-cost 
process for the production of road deicers making use of water plant 
residuals typically containing calcium and magnesium compounds. More 
particularly, the invention pertains to such a method wherein the water 
plant residuals are initially concentrated and reacted with organic acids 
such as acetic acid to yield calcium acetate, calcium magnesium acetate 
(CMA) or a mixture of such acids such as acetic and propionic acids to 
give a calcium magnesium acetate/calcium magnesium propionate (CMA/CMP) 
deicer. 
2. Description of the Prior Art 
The importance of the effectiveness removal of snow and ice from roadways 
during winter can not be overemphasized in terms of reducing traffic 
accidents and fatalities. Approximately ten million metric tons of salts, 
principally sodium chloride and some calcium chloride, are used annually 
on roadways in the United States for this purpose. While these salts are 
inexpensive and effective deicers, there are numerous harmful effects 
attendant to their use. These include the accelerated corrosion of 
automobiles, degradation of structural and reinforcing steel, damage to 
roadside vegetation, and harm to drinking water supplies. Among the 
various alternative compounds heretofore proposed as substitute deicers, 
calcium magnesium acetate (CMA) has emerged as the leading candidate, 
based on its effectiveness in deicing and reducing corrosion rates and 
environmental effects. However, the cost of this product is approximately 
twenty times or more than that of rock salt. Therefore, the use of CMA by 
various local and state governmental agencies has been quite minimal owing 
to cost considerations. 
U.S. Pat. No. 4,855,071 describes a slurry process for preparing CMA 
wherein alkaline earth or alkali metal carboxylates are formed and the 
slurry is heated and distributed as a thin layer on substrate particles. 
The basic raw materials suggested for such CMA production are dolomitic 
lime and glacial acetic acid. Similarly, U.S. Pat. No. 4,636,467 suggests 
use of dolomitic lime with acetic acid for the production of CMA. 
A principal drawback to the widespread use of CMA and similar road deicers 
is the cost thereof. While dolomite is relatively inexpensive, the overall 
cost of synthetic road deicers could be substantially lowered if still 
less expensive starting materials could be employed. 
It is known that residuals (sludge) from water softening plants contain 
calcium and magnesium salts as well as inorganic and organic impurities. 
The amount of precipitates such as ferric or aluminum hydroxide in the 
sludge can be virtually negligible if the plant has primary and secondary 
clarifiers and lime sludge is withdrawn from the primary basin. The 
amounts of inorganic constituents such as clay and organic compounds will 
depend on the raw water characteristics. In most cases, these amounts of 
impurities will be quite small for ground waters; for surface waters, 
these impurities may also be present only in small quantities if 
presedimentation basins are used. In most cases, therefore, the main 
constituents of water plant sludge from softening plants will be calcium 
carbonate, calcium hydroxide and magnesium oxide, although other 
impurities will almost always be present in greater or lesser amounts. 
Of course, water plant sludges have almost no economic value and indeed can 
present a disposal problem in their own right. Therefore, any economically 
viable use of such sludges would represent a distinct breakthrough in the 
art. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems outlined above, and provides a 
new and inexpensive process for the production of road deicers containing 
calcium acetate, CMA or more preferably a mixture of CMA and CMP. The 
invention represents an improvement over the processes described in U.S. 
Pat. No. 5,324,442 which is incorporated by reference herein. 
Broadly speaking, the invention relates to the discovery that efficient 
road deicer products can be manufactured using water plant residual 
sludges as a starting material containing at least the desired calcium 
ions, and usually magnesium ions as well. This represents a distinct 
advantage over other processes in that a very inexpensive starting 
material is employed which would otherwise be a burden on the environment. 
In more detail, the process of the invention involves first providing a 
quantity of water plant residual sludge which contains at least about 50% 
by weight on a dry basis of a mixture having compounds selected from the 
group consisting of calcium carbonate, calcium hydroxide, magnesium 
hydroxide and mixtures thereof; preferably the mixture has respective 
amounts of at least calcium carbonate and calcium hydroxide (and usually 
magnesium hydroxide) therein. If the sludge contains less than the desired 
amount of calcium and magnesium compounds, supplemental additions may be 
made in the form of dolime, dolomite or other calcium and magnesium 
compounds. More preferably, the residual sludge should contain at least 
about 70% by weight of such a mixture on a dry basis. Ideally, the 
residual sludge should contain (on a dry basis) about 40% by weight 
calcium in the form of carbonate or hydroxide, and about 60% by weight 
magnesium in the form of magnesium hydroxide. However, with waters that 
contain only small amounts of magnesium, removal of magnesium from the 
water may not be practiced. This may result in water plant sludges 
containing negligible amounts of magnesium hydroxide. In this case, the 
sludge containing only calcium compounds can be used to produce a calcium 
acetate deicer. Generally, this residual sludge is provided by first 
taking a quantity of as is water plant sludge or slurry and separating a 
portion of the liquid fraction thereof to yield a residual sludge having 
the desired mixture of calcium and magnesium compounds therein. 
In the next step, the residual sludge is treated with at least acetic acid 
to form a reaction product containing at least calcium acetate. Again in 
preferred forms, residual sludge containing appreciable quantities of both 
calcium and magnesium compounds is treated with a mixture of acetic and 
propionic acids to form a dual salt reaction product containing both CMA 
and CMP. The acids are provided at stoichiometric ratios depending upon 
the content of CMA or CMA/CMP desired in the final road deicer product. 
The reaction can be carried out under a variety of conditions, but 
generally involves merely contacting the acid(s) and the residual sludge 
with mixing at ambient temperatures and pressures; the reaction may be 
allowed to proceed overnight with mixing and heat dissipation and with 
provision to allow release of carbon dioxide produced as a byproduct. 
The final step of the process involves recovering a road deicer product, 
preferably in dry form, which contains the desired calcium acetate, CMA or 
CMA/CMP salts. In most instances, the recovery would involve drying, 
grinding and granulating. In the case of a calcium acetate or CMA road 
deicer, the deicer product should have at least about 50% by weight 
calcium acetate or CMA therein, and more prerferably from about 70-100% by 
weight CMA. In the preferred dual salt product, the road deicer should 
have an excess by weight of CMP as compared with CMA, and more preferably 
from about 50-75% by weight CMP and from about 25-40% by weight of CMA. 
The final product is normally a dried and granulated flowable road deicer 
which can be applied in the conventional manner. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following example describes representative processes for the production 
of road deicer products using water plant residual sludges. It is to be 
understood that the example is provided by way of illustration only, and 
nothing therein should be taken as a limitation upon the overall scope of 
the invention.

EXAMPLE 
In this example, lime sludge from the primary sedimentation basin at the 
Manhattan, Kans. water plant was used. The as is lime slurry had a solids 
content of approximately 12% by weight, and contained calcium hydroxide, 
calcium carbonate, magnesium hydroxide, iron (III) hydroxide, aluminum 
hydroxide, clay particles and other impurities. It was first vacuum 
filtered at 25 inches of Hg to achieve a solids content of about 60% by 
weight. The resulting filter cake was reacted with organic acids over a 
period of several hours with mixing, as follows: A sludge-CMA product was 
produced by reacting the filter cake and acetic acid at a ratio of 0.48 
grams acetic acid per gram of filter cake. A sludge-CMP product was 
produced by reacting the filter cake and propionic acid at a rate of 0.59 
grams propionic acid per gram of filter cake. This reaction product had a 
calcium to magnesium molar ratio found by analysis to be 12:1. The 
comparative sludge-CMP/CMA product was obtained by mixing three parts CMP 
product with 2 parts CMA product to give the sludge-CMP/CMA product a 
CMP:CMA ratio of 3:2. 
These two products were tested to determine their relative ice melting 
capacities. This test was conducted in accordance with the "Test Method 
for Ice Melting of Solid Deicer Chemicals" appearing in Handbook of Test 
Methods for Evaluating Chemical Deicers, published as a part of the 
Strategic Highway Research Program, National Research Council, Washington, 
D.C. (1992). This test method is incorporated by reference herein. In 
summary, however, the test utilizes a sheet of ice of uniform thickness 
(1/8") frozen in a flat circular Plexiglass dish. After equilibrium to the 
desired temperature, deicer particles are broadcast-spread over the ice. 
At specified time intervals, generated brines are decanted by tilting the 
specimen to the perimeter of the dish, withdrawn via a syringe, measured 
for volume, and reintroduced to the test specimen so the melting process 
can continue. 
The following table sets forth the results of these ice melting tests. 
______________________________________ 
Ice Sludge-CMA Sludge-CMP/CMA 
Time Temperature 
(Brine Vol. (Brine Vol. 
(Min.) (.degree.C.) 
ml/mg Deicer) 
ml/mg Deicer) 
______________________________________ 
0 -7 0.00 0.00 
15 -7 2.24 2.27 
30 -7 2.98 3.06 
45 -7 3.51 3.54 
60 -7 3.78 3.80 
0 -11.5 0.00 0.00 
15 -11.5 0.83 1.16 
30 -11.5 1.45 1.70 
45 -11.5 1.87 1.83 
60 -11.5 2.18 1.96 
0 -15 0.00 0.00 
15 -15 0.00 0.00 
30 -15 .sup. 0.00.sup.1 
0.52 
45 -15 .sup. 0.00.sup.1 
0.84 
60 -15 0.52 0.89 
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.sup.1 Some melting but negligible brine volume. 
As can be seen, while both the sludge-CMA and sludge-CMP/CMA products can 
be used for deicing, the dual salt mixture is more effective, especially 
at -15.degree. C.