Chemical fixation of desulfurization residues

Chemical fixation of industrial desulfurization residues by forming an intimate admixture of the desulfurization residue, such as a flue gas desulfurization sludge, with an alkaline calcination stack dust, such as a cement kiln dust, in the presence of sufficient water to provide a mass of workable consistency, adjusting the pH of the alkaline mass with acid, such as sulfuric or phosphoric acid, to a value of below about 7, e.g. in the acid range of from below about pH 7 down to about pH 5, and drying the pH adjusted mass to constant weight, preferably after forming the pH adjusted mass into a selectively shaped article, whereby to produce an environmentally stable and leach-resistant hardened solid, cement-like fixed product.

The present invention relates to the chemical fixation of industrial 
residues, and more particularly to the chemical fixation of industrial 
desulfurization residues by combining such residues with alkaline 
calcination stack dust and water, acidifying to an acid pH, and drying the 
resulting mass to form an optionally selectively shaped, environmentally 
stable and leach-resistant hardened, solid, fixed product. 
Industrial desulfurization residues or sludges are produced on a wide scale 
as a result of present day auxiliary scrubbing or absorbing processes 
designed to remove pollutants such as sulfur constituents from air and 
water discharge effluents of basic manufacturing operations, generally 
pursuant to air and water pollution control governmental regulation. While 
these auxiliary scrubbing or absorbing processes may be said to effect the 
removal of such pollutants to prevent their direct escape into the 
surrounding environment, the removed pollutants, which are usually in the 
form of concentrated solid material reaction products constituting 
industrial sludges or residues, must still be disposed of in a manner 
which minimizes the chance of immediate indirect escape thereof into the 
surrounding environment if the overall objectives of such governmental 
regulation are to be attained. Indeed, improper disposal of such sludges 
or residues may well undermine these objectives because of the ultimate 
adverse effect on the environment occasioned thereby. Many of these 
sludges or residues are so sufficiently rich in these pollutants, 
consonant with the demands for economically efficient removal procedures, 
that they may be said to constitute hazardous concentration waste 
materials necessitating extra precautions in regard to their handling and 
disposal. 
Aside from ocean dumping, which can only be safely used in those special 
cases where there is little or no adverse environmental impact and which 
as a practical matter is limited in application to coastal area 
manufacturing facilities, land disposal is primarily the manner by which 
industry rids itself of these hazardous and non-hazardous waste materials. 
Land disposal for instance includes landfilling, lagooning or ponding, 
abandoned mine filling, and like methods. Of course, any method which 
contemplates recovery of these waste materials for reuse in some form is 
to be preferred over disposal per se, depending upon the pertinent 
practical economical considerations, since not only is the basic 
environmental impact problem essentially obviated thereby but also a new 
product is created which would not otherwise have existed. 
A natural concomitant of land disposal is that the pollutants and other 
waste materials in the residue or sludge will eventually find their way 
back to the surrounding environment. Hence, especially regarding hazardous 
pollutant type waste materials or residues, i.e. those in which the 
pollutant constituents are present in high concentrations, the rate of 
dissipation or migration of the pollutant from the point of landfill or 
other disposal to the surrounding area must not be greater in time or 
quantity than an acceptably safe rate for the particular environment in 
question. Many factors affect such rate including for instance the 
concentration and form of the particular pollutant, its degree of mobility 
in the waste residue, and the influence thereon of physical, chemical 
and/or biological agencies and mechanisms in the adjacent areas. In view 
of these often imponderable factors, steps must be affirmatively taken to 
assure that the waste residues in question, particularly in the case of 
hazardous pollutants such as concentrated desulfurization residues, are 
rendered more or less innocuous prior to land disposal. Only in this way 
can avoidance of adverse environmental impact be purposefully attained. 
One proposal has been to provide a physical liner between the waste residue 
and the surrounding land. In addition to the labor and equipment costs for 
such liner, and possible supplemental adverse environmental impact due to 
the precursor production of the liner itself and attendant pollution 
generated as a consequence of such production, in practice the propensity 
for occurence of breaks in the liner and in turn leakage of the contents, 
render such proposal undesirable. Such breaks are not limited to 
mechanical punctures or rips which might occur during installation or as a 
result of incorrect disposition of the liner so as to render it vulnerable 
to local excessive stresses in use, but include as well those due to 
adverse reactions between potentially active constituents in the residue 
contents and in the liner itself. 
A further proposal involves encapsulation by chemical fixation, whereby 
similarly to the above physical liner proposal, a chemically produced 
liner encapsulates portions of the waste residue. The same disadvantages 
as noted for physical liners are generally applicable in the case of these 
encapsulating chemical liners as well. 
Other proposals involve basic chemical fixation of the waste residue with 
chemical reagents so that the dissipation or migration of the pollutant 
constituents from the residue into the surrounding area will be prevented 
or acceptably retarded. Such chemical reagents may be organic or inorganic 
substances, all of which are generally designed to change the chemical and 
physical properties of the waste residue in some way, in order to reduce 
or eliminate mobility or migration of the pollutant constituents from the 
residue into the environment. Usually, such fixation is accompanied by a 
decrease in the surface area to volume ratio of the residue by the 
solidification of the residue mass, which in turn results in an arresting 
of pollutant constituent migration in accordance with mass transport 
phenomena. The latter normally presupposes a leaching liquid diffusion 
mechanism for the residue which is surface area dependent. Important to 
such chemical fixation is the extent, if any, or rate of leaching of such 
pollutant constituents from the chemically fixed residue as a function of 
time in the particular land disposal environment. The relative cost of 
such chemical reagents, the relative complexity of the chemical fixation 
process manipulations and equipment, and the relative degree of attaining 
leach-resistance in the chemically fixed residue, are factors which 
generally have rendered these prior proposals unattractive, due to their 
performance shortcomings from an industrial scale, practical, economical, 
and efficiency point of view. 
It is among the objects of the present invention to overcome the various 
foregoing disadvantages and drawbacks, and to provide a simple, economical 
and efficient method for chemical fixation of hazardous and non-hazardous 
industrial waste residues, and especially industrial desulfurization 
residues, and to produce thereby an environmentally stable and 
leach-resistant hardened solid fixed product. 
It is among the additional objects of the invention to provide such a 
method employing a minimum of steps of corresponding minimum duration, a 
choice of readily available and comparatively inexpensive chemical 
reagents including waste alkaline calcination stack dust and even spent 
acid liquor, and involving uncomplicated mixing, shaping and drying 
containers and economical energy requirements. 
It is among the further objects of the invention to provide environmentally 
stable and leach-resistant hardened solid cement-like fixed composite 
products having serviceable mechanical structural rigidity and selective 
article shape, produced by the foregoing method. 
Other and still further objects of the invention will become apparent from 
a study of the within specification and accompanying examples. 
It has been found in accordance with the present invention that a simple, 
economical and efficient method may now be provided for chemical fixation 
of hazardous and non-hazardous industrial waste residues of the 
desulfurization type, while simultaneously producing environmentally 
stable and leach-resistant, optionally shapable, solid fixed products. 
In accordance with one embodiment of the invention, a method is provided 
for chemical fixation of industrial desulfurization residues which 
comprises intimately admixing the industrial desulfurization residue with 
alkaline calcination stack dust in the presence of sufficient water to 
provide a mass of workable consistency, adjusting the pH of the alkaline 
mass with acid to a value of below about 7, and drying the pH adjusted 
mass to constant weight. In this manner, an environmentally stable and 
leach-resistant hardened, solid, fixed product will be produced. 
The stack dust is preferably a cement kiln dust or similar argillaceous and 
calcareous material reagent, which is generally alkaline reacting and 
contains constituents such as calcium, silicon, aluminum, iron, magnesium, 
sodium, potassium and associated constituents found in cement-making and 
similar stack dust, for example in the form of oxides and salts. For 
instance, calcium oxide (lime), calcium carbonate (limestone), calcium 
sulfate (gypsum), and the like type constituents may be present, all of 
which might occur as waste products in the stack dust generated during 
industrial calcination of alkaline materials as in cement making. 
The industrial waste residue may be any such residue or sludge as for 
example is produced in the after-removal of pollutants such as sulfur 
constituents from the effluent or flue gas of basic manufacturing 
processes. These manufacturing processes often generate sulfur oxides, 
e.g. SO.sub.x and especially sulfur dioxide, which must be removed from 
the reaction effluent by scrubbing or absorbing techniques before venting 
to the atmosphere in order to avoid environmental pollution. Such 
manufacturing process may be a fossil fuel consumption reaction, e.g. the 
burning of oil or coal for producing power or for producing heat for an 
associated reaction, or an effluent generating reaction, e.g. a cement 
making or ore roasting reaction in which sulfur constituents are liberated 
or generated as sulfur oxides, especially sulfur dioxide, during the 
calcination or roasting. Also, the source of the sulfur constitutents 
might include a combination of both types of reactions, i.e. sulfur in the 
fossil fuel used to produce heat for a cement type calcination combined 
with sulfur present in the material being calcined. All of such sources 
may be considered sulfur constituent containing flue gas sources herein. 
In any case, such sulfur constituent containing effluents or flue gases are 
subject to after-treatment for desulfurization. For instance, the gaseous 
effluent from such a fossil fuel heated calcination or cement making 
process can be contacted with an aqueous lime slurry in a rotary kiln to 
produce calcium-sulfur salts, especially calcium sulfite by scrubbing or 
absorbing techniques. Even where such after-removal treatment is most 
efficient, the concentrated content of sulfur constitutents in the slurry 
must still be disposed of and in such high concentration can represent a 
hazardous industrial waste residue or sludge. Any such industrial 
desulfurization residue or sludge is potentially reactive since the 
absorbed sulfur oxides, especially sulfur oxide, are often unstable and 
can migrate from the mass and enter the environment. This tendency is 
advantageously overcome by the present invention. 
Generally, any inorganic or mineral acid may be used for adjusting the pH 
of the mixed wet alkaline mass of desulfurization residue and stack dust 
to neutralize and acidify such mass. As used herein, inorganic acid and 
mineral acid are considered interchangeable terms since any acid 
contributing hydrogen ions to acidify the alkaline content of the wet mass 
will effectively serve the purpose in question. The nature of the acid 
anion is not essentially significant so long as operative pH adjustment of 
the alkaline wet mass is able to be accomplished to change the same to the 
acid range (below 7). The pH of the final mass must be in the acid range 
as this is necessary to obtain a solid non-leaching composite product. 
Accordingly, sulfuric, phosphoric, hydrochloric, nitric and/or like 
inorganic or mineral acids are usable herein. Sulfuric acid and phosphoric 
acid have been found to be particularly advantageous, not only from a cost 
and source point of view but also because the corresponding anions thereof 
react with calcium constituents present to form essentially insoluble, 
inert and leach-resistant substances in the final product. Although any pH 
below about 7 will suffice for the purposes of the present invention, 
according to one feature thereof such pH is preferably adjusted to any 
value in the acid range of from below about 7 down to about 5, e.g. from 
about 6.9 to about 5.5 or 5 pH, since lowering the pH further merely 
wastes acid needlessly without apparent additional beneficial effect. In 
this regard, any acid concentration may be employed, either concentrated, 
e.g. 10 Normal, or dilute, e.g. 3 to 10% by weight concentration in water, 
and from any original or waste source. An advantageous source is a spent 
industrial waste acid liquor such as a spent pickle liquor of about 3-10% 
concentration. These waste acid liquors are obtainable from various 
industrial processes such as the pickling of steel and may be 
preconcentrated by heating, if desired. In any case, the amount of acid 
required is that amount needed to react with the acid reactive 
constituents of the desulfurization waste residue and the stack dust to 
produce a chemical fixation product of acid pH. 
The drying step is employed to dry the pH adjusted acid mass to constant 
weight. This substantially completes and finalizes the chemical fixation. 
Water removal in this regard can be effected by merely allowing the mass 
to dry in ambient air. Depending on the nature of the mass and its water 
content, room temperature or ambient air drying for about 1 to 2 weeks is 
generally sufficient to achieve water loss to constant weight and a 
product attaining cement-like hardness. Alternatively, however, 
accelerated drying can be effected by heating the mass, e.g. at about 
100.degree. C. While ambient air drying is simple and avoids the cost of 
applying heat to the system in this step, the use of applied heat is 
advantageous in those cases where rapid drying for a considerably reduced 
period is a more important consideration. 
Preferably, the pH adjusted mass is formed into a selectively shaped 
plastic mass article or object prior to the the drying step. In this way, 
the dried product will not only be an environmentally stable and 
leach-resistant hardened or cured solid composite fixed, cement-like 
product, but advantageously also will constitute a selectively shaped 
article or object usable as a per se serviceable mechanically rigid 
structural element as desired. Since the pH adjusted wet plastic mass is 
easily shapable to conform to the contour of any suitable mold or 
container, e.g. that used for mixing and pH adjustment, this intervening 
optional step can be conveniently carried out with a mimimum of effort. 
The drying step can be effected, with the shaped mass simply situated in 
the open mold or container of any desired shape, either by way of ambient 
air drying or by applying drying heat to or through the container walls. 
The weight ratio of the desulfurization residue and stack dust to one 
another may vary over a wide range depending upon the nature of the waste 
material. For instance, the desulfirization waste residue and stack dust 
may be present in the weight ratio of about 1 to about 3-4:1, i.e. in the 
weight ratio range of between about 80:20 to about 50:50. Convenient 
ratios of about 50:50 are particularly desirable since this enables both 
waste products to be equally converted into reusable form and thus 
eliminates their respective disposal problems. However, ratios toward the 
higher end of the range, i.e. 80:20, are preferred where the 
desulfirization sludge constitutes the main disposal problem. 
Generally, the content of stack dust is believed to contribute calcium 
oxide and silicate to the mass to improve the hardening or curing of the 
product and enhance its structural integrity. This inherently also aids in 
providing a leach-resistant cement-like composite product. Thus, apart 
from the foregoing, such stack dust may be employed in lesser amounts in 
terms of the weight ratio range in question where a harder product is 
desired and the industrial waste residue itself contains a significant 
content of these hardenable constituents such as calcium oxide, e.g. in 
calcium hydroxide form in the aqueous lime slurry used for desulfurization 
of flue gas or the like. 
In this regard, while the various mechanisms by which the advantageous 
results of the present invention are obtained are not fully understood, by 
way of possible non-limiting explanation, one plausible theory may be that 
the sulfur constituents in the desulfurization waste residue or sludge are 
at least in part in the form of calcium sulfite, which upon acidification 
will be converted to calcium sulfate. Any calcium sulfite contributed by 
the constituents in the stack dust would be similarly converted. 
Furthermore, other calcium constituents in the desulfurization waste 
residue and/or stack dust such as calcium oxide and silicates are believed 
to be converted under the wet conditions present to hardenable products. 
Upon drying of the composite intimately mixed mass, and eventual curing or 
hardening thereof, these hardenable products will harden to final 
cement-like solid products. 
Thus, it is probable that the desulfurization waste residue contributes 
mainly any calcium sulfate precursor constituents while the stack dust 
contributes mainly the silicate and other cement-like precursor 
constituents. Accordingly, the particular weight ratio of the industrial 
desulfurization waste residue or sludge to the alkaline calcination stack 
dust in the final analysis will depend on the nature and extent of the 
calcium, silicon and sulfur constituents, especially oxides and silicates, 
in the waste residue and the complemental nature and extent of these types 
of hardenable constituents in the stack dust. 
The loaded industrial desulfurization waste residue or flue gas sludge, 
e.g. which results from scrubbing sulfur oxide type liquid and gaseous 
effluent in the usual manner, is preferably intimately physically mixed 
with the alkaline calcination stack dust, e.g. cement kiln dust, in the 
presence of sufficent overall water content to provide a solids mass of 
workable consistency, whereupon adjusting the pH with acid to neutralize 
the alkaline reacting mass and change the same to acid reacting condition, 
will provide a hardenable chemical fixation mass. 
It will be noted that where sulfuric acid is used for the pH adjustment, 
calcium sulfate will be produced as a by-product from the sulfate anion of 
the acid used and calcium present in the mixed mass. This generally 
insoluble gypsum type salt will enhance the cement-like nature of the 
resulting composite mass system which resists leaching. In the case of 
phosphoric acid, calcium phosphate will be similarly produced as an 
insoluble by-product, which will also enhance resistance to leaching. 
Accordingly, these acids are preferred, although other acids are still 
usable, such as hydrochloric and nitric since the overall quantity of acid 
anion contributed salt in the final product is comparatively small whereas 
sufficient insoluble and cement-like imparting salts and similar 
argillaceous and calcareous constituents will be overwhelmingly otherwise 
provided to satisfy the needs of the calcium and other precursors for the 
calcium sulfate, oxide and silicate and similar cement-like ingredients of 
the final composite cured or hardened product. 
In this latter regard, it will be realized of course, that the industrial 
desulfurization waste residue or sludge and the industrial alkaline 
calcination stack dust generally will contain other mineral ingredients 
similarly beneficial to the hardness, structural rigidity and 
leach-resistant qualities desired, such as silicon, aluminum, magnesium, 
iron, etc., oxides, and the like, and various impurities normally found in 
cement clinker material or the like, and in cement calcination stack dust, 
fly ash or the like, as the case may be. Other constituents such as sodium 
and potassium oxides will often also be present. Additionally, impurities 
such as cadmium may find their way into the desulfurization sludge from 
the fossil fuel used in the basic manufacturing operation. 
The amount of water present in the system may vary over a wide range, 
depending upon the amount needed to reduce the viscosity of and provide a 
workable consistency for the particular weight ratio mixture of the waste 
residue and stack dust, considering the normally fine-grained or silt-like 
quality of the raw waste residue and the normally powder-like quality of 
the raw stack dust. This water may be contributed at least in part by the 
moisture content of the starting waste residue or sludge, e.g. an aqueous 
lime slurry sludge containing sulfur oxides such as sulfur dioxide in 
absorbed condition in high concentration therein, and/or by the aqueous 
content or extent of diluteness of the treating acid used for pH 
adjustment. Generally, the overall water content of the system may amount 
to between about 10 to 95% by weight based on the combined weight of the 
solids content of the desulfurization waste residue and stack dust. 
Where the waste residue is a comparatively dry or moisture-deficient 
material and/or the acid used is in concentrated form, e.g. 10 Normal, the 
water content will be provided primarily by addition of extraneous water 
in the necessary amount. Such amount will be that needed to achieve a 
workable consistency for readily mixing and blending waste residue, stack 
dust and acid components, to effect adjustment of the pH of the composite 
mass to within the acid range, i.e. below about 7, and to permit the pH 
adjusted mass to be selectively shaped or molded, or to take the shape of 
its container, optionally prior to the drying step. Naturally, the amount 
of water will be desirably no more than that required for the above 
purposes as excess water will add to the drying time, and in the case of 
accelerated drying by applying heat to the system will correspondingly 
increase the amount of energy needed to provide such heat. The amount of 
water added thus regulates the percentage of total solids in the final 
composite mass and concomitantly the extent of drying needed to achieve 
constant weight. 
The various components can be intermixed and contacted in sequence or in 
alternate increments as is convenient. Desirably, the stack dust and water 
components are added in alternate successive increments or batches to the 
waste residue or sludge component to facilitate uniformity of mixing, 
whereupon the acid component is added in successive increments to the 
resulting wet plastic mass to effect neutralization and consonant shift to 
the acid pH range. Appropriate agitation by stirring or the like is 
undertaken to achieve concurrent mixing of the components during these 
addition steps. Where the waste residue or sludge component contains 
sufficient water component at the start, the stack dust component need 
merely be added, preferably in successive increments, with attendant 
agitation. Any water not provided for in the waste residue or sludge 
component, or by independent addition, is concomitantly compensated for by 
including the same with or in the acid component. For this purpose, the 
acid may be used in dilute aqueous form or more concentrated acid may be 
diluted with any necessary water content prior to acid addition. 
In accordance with one embodiment, the pH is adjusted by adding sulfuric 
acid in an amount of between about 12 to 20% by weight H.sub.2 SO.sub.4 
based on the combined weight of the waste residue and stack dust, or by 
adding phosphoric acid in an amount of between about 2 to 7% by weight 
H.sub.3 PO.sub.4 based on the combined weight of said residue and dust, 
for example as the case may be with the waste residue and stack dust being 
present in the weight ratio of about 50:50, i.e. in about equal amounts, 
and correspondingly for example with the water being present in an amount 
of between about 10 to 95% by weight based on the combined weight of said 
residue and dust. 
A further feature of the present invention concerns the fact that during 
the pH adjustment, whether by acid addition in sequence after admixing the 
waste residue with the stack dust and water or in alternate successive 
increments with the addition of the other components, a considerable 
amount of heat is generated. To permit such heat to be distributed and 
dissipated, such agitation is desirable as well as incremental acid 
addition. Nevertheless, the neutralization heat so generated can be 
advantageously recovered by appropriate means for use in other processes 
as the artisan will appreciate. In terms of industrial scale operations, 
such generated heat, e.g. in closed pH adjustment systems and apparatus, 
can be recovered for use in the subsequent drying of the acidified 
chemical fixation product itself. Since the neutralization which occurs 
during the pH adjustment also causes carbon dioxide liberation, i.e. 
mainly from limestone constituents present, this gas may be recovered as 
well, especially in industrial scale operations, as an economic 
by-product. 
Advantageously, therefore, the present invention provides for the chemical 
fixation of hazardous and non-hazardous raw industrial waste residues, 
including flue gas desulfurization sludges in particular, to produce 
environmentally stable and leach-resistant solid fixed composite products 
readily adapted for safe land disposal, e.g. in landfills, without adverse 
environmental impact or pollution, or for use per se as structural 
materials or articles of optionally selective shape and practical 
mechanical rigidity. This is accomplished with the concurrent disposal 
along with the desulfurization waste residue, of large corresponding 
quantities of other waste materials representing the other components of 
the system, including stack dust, especially raw cement kiln dust 
(approximately 4.5 million tons of which are generated annually in the 
United States), and various industrial waste acids of many types and 
concentrations. Thus, not only is an environmentally beneficial invention 
involved, but the method can be practiced entirely with nuisance type and 
often difficultly disposable waste materials which constitute inexpensive 
starting materials in readily available abundant supply, while employing a 
minimum of steps and manipulations, as well as inexpensive and simple 
reaction or treatment equipment of almost negligible energy consumption, 
and in a manner free from production of by-products except carbon dioxide 
liberated during pH adjustment and water lost during drying of the 
chemical fixation composite product beneficially produced.

The following typical examples of the concept of the present invention are 
set forth by way of illustration and not limitation, all parts herein 
being parts by weight unless otherwise apparent or specifically indicated. 
EXAMPLE 1 
Eastern Coal Lime Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.2 SO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 24.7 129 
% by Wt. 28.4 28.4 7.0 36.2 
______________________________________ 
To 100g. alkaline SO.sub.2 scrubber sludge (about 40% total solids) 
obtained from eastern coal lime process flue gas desulfurization, are 
alternately added 100g. alkaline calcination stack dust in the form of 
cement kiln dust in approximately 10g. increments and 93 ml. H.sub.2 O in 
approximately 9 ml. increments, while stirring the wet mass. 
To the resulting intimate wet solids mixture of workable consistency are 
then added, with continued stirring, 50 ml. of 10 N H.sub.2 SO.sub.4 in 
approximately 5 ml. increments, allowing effervescence (CO.sub.2) to 
subside between the acid increment additions. Heat is generated during the 
acid addition, indicating that the initial alkaline reacting solids 
mixture is being neutralized. The amount of acid added is sufficient to 
adjust the pH to the acid range. The pH of the final smooth, viscous 
chemical fixation mixture is approximately 6.7. 
The 10 N H.sub.2 SO.sub.4 is prepared by adding 278 ml. of 96% conc. 
H.sub.2 SO.sub.4 (about 1762 g/l) to water, and diluting to 1000 ml. to 
provide a final concentration of approximately 493 g/l H.sub.2 SO.sub.4. 
The 50 ml. of acid used include 24.7 g. H.sub.2 SO.sub.4 (gram equivalent 
weight = 49 g) and 36 g. H.sub.2 O. Based on the combined weight of the 
sludge and stack dust solids components, the amount of water component 
added is about 64.5% by weight and the amount of dibasic acid component 
added is about 12.3% by weight. 
The chemical fixation mixture is allowed to dry in ambient air at room 
temperature to constant weight. This step takes approximately 22 days to 
reach equilibrium, and results in a cumulative weight loss of about 51.5%, 
although the product attains hardness after only 7 days of drying. The 
mixture upon drying will be molded in the shape of the container in which 
it is held. The so-produced chemically fixed composite article constitutes 
an environmentally stable and leach resistant hardened solid cement-like 
product having serviceable mechanical structural rigidity and a selective 
article shape. 
A 100 g. sample of the air dried solid product, produced according to the 
above procedure, is leached after drying for 33 days, by contact at room 
temperature with 500 ml. distilled water. The sample is placed in a 
plastic container and the water is poured down the container wall to avoid 
disturbance of the sample. After reaching equilibrium as determined by 
stabilization of pH and maximum specific conductance, the leachate is 
discarded and successive releaching by contact in each instance with a 
fresh 500 ml. quantity of distilled water is undertaken. No. 
disintegration occurs on leaching and the leachate progressively exhibits 
an environmentally acceptable low specific conductance (in micromhos/cm, 
25.degree. C.) as shown in Table 1, indicating minimal loss of pollutant 
constituent residue into the leachate, after a total of 33 days of drying 
and thereafter 15 days of cumulative leaching. 
EXAMPLE 2 
Eastern Coal Lime Sludge 
______________________________________ 
Composition: 
Sludge Slack Dust H.sub.3 PO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 9.9 50 
% by wt. 38.5 38.5 3.8 19.2 
______________________________________ 
The procedure of Example 1 is repeated but in this case adding 20 ml. 
H.sub.2 O in approximately 2 ml. increments alternately with the stack 
dust 10 g. increments to the same type sludge as in Example 1 and then 
adding 37 ml. of 10 N H.sub.3 PO.sub.4 in approximately 5 ml. increments 
to the resulting solids mixture, while allowing effervescence to subside 
between the acid increment additions. The same essential results are 
obtained. The pH of the final smooth, viscous chemical fixation mixture is 
approximately 6.4. 
The 10 N H.sub.3 PO.sub.4 is prepared by adding 185 ml. of 85% conc. 
H.sub.3 PO.sub.4 (about 1445 g/l) to water, and diluting to 1000 ml. to 
provide a final concentration of approximately 268 g/l H.sub.3 PO.sub.4. 
The 37 ml. of acid used include 9.9 g. H.sub.3 PO.sub.4 (gram equivalent 
weight = 32.7 g.) and 30 g. H.sub.2 O. Based on the combined weight of the 
sludge and stack dust components, the amount of water component added is 
about 25% by weight and the amount of tribasic acid component added is 
about 5% by weight. 
The drying step takes approximately 24 days to reach equilibrium and 
results in a cumulative weight loss of about 38.8%, although the product 
attains hardness after only 6 days of drying. The drying step provides a 
chemically fixed composite article of the same type as described in 
Example 1. 
Upon leaching a 100 g. sample of the air dried solid product, produced 
according to the above procedure, after drying for 31 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 31 days of drying and thereafter 33 days 
of cumulative leaching. 
EXAMPLE 3 
Eastern Coal Limestone Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.2 SO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 37 94 
% by Wt. 30.2 30.2 11.2 28.4 
______________________________________ 
The procedure of Example 1 is repeated, but in this case starting with 100 
g. alkaline SO.sub.2 scrubber sludge (about 23% total solids) obtained 
from eastern coal limestone process flue gas desulfurization, adding 40 
ml. H.sub.2 O in approximately 4 ml. increments alternately with the stack 
dust 10 g. increments, and then adding 75 ml. of 10 N H.sub.2 SO.sub.4 in 
approximately 8 ml. increments to the resulting solids mixture, while 
allowing effervescence to subside between the acid increment additions. 
The same essential results are obtained. The pH of the final smooth, 
viscous chemical fixation mixture is approximately 6.6. 
The 75 ml. of acid used include 37 g. H.sub.2 SO.sub.4 and 54 g. H.sub.2 0. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 47% by weight and the amount of 
dibasic acid component added is about 18.5% by weight. 
The drying step takes approximately 21 days to reach equilibrium and 
results in a cumulative weight loss of about 54.9%, although the product 
attains hardness after only 7 days of drying. The drying step provides a 
chemically fixed composite article of the same type as described in 
Example 1. 
Upon leaching a 100 g. sample of the air dried solid product, produced 
according to the above procedure, after drying for 22 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 22 days of drying and thereafter 32 days 
of cumulative leaching. 
EXAMPLE 4 
Eastern Coal Limestone Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.3 PO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 7.5 23 
% by Wt. 43.4 43.4 3.3 9.9 
______________________________________ 
The procedure of Example 2 is repeated, but in this case starting with 100 
g alkaline SO.sub.2 scrubber sludge obtained from eastern coal limestone 
process flue gas desulfurization as in Example 3, and alternately adding 
28 ml. of 10 N H.sub.3 PO.sub.4 in approximately 3 ml. increments with the 
stack dust 10 g increments, with appropriate stirring of the resulting 
incremental solids mixture and while allowing effervescence to subside 
between the acid increment alternate additions. The same essential results 
are obtained even though no water is separately added to the system. The 
pH of the final smooth, viscous chemical fixation mixture is approximately 
6.6. 
The 28 ml. of acid used include 7.5 g H.sub.3 PO.sub.4 and 23 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 11.5% by weight and the amount of 
tribasic acid component added is about 3.75% by weight. 
The drying step takes approximately 27 days to reach equilibrium and 
results in a cumulative weight loss of about 46.8%, although the product 
attains hardness after only 7 days of drying. The drying step provides a 
chemically fixed composite product of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 31 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 31 days of drying and thereafter 43 days 
of cumulative leaching. 
EXAMPLE 5 
Eastern Coal Double Alkali Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.2 SO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 39 187 
% by Wt. 23.5 23.5 9.2 43.8 
______________________________________ 
The procedure of Example 1 is repeated, but in this case starting with 100 
g SO.sub.2 scrubber sludge (about 37% total solids) obtained from eastern 
coal double alkali process flue gas desulfurization, adding 130 ml. 
H.sub.2 O in approximately 13 ml. increments alternately with the stack 
dust 10 g increments, and then adding 79 ml. of 10 N H.sub.2 SO.sub.4 in 
approximately 8 ml. increments to the resulting solids mixture, while 
allowing effervescence to subside between the acid increment additions. 
The same essential results are obtained. The pH of the final smooth, 
viscous chemical fixation mixture is approximately 6.8. 
The 79 ml. of acid used include 39 g H.sub.2 SO.sub.4 and 57 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 93.5% by weight and the amount of 
dibasic acid component added is about 19.5% by weight. 
The drying step takes approximately 20 days to reach equilibrium and 
results in a cumulative weight loss of about 55.4%, although the product 
attains hardness after only 7 days of drying. The drying step provides a 
chemically fixed composite article of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 21 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 21 days of drying and thereafter 29 days 
of cumulative leaching. 
EXAMPLE 6 
Western Coal Limestone Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.2 SO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 32.6 183 
% by Wt. 24.1 24.1 7.8 44.0 
______________________________________ 
The procedure of Example 1 is repeated, but in this case starting with 100 
g alkaline SO.sub.2 scrubber sludge (about 14% total solids) obtained from 
western coal limestone process flue gas desulfurization, adding 135 ml. 
H.sub.2 O in approximately 15 ml. increments alternately with the stack 
dust 10 g increments, and then adding 66 ml. of 10 N H.sub.2 SO.sub.4 in 
approximately 5 ml. increments to the resulting solids mixture, while 
allowing effervescence to subside between the acid increment additions. 
The same essential results are obtained. The pH of the final smooth, 
viscous chemical fixation mixture is approximately 6.7 
The 66 ml. of acid used include 32.6 g H.sub.2 SO.sub.4 and 48 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 91.5% by weight and the amount of 
dibasic acid component added is about 16.3% by weight. 
The drying step takes approximately 28 days to reach equilibrium and 
results in a cumulative weight loss of about 52.9%, although the product 
attains hardness after only 11 days of drying. The drying step provides a 
chemically fixed composite article of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 32 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 32 days of drying and thereafter 13 days 
of cumulative leaching. 
EXAMPLE 7 
Western Coal Limestone Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.3 PO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 13.9 92 
% by Wt. 32.8 32.8 4.6 29.8 
______________________________________ 
The procedure of Example 2 is repeated, but in this case starting with 100 
g alkaline SO.sub.2 scrubber sludge obtained from western coal limestone 
process flue gas desulfurization as in Example 6, adding 50 ml. H.sub.2 O 
in approximately 5 ml. increments alternately with the stack dust 10 g 
increments, and then adding 52 ml. of 10 N H.sub.3 PO.sub.3 in 
approximately 5 ml. increments to the resulting solids mixture, while 
allowing effervescence to subside between the acid increment additions. 
The same essential results are obtained. The pH of the final smooth, 
viscous chemical fixation mixture is approximately 6.7. 
The 52 ml. of acid used include 13.9 g H.sub.3 PO.sub.4 and 42 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 46% by weight and the amount of 
tribasic acid component added is about 7% by weight. 
The drying step takes approximately 26 days to reach equilibrium and 
results in a cumulative weight loss of about 40.3%, although the product 
attains hardness after only 7 days of drying. The drying step provides a 
chemically fixed composite product of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 46 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 46 days of drying and thereafter 38 days 
of cumulative leaching. 
EXAMPLE 8 
Western Coal Double Alkali Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.2 SO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 28.1 136 
% by Wt. 27.5 27.5 7.7 37.3 
______________________________________ 
The procedure of Example 1 is repeated, but in this case starting with 100 
g alkaline SO.sub.2 scrubber sludge (about 44% total solids) obtained from 
western coal double alkali process flue gas desulfurization, adding 95 ml. 
H.sub.2 O in approximately 10 ml. increments alternately with the stack 
dust 10 g increments, and then adding 57 ml. of 10 N H.sub.2 SO.sub.4 in 
approximately 5 ml. increments to the resulting solids mixture, while 
allowing effervescence to subside between the acid increment additions. 
The same essential results are obtained. The pH of the final smooth, 
viscous chemical fixation mixture is approximately 6.7. 
The 57 ml. of acid used include 28.1 g H.sub.2 SO.sub.4 and 41 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 68% by weight and the amount of 
dibasic acid component added is about 14% by weight. 
The drying step takes approximately 22 days to reach equilibrium and 
results in a cumulative weight loss of about 60.3%, although the product 
attains hardness after only 11 days of drying. The drying step provides a 
chemically fixed composite article of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 32 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 32 days of drying and thereafter 15 days 
of cumulative leaching. 
EXAMPLE 9 
Western Coal Double Alkali Sludge 
______________________________________ 
Composition: 
Sludge Stack Dust H.sub.3 PO.sub.4 
H.sub.2 O 
______________________________________ 
Grams 100 100 4.6 14 
% by Wt. 45.8 45.8 2.1 6.3 
______________________________________ 
The procedure of Example 2 is repeated, but in this case starting with 100 
g alkaline SO.sub.2 scrubber sludge obtained from western coal double 
alkali process flue gas desulfurization as in Example 8, and alternately 
adding 17 ml. of 10 N H.sub.3 PO.sub.4 in approximately 2 ml. increments 
with the stack dust 10 g as in Example 4, with appropriate stirring of the 
resulting incremental solids mixture and while allowing effervescence to 
subside between the acid increment alternate additions. The same essential 
results are obtained even though no water is separately added to the 
system. The pH of the final smooth, viscous chemical fixation mixture is 
approximately 6.4. 
The 17 ml. of acid used include 4.6 g H.sub.3 PO.sub.4 and 14 g H.sub.2 O. 
Based on the combined weight of the sludge and stack dust components, the 
amount of water component added is about 7% by weight and the amount of 
tribasic acid component added is about 2.3% by weight. 
The drying step takes approximately 22 days to reach equilibrium and 
results in a cumulative weight loss of about 43.1%, although the product 
attains hardness after only 13 days of drying. The drying step provides a 
chemically fixed composite product of the same type as described in 
Example 1. 
Upon leaching a 100 g sample of the air dried solid product, produced 
according to the above procedure, after drying for 26 days, the same 
beneficial results are obtained following the leaching procedure of 
Example 1. No disintegration occurs on leaching and the leachate has an 
environmentally acceptable similarly low specific conductance, as shown in 
Table 1 below, after a total of 26 days of drying and thereafter 45 days 
of cumulative leaching. 
TABLE 1 
______________________________________ 
H.sub.2 O LEACH EQUILIBRIUM 
______________________________________ 
Leach Duration Leachate Specific Conductance in 
No. In Days pH micromhos/cm 25.degree. C 
______________________________________ 
Ex. 1 
1 5 8.3 9500 
2 3 8.1 2910 
3 7 8.0 2410 
Ex. 2 
1 7 7.8 10000 
2 9 7.7 3400 
3 7 7.7 2000 
4 10 7.7 1770 
Ex. 3 
1 10 8.2 8700 
2 12 8.1 3150 
3 10 7.9 2390 
Ex. 4 
1 5 7.8 13000 
2 7 7.7 3650 
3 14 7.7 2500 
4 7 7.8 1390 
5 10 7.8 1380 
Ex. 5 
1 6 8.6 11000 
2 13 8.2 3280 
3 10 8.1 2450 
Ex. 6 
1 5 8.5 12500 
2 4 8.1 3250 
3 4 8.0 2400 
Ex. 7 
1 4 7.8 13000 
2 6 7.7 4400 
3 6 7.7 2520 
4 9 7.6 2270 
5 5 7.7 1740 
6 8 7.7 1975 
Ex. 8 
1 7 8.1 12000 
2 4 8.0 3200 
3 4 7.9 2350 
Ex. 9 
1 5 8.1 15000 
2 6 7.9 3570 
3 11 7.8 2650 
4 6 7.9 1950 
5 7 7.8 1730 
6 10 7.9 1700 
______________________________________ 
EXAMPLE 10 
Sludge specimens correspondingly chemically fixed according to the 
procedures of Examples 1 to 9 were cured or dried to constant weight (20 
to 30 days) and then leached by contact with deionized water for 72 hours. 
The conductivity was periodically measured during the leaching process and 
reached equilibrium value after 24 hours. The results as set forth in 
Table 2 below indicate the beneficial environmentally stable and 
leach-resistant nature of the hardened solid composite chemically fixed 
products obtainable according to the present invention. The cadmium 
content (in micrograms) is traceable to the fossil fuel (coal) used in the 
basic manufacturing process that generates the effluent with respect to 
which desulfurization is required. 
TABLE 2 
______________________________________ 
LEACHATE DATA 
______________________________________ 
Type Type Calcium Sulfate 
Cadmium 
Specimen 
Sludge Acid mg/1 mg/1 ug/1 Weight g 
______________________________________ 
Ex. 1 H.sub.2 SO.sub.4 
500 3520 0.8 160 
Ex. 2 H.sub.3 PO.sub.4 
490 2400 1.1 143 
Ex. 3 H.sub.2 SO.sub.4 
380 1760 1.8 140 
Ex. 4 H.sub.3 PO.sub.4 
490 3850 2.6 118 
Ex. 5 H.sub.2 SO.sub.4 
510 4400 3.8 164 
Ex. 6 H.sub.2 SO.sub.4 
470 3750 5.0 129 
Ex. 7 H.sub.3 PO.sub.4 
510 3960 3.3 130 
Ex. 8 H.sub.2 SO.sub.4 
500 7000 5.5 161 
Ex. 9 H.sub.3 PO.sub.4 
450 6000 6.0 146 
______________________________________ 
EXAMPLE 11 
The procedure of Examples 1 to 9 is repeated respectively, except that in 
each instance instead of air drying the chemical fixation product the same 
is dried by applying heat thereto to accelerate the drying and curing 
procedure. For this purpose, the product is heated at about 100.degree. C. 
and the drying time is correspondingly considerably reduced. 
As contemplated in the examples, the eastern coal lime sludge is an 
SO.sub.x scrubber sludge, i.e. of unknown sulfur-oxygen ratio content, 
which contains about 40% total solids including calcium, sulfate and 
sulfite as major contaminants, with the presence of the additional 
elements Sr, Mn, Na and Cd as well as Mg, Cr, Hg, Ni and Zn also being 
indicated. In the same way, the eastern coal limestone sludge is an 
SO.sub.x scrubber sludge which contains about 23% total solids including 
calcium, sulfate and sulfite as major contaminants, with the presence of 
the additional elements Zn, Si, Mg, Fe, Al, Cr and Sr as well as Be, Cu, 
Hg, Mn and Ni also being indicated. In turn, the eastern coal double 
alkali sludge is an SO.sub.x scrubber sludge which contains about 37% 
total solids including sodium, calcium, sulfate and sulfite as major 
contaminants, with the presence of the additional elements Sn, Cr, Si, B, 
Zn, Mn, Fe, Sb, Pb, Ti and Mg as well as Be, Hg, Ni and Zn also being 
indicated. Correspondingly, the western coal limestone sludge is an 
SO.sub.x scrubber sludge which contains about 14% total solids including 
calcium, sulfate and sulfite as major contaminants, with the presence of 
the additional elements Si, Mg, Fe, Al, Cr and Sr as well as Be, Cu, Hg, 
Mn, Ni and Zn also being indicated. Lastly, the western coal double alkali 
sludge is an SO.sub.x scrubber sludge which contains about 44% total 
solids including calcium, sodium, sulfate and sulfite as major 
contaminants, with the presence of the additional elements B, Si, Hg, Fe, 
Mg, Pb, Zn, Cu, Ti, Cr, Sr, Be as well as Mn and Ni also being indicated. 
The stack dust or kiln dust in this same regard has a general make-up 
roughly 14-16% SiO.sub.2, 3.5-4% Al.sub.2 O.sub.3, 2.5-3% Fe.sub.2 
O.sub.3, 45-53% CaO, 0.5-1% MgO, and total sulfur and alkalies of roughly 
9-15.5% SO.sub.3, 4% K.sub.2 O and 0.5% Na.sub.2 O, with the remainder 
correspondingly representing a loss on ignition content of about 9-19%. On 
an ignited basis these analysis values are roughly 17-17.5% SiO.sub.2, 
3-4.5% Al.sub.2 O.sub.3, 3-4.5% Fe.sub.2 O.sub.3, 53-59% CaO, 0.5-1% MgO, 
10-17% SO.sub.3, 4-5% K.sub.2 O and 0.5-0.6% Na.sub.2 O. 
Specific type calcium and silicate containing cement kiln dusts typically 
usable herein, with ingredients noted in percent by weight for each 
specimen both before and after analysis ignition, are set forth in Table 3 
below, the first set of values relating to the specimen as received and 
the second set of values (in parentheses) relating to the specimen on an 
ignited basis: 
TABLE 3 
__________________________________________________________________________ 
Kiln Dust Analysis 
__________________________________________________________________________ 
Constituent 
Specimen 1 
Specimen 2 Specimen 3 Specimen 4 
__________________________________________________________________________ 
SiO.sub.2 
15.7 
(17.6) 
14.2 (17.4) 
15.9 (17.4) 
15.8 (17.2) 
Al.sub.2 O.sub.3 
3.7 ( 4.1) 
3.6 ( 3.2) 
4.0 ( 4.3) 
4.2 ( 4.6) 
Fe.sub.2 O.sub.3 
2.7 ( 3.0) 
2.6 ( 4.6) 
3.1 ( 3.4) 
2.8 ( 3.1) 
CaO 52.6 
(58.8) 
45.9 (56.6) 
48.8 (53.4) 
48.7 (53.3) 
MgO 0.68 
( 0.76) 
0.81 ( 1.0) 
0.49 ( 0.54) 
0.54 ( 0.59) 
SO.sub.3 * 
9.3 (10.4) 
9.6 (11.8) 
15.1 (16.6) 
15.4 (16.8) 
K.sub.2 O* 
4.2 ( 4.8) 
3.9 ( 4.7) 
4.0 ( 4.4) 
3.9 ( 4.3) 
Na.sub.2 O* 
0.55 
( 0.62) 
0.50 ( 0.62) 
0.50 ( 0.55) 
0.50 (0.55) 
Loss 10.1 
-- 18.8 -- 8.7 -- 8.7 -- 
Total Before 
99.53 100.10 100.59 100.54 
After (100.08) (99.92) (100.59) (100.44) 
__________________________________________________________________________ 
*Total sulfurs and alkalies 
It will be appreciated that the foregoing specification and examples are 
set forth by way of illustration and not limitation, and that various 
modifications and changes may be made therein without departing from the 
spirit and scope of the present invention which is to be limited solely by 
the scope of the appended claims: