Method for processing residues of barium sulfide or strontium sulfide leaching

A method for treating residues of barium sulfide or strontium sulfide leaching with waste sulfuric acid and hydrochloric acid, which method significantly decreases the amount of residual material which must be disposed of.

BACKGROUND OF THE INVENTION 
This invention relates to a method for processing residues of barium 
sulfide leaching or strontium sulfide leaching. 
Barium sulfide leaching is a step in the process for producing barium 
compounds from the naturally occurring starting material, namely heavy 
spar (baryta). Heavy spar contains about 92 to 98% by weight of barium 
sulfate, varying amounts of calcium salts and iron salts, as well as 
silicate. Ground heavy spar is mixed with finely ground charcoal and 
reduced at high temperatures (approximately 1200.degree. C.) to barium 
sulfide. The cooled melt containing barium sulfide is extracted with hot 
water to form barium sulfide lye. The barium sulfide lye obtained by this 
barium sulfide leaching is an intermediate in the manufacture of many 
different organic and inorganic barium compounds. To form these compounds 
the lye is further reacted. For example, barium carbonate, which is used 
in the glass industry, can be prepared by introducing carbon dioxide. 
Leaching of barium sulfide leaves a residue which contains mostly barium 
salts plus iron and calcium salts. The iron and calcium salts are 
especially in the form of the silicates and sulfides. 
The preparation of strontium compounds is comparable to the preparation of 
barium compounds from barium sulfate. The starting material in this case 
is the naturally occurring mineral celestine, which consists primarily of 
strontium sulfate. The production of strontium compounds from this mineral 
is similar to the above-described production of barium compounds. The 
ground celestite is mixed with charcoal, reduced at high temperatures to 
strontium sulfide, and the cooled melt is extracted with hot water to form 
a strontium sulfide lye. This strontium sulfide lye obtained by leaching 
strontium sulfide is an intermediate in the preparation of a great number 
of strontium compounds. The far greatest part of this lye is further 
reacted to strontium carbonate, for example by introducing carbon dioxide. 
Strontium carbonate is used on a large scale in making television picture 
tubes. The leaching of strontium sulfide also leaves a residue. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a new method for processing 
residues of barium sulfide leaching or strontium sulfide leaching. 
A further object of the invention is to provide a method of processing 
residues from barium sulfide leaching or strontium sulfide leaching which 
is simple and applicable on a large industrial scale. 
Another object of the invention is to provide a method of processing 
residues from barium sulfide leaching or strontium sulfide leaching which 
will facilitate partial or complete utilization of these residues and 
decrease the amount of residue to be disposed of. 
These and other objects of the invention are achieved by providing a method 
for processing residues of barium sulfide leaching or strontium sulfide 
leaching, comprising reacting the residues with hydrochloric acid and 
sulfuric acid to form an acid solution and a solid, and separating the 
acid solution from the solid, wherein a waste sulfuric acid or a sulfuric 
acid obtained by the treatment of exhaust gases containing SO.sub.2 and/or 
H.sub.2 S is used as the sulfuric acid.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The method of the invention for processing residues of barium sulfide 
leaching or strontium sulfide leaching, in which the residues are reacted 
with sulfuric acid to form an acid solution and a solid, and the acid 
solution is separated from the solid, is characterized by the fact that a 
waste sulfuric acid, or a sulfuric acid obtained by the processing of 
exhaust gases containing sulfur dioxide and/or hydrogen sulfide, is used 
as the sulfuric acid. 
Sulfuric acid is used on a large scale for a great number of purposes in 
the chemical industry. Sulfuric acid is used, for example, as a reactant 
(e.g., in the sulfonation of aromatic compounds), as a drying agent (e.g., 
the drying of chlorine), as a catalyst (e.g., in the nitration of aromatic 
compounds), for taking up water of reaction (e.g., in condensation 
reactions), as a means of purification (e.g., of acetylene or in the acid 
refinement of lubricating oils), or in digestion processes (e.g., in 
pigment production). In these and other processes waste sulfuric acid is 
produced. Such waste sulfuric acids, which in some cases may also contain 
dissolved salts, can, according to one variant, be used in the method of 
the invention. Often waste sulfuric acids exist in more or less diluted 
form, containing, e.g., 10 to 80% of H.sub.2 SO.sub.4 by weight. The 
sulfuric acids can, if desired, be used in the method of the invention in 
the concentration in which they occur in the process that creates them. If 
desired, however, they can be concentrated, e.g., to a minimum content of 
40 weight-percent H.sub.2 SO.sub.4 . 
The above-described embodiment of the method of the invention solves both 
the problem of making rational use of leaching residues and the problem of 
making rational use of waste sulfuric acid produced in chemical processes. 
A preferred embodiment of the method of the invention relates to the use 
of a sulfuric acid obtained by processing waste gases containing SO.sub.2 
and/or H.sub.2 S. Waste gases which contain sulfur dioxide and/or hydrogen 
sulfide are produced in a number of chemical processes, e.g., in waste 
gases from sulfide roasting, in waste gases from sulfide leaching, in 
waste gases from Claus apparatus, and in furnace exhausts. In the method 
according to the invention it is especially preferred to use sulfuric acid 
which has been obtained by the processing of such waste gases. The 
processing of such waste gases is known. Hydrogen sulfide that may be 
present in the exhaust gases is first oxidized to sulfur dioxide. The 
sulfur dioxide contained in the exhaust gases is catalytically oxidized, 
usually by contact with vanadium pentoxide, and the resulting sulfur 
trioxide is then finally converted to sulfuric acid, e.g., by washing it 
out with water. Thus "wash acid" is formed. 
Sulfuric acid which has been obtained by processing exhaust gases from 
Claus apparatus, which contain SO.sub.2 and/or H.sub.2 S, is very 
especially preferred in the method of the invention. The Claus process 
serves for producing elemental sulfur from hydrogen sulfide. In this 
process gases containing hydrogen sulfide and oxygen, e.g., air containing 
hydrogen sulfide, are passed over a catalyst in two stages. This 
ultimately results in the formation of elemental sulfur and water. The 
reaction usually is not quantitative. Exhaust gases are formed which are 
referred to hereinafter as "Claus exhaust gases," which still contain 
sulfur dioxide and/or hydrogen sulfide. As stated, sulfuric acid from the 
processing of such "Claus exhaust gases" is used to very special advantage 
in the method of the invention. The concentration of the sulfuric acid 
from the processing of "Claus exhaust gases" can vary widely. In 
principle, the sulfuric acid can be used here too in the concentration in 
which it occurs in the process. Advantageously, however, the sulfuric acid 
should contain at least 40% by weight sulfuric acid. The sulfuric acid 
preferably amounts to from 50 to 100% by weight. If desired, the sulfuric 
acid can be concentrated or diluted. 
The hydrochloric acid advantageously should contain at least 15% by weight 
HCl. Hydrochloric acid containing from 20 to 32% by weight HCl is 
especially preferred. 
The sulfuric acid or hydrochloric acid could also be used in a 
concentration lower than 40 and 15 wt.-%, respectively. However, too high 
of a water content in the reaction mixture is not desirable. 
The reaction of the leaching residues with the acids is advantageously 
performed at a temperature of at least 50.degree. C. It is preferable to 
operate at between 60.degree. C. and the boiling temperature, but very 
especially preferred between 65.degree. C. and 100.degree. C. One can also 
operate at a lower temperature, but the reaction times are longer. It is 
possible to operate at a temperature higher than boiling, but it is 
generally uneconomical. In addition to the thermal energy that may be 
released by an exothermic reaction, the reaction mixture can be 
appropriately heated. 
The separated solids can be treated with alkali if desired. The acid 
solution and the solids can be neutralized if desired, and dewatered if 
desired. 
The addition of hydrochloric acid and sulfuric acid to the leaching 
residues can be performed simultaneously. In that case a hydrochloric acid 
containing sulfuric acid can be added. Preferably, however, the residues 
are first reacted with hydrochloric acid, and afterward the sulfuric acid 
is added. 
The sulfuric or the hydrochloric acid can contain salts if desired. Also, a 
portion, e.g., up to 20%, of the sulfuric or hydrochloric acid can be 
replaced by sulfate or hydrogen sulfate salts or chlorides, especially of 
alkali metals. Preferably the addition of such salts is omitted. 
The sulfate or sulfuric acid, as the case may be, is advantageously 
supplied in at least the amount necessary to completely precipitate the 
barium or strontium contained in the residue, in the form of barium 
sulfate or strontium sulfate. Lesser amounts of sulfate or sulfuric acid 
can also be added, but the effectiveness of the method of the invention is 
then diminished. 
Sulfate or sulfuric acid can also be introduced in amounts greater than are 
stoichiometrically necessary for the barium or strontium precipitation. 
Particularly good results are obtained if sulfuric acid is added in an 
amount that is 1.02 to 1.1 times the stoichiometrically required amount. 
The hydrochloric acid is advantageously metered in at least the amount 
stoichiometrically required to dissolve the calcium and iron contained in 
the residue. The effectiveness is reduced if less than stoichiometric 
amounts of hydrochloric acid are used. The hydrochloric acid, however, can 
be used in greater than stoichiometric amounts. Particularly good results 
are obtained if the hydrochloric acid is introduced in an amount that is 
1.01 to 1.1 times the amount stoichiometrically required for dissolving 
the calcium and iron content in the residue. 
Advantageously, the reaction components are thoroughly mixed. 
The leaching residue is preferably reacted with the hydrochloric acid and 
the preferably utilized aqueous sulfuric acid at temperatures from 
65.degree. C. to 100.degree. C. In addition to the thermal energy released 
by an exothermic reaction, the reaction mixture can be appropriately 
heated. 
The duration of the reactions between the leaching residue and hydrochloric 
acid or sulfuric acid can vary widely. Good results are achieved if the 
reaction is performed for a period totalling 15 minutes to 3 hours. The 
reaction can, of course, be performed for a shorter or longer time of up 
to 24 hours or more. 
For reasons relating to the apparatus it is preferable to operate at 
standard pressure (ambient pressure). However, if suitable apparatus is 
used, a lower or a higher pressure can be employed. 
Hydrogen sulfide released in the reaction of the leaching residue with 
acid, the release of which can be further increased by stripping, is 
removed and can be processed, for example, to sulfur. 
In the reaction, an acid solution and a solid are formed in the reactor. 
After the reaction of the leaching residue with hydrochloric acid and 
sulfuric acid, the acid solution is separated from the solid. Known 
methods can be used for this purpose, e.g, filtration, centrifugation or 
decantation. 
The separated solid, which also can be washed with water, consists, in the 
case of application to barium sulfide leaching residue, substantially of 
barium sulfate and silicon dioxide. It can be used moist from the filter 
or it can be dried, if desired. In particular, it can be recycled to the 
barium sulfate reduction. According to a variant described hereinafter, it 
can also be further processed. 
In the application to strontium sulfide leaching residue, the solid 
consists essentially of strontium sulfate. This residue can also be 
recycled to the sulfate reduction or further processed. 
The aqueous acid solution contains essentially the iron and calcium 
components of the leaching residue. Due to the use of hydrochloric acid, 
an aqueous solution of iron chloride and calcium chloride as well as the 
corresponding sulfates is obtained. The mixture of iron chloride and 
calcium chloride which remains after optionally concentrating or drying 
the solution or after separating the acid, can be used as a flocculating 
agent or as a precipitating agent, for phosphates for example, in the 
treatment of waste water. 
In one preferred embodiment of the method of the invention, the solid that 
forms in the acid treatment, which consists essentially of barium sulfate 
or strontium sulfate and silicon dioxide, is further treated. For this 
purpose the solid is decomposed with aqueous alkali solution, and soluble 
components are dissolved. An alkaline extract solution and an extraction 
residue are formed. 
This embodiment is characterized in that the solid separated from the acid 
solution is decomposed with aqueous alkali solution with the formation of 
an alkaline extract solution and an extraction residue, and the alkaline 
extraction solution is separated from the extraction residue. 
The concentration of the alkali solution used can vary widely, e.g., 
between 1% by weight and the saturation limit. However, it is 
advantageously between about 15 and 50 wt.-%. Caustic soda solution is 
particularly suitable. 
The reaction of the solid with alkali solution in the decomposition can be 
performed at temperatures between about 10.degree. C. and the boiling 
point. It is preferably performed, however, above about 50.degree. C. and 
particularly preferably between about 80.degree. C. and the boiling point 
of the reaction mixture. 
Leaching with alkali solution can be performed at standard pressure (1 
atm). If desired, an elevated pressure can be used, for example a pressure 
up to 5 bar, and a temperature of up to 160.degree. C. or higher. In that 
case pressure-resistant apparatus (autoclaves) are used. If suitable 
apparatus is used, it is also possible to operate at even higher pressures 
than 5 bar and higher temperatures than 160.degree. C. 
After the reaction the alkaline extract solution contains the silicon 
dioxide in the form of dissolved sodium silicate. The alkaline solution is 
separated by known methods, for example by filtration, centrifugation or 
decantation, from the alkali-insoluble solid which consists primarily of 
barium sulfate or strontium sulfate. 
The alkaline solution containing alkali silicate can be used as a water 
glass solution or, optionally after drying, for producing silica gel. The 
alkali-insoluble solid can be recycled, optionally after drying, to the 
sulfate reduction. 
This variant of the method of the invention thus makes it possible to fully 
utilize the residue of barium sulfide leaching or the residue of strontium 
sulfide leaching. 
If there is no need to utilize the iron- and calcium-containing acid 
solution or the alkali silicate solution, the solution in question can, of 
course, also be neutralized, dried to a greater or lesser extent, and the 
resulting residue can be delivered to a dump. 
If there is no interest in utilizing the iron- and calcium-containing acid 
solution or in the utilization of the alkali silicate solution, the two 
solutions are advantageously combined. If desired, they are neutralized 
and the resulting residue delivered to a dump. The residue can be dumped 
either filter-moist or more or less dry. 
An especially advantageous embodiment of the method of the invention is 
characterized by using a hydrochloric acid containing about 20 to 32 wt.-% 
of HCl, a sulfuric acid containing about 50 to 100wt.-% of H.sub.2 
SO.sub.4 obtained from Claus exhaust gases containing SO.sub.2, performing 
the reaction of the leaching residues with hydrochloric acid and sulfuric 
acid at a temperature between 60.degree. C. and the boiling point, 
treating the solid matter separated from the acid solution with aqueous 
alkali solution to form an alkaline extract solution and an extraction 
residue, separating the alkaline extract solution from the extraction 
residue, combining the separated acid solution with the separated extract 
solution, and neutralizing and dewatering them. 
Even in this embodiment of the method according to the invention, in which 
only the barium or strontium content of the leaching residues is utilized, 
the amount of residual material that has to be disposed of is even more 
greatly reduced, namely by as much as 90% by weight. Furthermore, good use 
is made of the Claus exhaust gas. 
The method of the invention thus makes it possible in a technically 
advantageous and simple manner on the one hand to recycle most or even all 
of the residue produced by the leaching of barium sulfide or strontium 
sulfide, thereby reducing the amount that has to be disposed of, and on 
the other hand to make good use of waste sulfuric acid or SO.sub.2 and/or 
exhaust gases containing hydrogen sulfide. 
The following example is intended to further illustrate the method of the 
invention without limiting its scope. 
EXAMPLE 
Production of the Sulfuric Acid 
Sulfur dioxide-containing exhaust gases from a Claus apparatus operated 
with hydrogen sulfide from the leaching of barium sulfide were oxidized 
catalytically over vanadium pentoxide, and the resulting sulfur trioxide 
was washed out with water. This produced a "wash acid" which contained 
about 60% by weight of sulfuric acid. 
Decomposition of the Residue 
3.5 tons of moist residue from barium sulfide leaching containing about 50% 
by weight water was mashed in a heated stirring tank with 1.8 tons of 
water. 620 kg of hydrochloric acid (concentration 32 wt.-% HCl) and 720 kg 
of the "wash acid" obtained in the preceding step (concentration 60 wt.-% 
H.sub.2 SO.sub.4) were added with stirring. The hydrogen sulfide that 
formed was absorbed with barium sulfide lye in a gas washer. 
After 1.5 hours of stirring at a temperature of 70.degree. C., the 
suspension was filtered through a filter press. A 2.6 ton filter cake 
containing 28 wt.-% water was obtained. The filter cake was then washed 
with 2.8 m.sup.3 of water. Then 170 kg of NaOH (concentration 30 wt.-%) 
were added, and the mixture was stirred for 2 hours at 90.degree. C. The 
suspension was filtered through a filter press. The resulting filter cake 
(25 wt.-% water) contained 1.8 t of dry mass containing approximately 95.2 
wt.-% BaSO.sub.4, 1.5 wt.-% SrSO.sub.4, 0.69 wt.-% SiO.sub.2, 1.6 wt.-% 
Fe.sub.2 O.sub.3, 0.78 wt.-% CaO. 
The product corresponded to a highly concentrated heavy spar, and was used 
in the rotary kiln process for producing barium sulfide. 
The collected filtrates from both filtration steps, combined with the 
respective wash waters were mixed in a stirring tank. A suspension 
resulted. By the addition of NaOH the suspension was adjusted to pH 7, and 
then dewatered in a filter press. 1.4 tons of moist filter cake containing 
70 wt.-% of water were obtained. The filter cake was so solid that it 
could be worked with a spatula. The dry mass consisted mainly of iron(III) 
oxide, calcium oxide, silicon dioxide and aluminum oxide. This residue can 
be disposed of without difficulty. 
In comparison with the residue introduced into the process, the amount of 
material to be disposed of was reduced by 76% with respect to the dry 
mass. Also, the "wash acid" was put to good use. 
The example shows how advantageously the method of the invention can be 
integrated into the leaching process for the production of barium sulfide, 
in that the sulfur content of the exhaust gases is recycled into the 
process. 
Of course, sulfuric acid from other sources can be added if, for example, 
not enough waste sulfuric acid or "wash acid" is available. 
The foregoing description and examples have been set forth merely to 
illustrate the invention and are not intended to be limiting. Since 
modifications of the described embodiments incorporating the spirit and 
substance of the invention may occur to persons skilled in the art, the 
scope of the invention should be construed to include all variations 
falling within the ambit of the appended claims and equivalents thereof.