CA1249416A - Process for the production of sulfur dioxide - Google Patents

Abstract

A PROCESS FOR THE PRODUCTION OF SULFUR DIOXIDE

ABSTRACT OF THE DISCLOSURE

This invention relates to a process for the production of sulfur dioxide by the thermal decomposition of metal sulfates and the simultaneous roasting of sulfidic ores in a reactor, the improvement which comrises intro-ducing the metal sulfates together with the metal sulfides into the reactors in the form of mixed granules.
Advantageously, the mixed granules are produced by pre-granulating the metal sulfate with at least one of sulfuric acid and/or a granulating aid and subsequently coating with metal sulfide.
The mixed granules may be powdered with a dry non-adhering substance such as coal dust, fine coal, sulfur and/or cinders prior to introduction into the reactor.

Description

A PROCESS FOR THE PRODUCTION OF SIJLFUR DIOXIDE

This invention relates to a process for the production of sulfur dioxide hy the thermal decomposition o~ metal sulfates and the simultaneous roasting of sulfidic ores.
The thermal decomposition of metal sulfates and sulfuric acid is a highly endothermic reaction, Therefore, coal, fuel oil or fuel gases or, advantageously, sulfur or sulfidic ores, preferably iron pyrites. are often burnt in the same reaction zone in order to supply the necessary hea~ of reaction. In that case, the thermal decomposition process is frequently carried out in multi-ple bed furnaces where fuel oil or fuel gas is used and also where sulfur is used.
The more recent process of thermal decomposition in fluidized-bed reactors requires the permanent presence of a fluidized bed, iron o~ide being a particu;larly suitable bed material. The losses caused by abrasion may be compensated using as the sulfur-yielding raw material iron pyrites which is also a relatively inexpensive energy source.
The problem associated with this process i5 to introduce the iron pyrites, particularly in the form of finely particulate flotation pyrites, and the sulfates, which frequently contain sulfuric acld and are hygro-scopic, into ~le fluidized bed reactors and to distribute them in the bed such that the consumption and release of energy are in equilibrium throughout the reactors which, in many cases, are ~e A 22 511 ~Z~41~;

- 2 - 23189-5787 extremely large. Only in -this way can maximum decomposition be ob-tained without metal sulfates being discharged from the reactors or sintered beds being formed through local overheating.
The object of the present invention is to improve the in-put both of the sulfidic ores and of the metal sulfates into the fluidized bed reactors, and also into other suitable reactors, such as multiple bed furnaces or rotary furnaces, and to guarantee opti-mum admixture of the energy-consuming and energy-yielding raw ma-terials.
According to the invention, this object is achieved by gra-nulating together the metal sulfates and the sulfidic ores, herein-after referred to as pyrites, optionally in conjunction with other energy sources, and introducing these materials into the reactors in the form of granules.
According to the present invention there is provided in the pr~duction of sulfur dioxide by the thermal decomposition of me-tal sulfates and the simultaneous roasting of sulfidic ores in a reactor, the improvement which comprises introducing the metal sul-fates together with the metal sulfides into the reactors in the form of mixed granules, wherein (i) the granules are produced by granulating the sulfidic ores with a metal sulfate filter cake containing adhering acid, or (ii) the mixed granules are produced by pregranulating the metal sulfate with at least one of sulfuric acid and/or a granula-ting aid and subsequently coating with metal sulfide.
Accordingly, the present invention provides a process for j =~.,.~

~Z~ i6 - 2a - 23189-5787 the production of sulfur dioxide by the thermal decomposition of metal sulfates and the simultaneous roasting of sulfidic ores in suitable reactors, the metal sulfates being granulated together with -the metal sulfides and subsequently introduced into the reactors in the form of granules. Whereas pyrites are very difficult to granu-late on their own, giving soft granules which are unsuitable for mechanical handling and introduction into furnaces, it has surpri-singly been found that metal sulfates and pyrites may together readily be granulated, providing stable granules whieh are suitable for mechanieal handling including introduetion into reaetors.
The granules obtained eonsist of an inner eore, whieh eon-sists of the metal sulfates, and of a shell of pyrites. Metal sul-fates containing sulfuric acid, of the type which accumulate in the concentration of -~ - 2a -~2~ 16

-3- 23189-5787 TiO2 waste acid or pickling solutions by evaporation, may be granulated wi-thout difficulty providing they are present in the form of "dry", i.e. friable filter cakes.
Accordingly, filter cakes containing adhering acid are particularly suitable for use as metal sulfates in accordance with the present invention. Their adhering acid content may be reduced before granulation by mechanical demoisting, for example using belt-type filter presses, thereby improving the friability of the filter cakes.
Granulation may be carried out in various apparatus, for example granulating plates are eminently suitable, in which case granulation may he carried out in one or more stages.
The granules very quickly become moist and tacky however, by virtue of the hygroscopic properties of the acid-containing salts. According to the invention, relatively firm, free-flowing granules are obtained by coating these granules with pyrites on a second granulating plate or in the outer ring of the same plate. The granules formed adsorb very little moisture, even in the event of prolonged standing in air, and they remain free-flowing.
In the process according to the invention, the metal sulfates are pregranulated with sulfuric acid and/or granulating aids and subsequently coated with metal sulfides.
Dry, free-flowing metal sulfates may be pregranulated l6 -3a- 23189-5787 by the addition of sulfuric acid, oil or known granulating aids, such as molasses, fish glue, bentonite or sulfite waste liquor, and subsequently coated with pyrites.
Fine pyrites having particle sizes of up to 5mm may be used as the pyrites. However, the process affords particular advantages where flotation pyrites having particle diameters considerably below 0.1 mm are used. The granules obtained in this case show extremely good flow and storage properties. MoreoYer, the considerable difficulties in~olved in the introduction of flotation pyrites into fluidized bed reactors are avoided.
The ratio of pyrites to metal sulfates ma~ be varied within ~ide limits in ~ccordance ~ith the operational requirements goYerning introduction of the raw materials. By adding conventional granulating aids, it is possible to increase the strength of the pyritic shell, particuarly in the case of a high pyrites-to-metal sulfate ratio, although this is generally not necessary for normal handling.
It has proved to be advantageous to powder the granules consisting of metal sulfates and metal sulfides with dry, non-adhering substances, such as coal dust, fine coal, sulfur and/or cinders. This produces a further impro~ement in the flow properties of the granules.
The use of the granules produced in accordance with the invention guarantees more uniform operation of the reactors and more effective decomposition of sulfates particularly in the case of fluidized-bed reactors.
The following examplesillustrate the advantages afforded by the invention without in any way limiting the scope of the invention.
ExAMæLE l (Comparison Example) Fxiable metal sulfates (predominantly Fe-sulfate with Mg-, Al-, Mn- and other sulfates~ containing approximately 30% by weight of adhering acid (salt - free acid containing 65% of H2S04~ (- filter salt I) were granulated on a granulating plate. After l to 2 minutes, granules from 3 to lO mm in diameter had been produced from the fine particles. Through further rolling on the plate~ the surface of the granules became wet and shiny and, after 4 to 5 minutes, the granules began to stick to one another.
~e A 22 511 l 2 4 ~3 L~

When the ~ranules were powdered ~ith lignite dust after a residence time of 2 to 4 minutes on the granulating plate, they remained free-flowing.
Some of the granules were le~t standing in a pan in the building. After 48 hours, they had ~ecome distinctly tacky. The increase in weight by the adsorption of atmospheric moisture was 13.6%.
EXAMPLE 2 (Comparison Example) An attempt was made to granulate iron pyrites A
(flotation pyrites with 70% of the particles smaller than 0.04 mm in diameter) on a granulating plate.
Only a few granules with diameters ranging from 3 to 20 mm were formed and could readily be crushed in the hand, i.e. were unsuitable for introduction into roasting furnaces using standard handling and metering equipment.
ExAMæLE 3 Filter salt I, which was produced by demoisting a rotary filter cake in a belt-type filter press and disintegrating the plate-like cake in a pinned disc mill, and iron pyrites A were simultaneously introduced in a ratio of l:l onto a granulating plate (d = 0.8 m). After 5 minutes, the dry, dull-looking, free-flowing granulate was removed. The sieve analysis is shown in Table l.

`25 2 kg of filter salt I were introduced onto the granulating plate. After 3 minutes, 2 kg of iron pyrites A were added to the moist and shiny granules cver a period of 30 seconds. After another 2 minutes, the granulating plate was stopped and the spherical ~ranules were sifted (Table l).
The granules were strong enough to withstand the stressing normally involved in handling, bunkering and metering. After three ~eeks in an open pan in the laboratory, they wPre still free flowing. The increase in weight through moisture adsorption was 3.2~.
Le A 22 511 -Filter salt I ~as granulated with iron p~rites B
(fine pyrites with an aYerage particle diameter of l.3 mm) in the same way as described in Example 4. The granules formed had a more shiny appearance and were also free-flowing.

Granules were produced as de~cribed in Example 4 and subsequently powdered with l~Og of lignite dust.
Immediately after production and also after storage for 3 weeks in an open pan, the powdered granules showed distinctly better free-flow properties than the unpowdered granules according to Example 4.

. _ lS Filter salt I and iron p~rites A in a ratio of 2:l (7a) and l:2 (7b) were granulated as described in Example 4. Sufficiently strong, free-flowing granules were obtained in both cases.
ExAMæLE 8 Filter salt II (rotary ~ilter cake containing 42 of 65% sulfuxic acid as adhering moisture~ were pre-granulated as in Example 4 and regranulated with the same quantity of iron pyrites A. The granules (sieve analysis Table l) were free-flowing, but showed distinctly less compressive strength than in Examples 3 to 7.

A granulating plate l m in diameter and 0.2 m deep had been fitted with an inner cylindrical ring 0.6m in diameter and 0.15 m deep. lO0 kg of filter salt I/hour were introduced into the inner zone. The glistening granules of filter salt I rolled into the outer zone ~ere 50 kg/hcur of iron pyrites A were added to them. The granules rolling o~f over the outer edge were free-flowing and extremely uniform (Table l).
EXAMPLE lO
_ .
Free-flo~ing iron sulfate monohydrate produced by he ~ 22 511 ~2'~ 6 the deh~dration of iron sulfate heptah.~drate was granulated in the inner zone of the granulating plate (correspondin~ to Example 9I by spraying with molasses solution. Iron pyrites A ~ere added to the ~ranules rolling into the outer zone ~f the plate in a ratio by weight of l:l. The free-flowina ~ranules rolling off from the outer zone were similar in their grain size and strength to the granules produced in accordance with Example 9.
Table Sieve analyses of various granulates Example ~2 mm % 2 - 6 mm 6 - lO ~lO mm %
No. ~y ~eight % by ~eight ~ by weight by weight

4 7 66 22 5 ll 57 24 8 7 a 12 59 25 4 7 ~ lO 6~ 23 5 8 37 47 ll 5 Le A 22 511

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the production of sulfur dioxide by the thermal decom-position of metal sulfates and the simultaneous roasting of sulfidic ores in a reactor, the improvement which comprises introducing the metal sulfates together with the metal sulfides into the reactors in the form of mixed granules, wherein (i) the granules are produced by granulating the sulfidic ores with a metal sulfate filter cake containing adhering acid, or (ii) the mixed granules are produced by pregranulating the metal sulfate with at least one of sulfuric acid and/or a granula-ting aid and subsequently coating with metal sulfide.

2. A process according to Claim 1, wherein the adhering acid content of the filter cake is reduced before granulation by mechani-cal demoisting.

3. A process according to Claim 1, wherein the mixed granules of metal sulfates and metal sulfides are powdered with dry non-adhering substance prior to introducing into the reactor.

4. A process according to Claim 3, wherein the dry non-adhering substance comprises at least one of coal dust, fine coal, sulfur and cinders.

5. A process according to Claim 1, wherein the mixed granules are produced on a granulating plate.

6. A process according to Claim 1, wherein the mixed granules are powdered with a dry non-adhering substance prior to introducing into the reactor, the dry non-adhering substance comprising at least one of coal dust, fine coal, sulfur and cinders.