Sulphur is present in the earth's crust as well as the ocean in its elemental state and as sulphides and sulphates.
The most important sulphides are pyrite, chalcolite, sphalerite and galena. The principle sulphates are gypsum and kieserite.
There are three major types of sulphur deposits:
- The gypsum type (95% of the world's sulphur), as found in the USA, Mexico and Sicily
- The solfataras (volcanic vents, formed by oxidation of hydrogen sulphide gas), as found in Japan and Chile.
- The fumarole deposits (sinter deposits formed by interaction of sulphur dioxide and hydrogen sulphide), as found in Mammoth Hot Springs, Yellowstone park.
Sulphur found in the USA (Louisiana and Texas) is mined by the Frasch process, which incorporates a multi-wall well, Hot water is pumped into the formation, melting the sulphur which is subsequently "pumped" to the surface by compressed air.
11.2.1) Sulphur Filtration
Elemental sulphur, used to produce sulphuric acid, is burned to produce a mixture of oxygen, nitrogen and sulphur dioxide and suspended solids and/or ash will ultimately foul boiler tubing, etc. and therefore requires filtration.
Molten sulphur filtration poses special demands upon filtration equipment since filtration takes place at elevated temperature and corrosion of the vessel and internals may occur unless the Ph is continuously monitored.
The sulphur viscosity is not a linear function with respect to temperature changes so that the operating temperature must be kept with in certain boundaries if relatively Constant filter rates are expected. All vessels, tanks, pits or piping must also be steam jacketed, in order to control above mentioned operating conditions.
Sulphur obtains its corrosive aspects from the contaminants and the introduction of moisture and/or air and acid formation may form under those conditions. To reduce air formation inside the filler part of the unfiltered sulphur may be recirculated back to the unfiltered pit.
Precoating the filler will normally be done with medium D. E, precoat grades, Perlite or combination of D. E. and fibrous precoat grades, the latter of value when leaves get old and screening show minor wear and tear. Body feed is recommended, as blinding of the filter cake otherwise will occur prematurely, Body feed grades are generally a little tighter (D. E. b diatomaceous earth) than the precoat grades. Bleaching clay, in some instances, may be used as body feed to assist in removal of carbonaceous, tarry solids present.
In order to reduce the acid content of the sulphur, neutralization is accomplished by adding hydrated lime, ammonium bicarbonate as neutralizing agents.
Depending upon the source of sulphur and its handling practices, in terms of filtration requirements, sulphur filtration is based on the following average flows and cycles.
Sulphur filtration is a fixed leaf filter application. For small plants, vertical filters with full top and bottom opening covers may be applied.
However, most flow rates will require a horizontal shell vertical fixed leaf design,
The cleaning step for sulphur filters needs further thought since the conventional HF type (High frequency/low amplitude) vibrator does not get the cake off the leaves too well. Most filter manufacturers have, in the past, been saddled with manual cleaning recommendations such as the use of rubber or wooden mallets or non metallic cake scrapers (to prevent screen damage), The reciprocating cylinder type vibrator has been used with some degree of success and should be offered as part of the filter package. Inspection of the leaves after cleaning however should be recommended.
The filter cake will release best, when left on the leaves after retraction for approx 15 minutes.
Filter cake is very combustible. Protect filter from wind to minimize Ignition of the cake resulting in damage to the fine outer screening. Keep fire extinguisher near the filter location.
Due to excessive strain placed upon the filter leaves in terms of operating temperature (between 120 and 150° C), cleaning method and filter cake nature, it is important that the filter leaves are of heavy duty design.
Even though residual amounts of precoat material (after leaf cleaning) will not impede the flow during subsequent cycles, occasional soaking of the leaves in hot clean sulphur
(or a suitable cleaning compound) is recommended in order to prevent long range blinding of the filter cloth.
More than anything else in Importance, the sulphur must be neutralized (to eliminate acid corrosion upon the filter leaves) in the precoat pit and/or unfiltered sulphur pit.
Materials of constructions:
- Carbon steel shell and internals
- Filter leaves 316 ss with 24x110 mesh filter screens suitable for D.E. precoating. Steam jackets for 5 to 6 bar working pressure.
Precoat and unfiltered sulphur pits provided with 5-6 bar steam coils. Conventional precoat piping system (also jacketed).
Special submerged sulphur pumps.
Good piping design provides for jacketed piping to keep filter free of air (from top vessel back to unfiltered sulphur pit) and jacketed piping to allow for filtered sulphur recirculation back to the unfiltered sulphur pit during production interruptions &o that the filter cake will not slough off the leaves.
11.2.2) Filtration Of Molten Sulphur
Basic filter design:
1. DRY CAKE DISCHARGE (preferred)
a. vertical tank with bottom opening head, minimum floor space, low first costs, leaves not fully exposed for cleaning
b. horizontal tank with retractable carriage, easier to clean (leaves fully exposed) more floor space, somewhat higher in costs in sqm of area than vertical tank.
2. SLURRIED CAKE DISCHARGE
Cake re-slurred in sulphur results in high sulphur load with cake or necessitates recycle to melting pit which must than be cleaned more frequently (vertical! tank) filter recommended with vibrator.
3. FILTER TANK
a. carbon steel (5 Bar) b. steam jacket (5 Bar)
e. jacketed connections (flanged) d. silicone or Viton gaskets. 4. LEAVES
a. 24x110 Mesh Plain Dutch Weave in SS 316
b. drainage screen and balance of leaf in carbon steel or SS 304 e. "O" ring gaskets in Viton
d. 4+5" leaf spacing. 5. ACCESSORIES
a. jacked carbon steel precoat tank (volume equal to 1,3 times the volume of the filter tank).
b. jacketed interconnecting pipe, valves and fittings c. jacketed submerged pumps for precoating and feed.
11.2.3) Operation
1. PRE TREATMENT OF SULPHUR IN PIT
a. Neutralize all free acid with hydrated lime (0,5 kg, lime per 0,5 kg free acid). b. Boil off all moisture (12 ÷ 24 hrs holding time in melting pit).
c. Maintain 140 ÷ 150° C temperature (minimum viscosity).
2. PRECOAT
a. moderately coarse filler aid (Celite 503 or Dicaiite Speedex) b. Hydrated time (0,3 ÷ 0,4 kg/sqm).
c. Mix filler aid and lime with previously filtered sulphur d. Recirculate 10 +15 minutes.
3. FILTRATION
a. Maintain Constant filtration rate somewhat higher than burning rate to balance filter down time and hold filtered sulphur level.
b. Variable speed pump drive recommended (steam turbine drive works out nicely).
11.2.4) Filter Sizing
1. BRIGHT SULPHUR
Area required 0,1 ÷ 0,3 sqm per ton S per day.
Cycles 1 ÷ 3 days between cleaning without filler aid admix. 2. DARK SULPHUR
a. With O,05 % filter aid admix.
Area required 0,15 ÷ 0,25 sqm per ton S per day. Cycles 8 ÷ 24 hours between cleaning.
11.2.5) General comments
1. Filter leaves should occasionally be soaked in clean molten sulphur to prevent gradual blinding of the wire screen.
2. Bolted leaf construction is recommended to allow field replacement of wire cloth if necessary.
3. Filter should be sheltered from wind to minimize change of ignition of the cake when the tank is opened. Sulphur flame can burn through 24x110 mesh wire cloth.
4. Filter cake and pit sulphur should be checked regularly for free acid which will corrode the filters.
5. Use of combustible filter aids such as Solka Floc (cellulose and carbon) will minimize ash in converter, should a broach occur in any of the leaves. These materials are somewhat more expensive than diatomaceous earth, however proper filter operation
For bright sulphur use Speedex or Celite 503. For dark sulphur use Hy-Flo.
Precoat quantity about 1 kg per 1 sqm of filter area. Lime is added to the unfiltered pit, Generally additional lime is added past the neutralization mark to get rid of any free acid.