GB2190909A

GB2190909A - Treatment of waste material

Info

Publication number: GB2190909A
Application number: GB08709985A
Authority: GB
Inventors: Richard William Watson
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1986-05-02
Filing date: 1987-04-28
Publication date: 1987-12-02
Anticipated expiration: 2007-04-28
Also published as: GB2190909B; GB8709985D0

Description

SPECIFICATION Treatment of waste material This invention relates to a method ofapparatusfor treatment of waste materials. It is particularly concerned with the treatment of "Sulphate Waste Material". By the term "Sulphate Waste Material" as used herein we mean waste material comprising sulphuricacidand/orasulphate(typicallyan inorganic sulphate). Examples of such sulphate waste material include petroleum refinery sludge, spent pickling liquor and other wastes from chemical or metallurgical processes in which sulphuric acid is employed.

One particular example with which this invention is concerned is the sulphate waste material that is typically obtained in the manufacture oftitanium dioxide pigments by the sulphate route. The sulphate waste material from this process particularly comprises relatively dilute sulphuric acid including dissolved iron sulphate. Typically, such sulphate waste includes from 14to 20% by weight of sulphuric acid and upto 20% by weight of iron sulphate may be included, and typically traces of other inorganic sulphates, the balance being water.

It has been proposed to treat this waste material by subjecting it to spray drying whereby to remove the water without liberating a substantial amount of sulphur dioxide. The resultant anhydrous sulphate salts are then removed from the acid and the acid is recondensed at a higher strength. One disadvantage ofthis method is that it requires relatively complex spray drying equipment and the spray drying stage is difficultto optimise. A second and more fundamental disadvantage is that in the event that the waste material includes metal ionsthat may have a deleterious effect on the titanium dioxide pigment, such metal ions may be carried with the acid vapour with the resultthatthe resulting concentrated sulphuric acid may not be sufficient purity for reuse in the manufacture oftitanium dioxide.

In ourco-pending application 2181 120Awe disclose a process for the treatment ofsulphate waste material that includes a significant proportion of inorganic sulphate in which the sulphatewaste mateiral may be cracked in a rotary or other kiln orin a fluidised bed. In the fluidised bed, an oxygen-rich gas such as pure oxygen or oxygen-enriched air is preferably employed to fluidise the bed, and a solid fuel such as coke may be burned to raisethe temperature necessaryto crack the sulphate waste material fed into the bed. With this method the oxygen-rich gas typically fulfils two functions. The firstfunction is that of providing the necessary oxygen for burning the fuel and thereby generating a temperature sufficiently high to effect the thermal cracking ofthe sulphate. The second function isto fluidisethe bed.Although in some examples, it is perfectly possible for the oxygen-rich gas to fulfil both these functions, it is to be appreciated that if undesirably high levels of residual oxygen are to be avoided in the gas leaving the fluidised bed, in other examples it will be necessaryforthe oxygen-rich gas to contain a relatively high proportion of nitrogen so as to obtain an adequate fluidising velocity and a stage may be reached where the proportion of nitrogen becomes so high that it is no longer possible to obtain the optimum temperaturefor thermal cracking or the overall requirements for thermal energy become undesirably high.

The invention relates to an alternative method and apparatus that overcomes or reduces these problems.

According to the invention there is provided a method of recovering from sulphatewaste material (as herein before defined) a gas mixture including sulphur dioxide, comprising burning a fuel to form a flame zone, introducing sulphate waste material into the flame zone, employing oxygen-rich gas (as hereinafter defined) to support combustion of the fuel and to generate a flame of sufficient temperature to crack said sulphate waste material and thereby liberate sulphur dioxide therefrom, and separating solids exiting the flame zone from the gaseous combustion products.

The invention also provides apparatus for use in performing such method, comprising a burner having afirstinletforfuel and a second inletfor oxygen-rich gas (as hereinafter defined), a chamber into which, in use, said burner fires, means for introducing sulphate waste material into the flame zone, an outlet for gaseous combustion products, and means for collecting solid particulate residue emanating from the flame zone ofthe burner.

Bytheterm oxygen-rich gas as used herein,we mean oxygen-enriched air or pure oxygen itself.

Preferably, pure oxygen is employed to support combustion of the fuel. By the use of pure oxygen, notwithstanding the endothermic nature of both the thermal cracking of sulphate and the dehydration and evaporation of the sulphuric acid and waste present, it is possible to maintain an adequate temperature forthe thermal cracking to proceed.

Preferably said temperature is in the range 900 to 1 ,300 C.

The burner is preferably disposed vertically or at an angleto the vertical whereby the pa rticles fall undergravityfromtheflamezoneandcollectatthe bottom of the chamber. The outlet for the residue may typically have a rota ry valve and may, if desired, be provided with cooling means to control the temperature.

If desired, the chamber may have a cyclonic particle separator associated with its outlet for gaseous combustion products.

In the event that the solid residue from the thermal cracking is substantiallyfree ofsulphate,the gaseous combustion products downstream of any cyclonic separator may be subjected to the necessary cleaning and water removal to make them suitable for use as a feed gas to a plant for producing sulphur dioxide by the contact process. In the event that the concentration of sulphur dioxide in the gas mixture after clean up and removal ofwatervapour is higherthan that normally encountered in the contact process, the gas mixture may be diluted with air.

An alternative method is to employ the kind of contact process described and claimed in our co-pending application 8524083 in which the gas mixture after drying contains at least 30% by volume of carbon dioxide and more than 16% by volume of sulphur dioxide. In this process the dried gas mixture is subjected to a plurality of steps of catalytic conversion to convert sulphur dioxide in the mixture to sulphurtrioxide. The temperature of the gas mixture upstream of each stage is adjusted and is the mole ratio of sulphur dioxide to oxygen in the gas mixture upstream ofthe first stage and intermediate the first and second stages, the mole ratio being adjusted bythe addition of oxygen-rich gas to gas mixture. The resulting sulphurtrioxide is then absorbed to form sulphuric acid.

In the event that the residue from the flame zone still contains sulphate, the residue and combustion gases are preferably passed into a fluidixed bed or turbulent phase reactor and the combustion gases are employed to heat the particles of solid residue to yield more sulphurdioxide. Typically, the heat content ofthe combustion gases will be sufficient to complete the cracking of the sulphate. In the event thatthe gases do not have this necessary heat content, they may be premixed with hot combustion gases from a supplementary burner. Whereas the liquid or gaseous fuel supplied to the main burner is preferably a hydrocarbon, if desired, sulphur may alternatively be burned in the supplementary burner.

Such combustion of sulphur has additional benefit in increasing the sulphuric acid output of a downstream plant for producing sulphuric acid by the contact process and may be particularly useful where the sulphur content of the sulphate waste material does not meet the requirements for sulphuric acid ofthe process from which the sulphatewaste material is taken.

Downstream of the fluidised bed reactor, the combustion gases from said flame zone are preferably passed through a separatoradapted to remove solid particles elutriating from the fluidised bed, and the gases may then be treated in a conventional precipitator and driers priorto being converted to sulphurtrioxide by the contact process.

In another example of the method and apparatus according to the invention, the solid residue from the burnerfalls under gravity into a treatment region which is preferably located in the same vessel as that into which the burner fires. The residue is then preferably heated by the hotgasesfrom an auxiliary burner so as to crack residual sulphate. The hot gas from the auxiliary burner preferably fluidise or renderthe residue turbulent. This example is advantageous intheeventthatthesulphatewaste contains a component such as manganese sulphate thattendsto melt in the burnerflame.

The sulphate waste material is preferably concentrated upstream of the burner by evaporating part of its water content, but without any cracking of sulphate taking place atthis stage.

The method and apparatus according to the invention will be described by way of example with reference to the accompanying drawings, in which Figure lisa schematic circuit diagram showing one plant for performing the invention and Figure2 is a schematic circuit diagram showing an alternative plantfor performing the invention.

Referring to Figure 1 ofthe drawings, a generally symmetrical combustion chamber 2 is disposed with the longitudinal axis vertical and has extending through its top an axial burner 4. The burner4 has a first inlet 6 for sulphate waste material from a process for forming titanium dioxide pigment by the sulphate route, a second inlet8forcommercially pure oxygen and a third inlet 10 for gaseous or liquid hydrocarbon fuel.

In operation, a flame zone 14 is created which extends generally vertically downwards within the chamber 2 and ends above a lower conical portion 16 of the chamber 2. At the bottom of the conical portion 16, the chamber 2 has a first outlet 18.A rotary valve 20 is disposed in outlet 18. The chamber 2 has a second outlet 22 near its top. The outlet 22 communicates with a cyclonic solids separator 24 having at its top an outlet communicating with a pipeline 26that leadsto a plenum chamber 28 of a fluidised bed reactor 30 having a grid 32 forming the top of the chamber 28 and supporting a bed 34 of particulate materials.The separator 24 has a bottom outlet with a rotary valve 36 disposed therein, the outlet communicating with a conduit 38 which at one of its ends terminates in the outlet 18 from the chamber2 and at its upper and terminates in the bed 34. By this means, in operation, a residue of solid particles of oxide and sulphate may be fed into the bed 34 and be fluidised by gas passing from the outlet 22 of the chamber 2 via the separator 24 and conduit 26. The hot gas will also serve to crack thermally the residual sulphate.

The apparatus preferably includes an auxiliary combustion chanber40 having a fuel burner 42 disposed therein. The chamber 40 has an outlet communicating with a condu it 48 for combustion gases which terminates in the conduit 26. If desired, the temperature in the chamber 40 may be controlled bythe addition of cooling medium such as colled gas from a downstream location in the process (in which example a recyclefan is used) orwaterorsteam.The coolant may be introduced through a conduit 44 having a flow control valve 46 disposed therein or directlythroughthe burner42.

The reactor 30 has an outlet 50 at its top communicating with a cyclonic separator 52 adapted to separate elutriating solids from the gas mixture.

The separator 52 has an outlet 54for gas at its top and an outlet 56 at its bottom for discharging separated solids.

In operation, hydrocarbon fuel such as methane or propane orfuel oil is burnt with a slight stoichiometric excess of oxygen in the burner 12 and is employed to crack the sulphate waste comprising sulphuric acid and iron sulphate. The rate offeeding the sulphate waste into the flame zone 14from the inlet 6 is chosen such that the gases leaving the chamber 2 through the outlet 22 have a temperature not less than 1,000 C and preferably in the range 1,000to 1,3009C. Intheflamezone 14the temperature generated by the combustion of the fuel firstly evaporates the sulphate waste material, secondary drives off water of crystallisation from the solid particles ofsulphate that result from the vaporisation, and thirdly cracks the sulphate and sulphuric acid to form sulphur dioxide. The gas mixture leaving the outlet 22 thus comprises water vapoursulphurdioxide and carbon dioxide. It may also contain small traces of sulphurtrioxide and typicallyfrom 1 % to 5% by volume of oxygen.

Typically, the gas mixture contains up to 14% by volumeofsulphurdioxideifthewatervapouris excluded.

The cracking ofthe solid sulphate can in some examples be completed in the zone 12 but is typically not completed in the zone 12. A solid residue comprising particles of oxide and sulphate thus falls under gravity to the bottom ofthe chamber 2 being collected in the conical portion 16. Operation of the rotary valve 20 is effective to feed these particles into the bed 34 of the fluidised bed reactor 30. If desired, iron sulphate heptahydrate crystals that have been separated from the reacting liquid in the process for forming titanium dioxide by the sulphate route may be fed into the bed 34 through a conduit 39. In addition, the separator 24, when in operation, feeds a small quantity of solids into the conduit 38.After passage through the separator 24 the gas mixture leaving the outlet 22 of the chamber 2 passes into the conduit 26 and then enters the plenum chamber 28 of the fluidised bed reactor 30. The gas flow is effective to fluidise the particles of oxides and sulphates that are estabiished as the bed materials. In view ofthe temperature of the gas mixture entering the chamber 28, furtherthermal cracking of the sulphate takes place. Typically, the gas mixture now incorporating an enhanced volume of sulphur dioxide and oxygen leaves the top of the bed 34 at a temperature of about 1,000 C. It passes out of the outlet 50 into the separator 52.Solids are separated from the gas in the separator 52 and returned to the bed 34 while the resultant gas mixture comprising sulphur dioxide, water vapour, carbon dioxide, oxygen and traces of sulphurtrioxide typically passes to the precipitators, heat recovery unit and driers of a plant (not shown) for producing sulphuric acid from its sulphur dioxide content by the contact process.

In orderto control the quantity of material in the bed 34, a portion of the solids leaving the separator 52 through the outlet 56 is typically discharged from the plant through an outlet 58 and, if desired, may be used in metallurgical processes.

In the event that there is not sufficient heat in the gases leaving the separator 24to maintain an exit temperature of 1,000 C above the fluidised bed 34 a temperature in excess of 1,00000 is created at the outlet of the chamber 40 thereby enabling the temperature of the gas leaving the bed 34 to be maintained at or above 1,00000.

Referring now to Figure 2 of the accompanying drawings, a stream of sulphate waste material from a processformanufacturingtitanium dioxidefrom ilmenite by the sulphate route, typically comprising 70% by weight of water, 1 5% by weight of sul phu ric acid and 15% byweightofinorganicsulphates, at least most of said inorganic sulphates comprising iron sulphate, is subjected to treatment in accordance with the invention. The stream is first passed through an evaporator 62 in which the water content of the stream is typically reduced to about 30% to 70% by weight of the stream. The evaporator 62 typically operates at a temperature in the order of 10000 such that substantially no thermal cracking of the sulphate or evaporation of the acid takes place.

Energy from the evaporator 62 may be recycled from a downstream part of the process so as to reduce the overall fuel requirements.

The concentrated stream exiting the evaporator 62 is passed to afirstinlet66 ofa burner 64. The burner extends into a combustion chamber 61 which is disposed with its longitudinal axis vertical and has the burner 64 extending axially through its top. The burner 64 has a second inlet 68 for commercially pure oxygen and a third inlet 70 for gaseous or liquid hydrocarbon fuel.

In operation, a flame zone 74 is created by the burner. The flame zone 74 extends generally vertically downwards within the chamber 61 and ends above the lower cylindrical portion 76 ofthe chamber 2 in which in operation a residue of solid particles collects, the solid particles comprising inorganic oxide and sulphate issuing from the flame zone 74. The temperature generated in the flame zone 74 is sufficient to crack thermally most ofthe sulphate in the waste material. The resulting gaseous combustion products including sulphur dioxide, carbon dioxide and water vapour leave the chamber 61 through an outlet 84. At the very bottom ofthe chamber 61 there is a plenum chamber 78 whose top is defined by a grid 80. The solid residue collects on the grid 80.The chamber 61 has afirst outlet 82 into which oxides substantially free of sulphate flow from the bed 78. The outlet 84 communicates with a cyclonic solid separator 86 having at its bottom an outlet 88 communicating with a pipeline 90 which also communicates with the outlet 82. In operation, waste solids are withdrawn from the plant th rough the pipeline 90.

The apparatus shown in Figure 2 also includes an auxiliary combustion chamber 92 having a burner 94 disposed therein. The burner 94 has a first inlet 96 for fuel and a second inlet 98 for oxygen-enriched air. In operation, the combustion products from the burner 94, comprising carbon dioxide, nitrogen and water vapour(andsulphurdioxideifthefuel comprises sulphur) pass through an outlet 100 ofthechamber 92 at a temperature in range of 1000 to 1 3000Cwhich is controlled in a manner analogousto that described with reference to Figure 1 and flow along a pipeline 104intothe plenum chamber78.The velocity ofthe combustion products entering the plenum chamber 78 is typically selected so as to be able tofluidise or renderturbulentthe solid residue from the flame zone 74 of the burner 64. The temperature of the combustion products is in any case sufficientto complete the thermal cracking of the sulphate in the residue and the resulting gas mixes with the gaseous combustion products from the flame zone 74.

The separator 86 has an outlet for solids-free gas which communicates with the pipeline 110 leading two a plant not shown for purifying the gas mixture prior to the conversion of its sulphur dioxide content to sulphurtrioxide and the subsequent absorption of the sulphurtrioxide in relatively dilute sulphuric acid to form a more concentrated sulphuric acid product.

In operation ofthe apparatus shown in Figure 2, the gas mixture leaving the outlet 84 of the chamber 61 typically has a temperature of about 1000"C and typicallycontains up to about 25% byvolume of sulphur dioxide.

Claims

1. Amethod of recovering from sulphatewaste material (as hereinbefore defined) a gas mixture including sulphur dioxide, comprising burning a fuel to form a flame zone, introducing sulphate waste material into the flame zone, employing oxygen-rich gas (as hereinbefore defined) to support combustion ofthefuel and to generate aflame of sufficient temperaturetocracksolid sulphate waste material and thereby liberate sulphur dioxide therefrom and separating solids exiting the flame zone from the gaseous combustion products.

2. A method as claimed in claim 1, additionally including the step of pre-concentrating the sulphate waste material by evaporating part of its water content.

3. A method as claimed in claim 1 or claim 2, in which the oxygen-rich gas is pure oxygen.

4. A method as claimed in any of the preceding claims, in which the thermal cracking proceeds at a temperature in the range of 900 to 1,300 C.

5. A method as claimed in any one of the claims 1 to 4, in which a burner, employed to burn said fuel is disposed vertically or at an angle to the vertical, whereby solid residue from the cracking of the sulphatewaste material falls under gravity, and additionally including the step of collecting said residue.

6. A method as claimed in any one of the preceding claims in which solid residue from the cracking of the sulphate waste material contains residual sulphate, additionally including the step of thermally cracking said residual sulphate in a region spaced from said flame zone.

7. A method as claimed in claim 6, in which gaseous combustion products from said flame zone are employed to provide heatforthethermal cracking of said residue.

8. Amethod as claimed in claim 7, in which the gaseous combustion products are employed to fluidisesaid residue.

9. A method as claimed in claim 7 or claim 8, in which additional heatforthethermal cracking is provided by gaseous combustion products from an auxiliary burner.

10. A method as claimed in claim 6, in which said residue is collected and is subjected to further cracking in thesame chamber as one in which saud flame zone is created.

11. A method as claimed in claim 10, in which heatforthe cracking of said residue is provided by hot combustion gas from an auxiliary burner.

12. A method as claimed in claim 11, in which said hot combustion gases from the auxiliary burner fluidises or otherwise renders said residue turbulent.

13. A method as claimed in any one ofthe preceding claims, additionally including the step of converting the sulphur dioxideto sulphurtrioxide and absorbing the sulphurtrioxide in an aqueous liquid to form sulphuric acid.

14. A method of recovering from a sulphate waste material (as hereinafter defined) a gas mixture including sulphur dioxide, substantially as herein described with reference to Figure 1 or Figure 2 ofthe accompanying drawings.

15. Apparatusfor performing the method claimed in claim 1, comprising a burner having a first inletforfuel, a second inlet for oxygen-rich gas, a chamber into which, in use, said burnerfires means for introducing sulphate waste material into the flame zone, an outlet for gaseous combustion products and means for collecting solid particulate residue emanating from the flame zone ofthe burner.

16. Apparatus as claimed in claim 15, additionally including means for pre-concentrating the sulphate waste material by evaporating part of its water content.

17. Apparatus as claimed in claim 15 orclaim 16, in which the burner is disposed vertically or at an angle to the vertical, whereby solid residue from the cracking ofthe sulphate waste material falls under gravity and collects at the bottom of said chamber.

18. Apparatus as claimed in claim 17, additionally including means for passing said residue to a separate vessel and means for passing said gaseous combustion productthrough said residue in said separate vessel, whereby to provide heatforthe thermal cracking of residual sulphate in said residue.

19. Apparatus as claimed in claim 18, additionally including an auxiliary burner and means for passing the gaseous combustion products from said burnerthrough said residue.

20. Apparatusasclaimed in claim 17, additionallyincluding an auxiliary burnerand means for passing the gaseous combustion products from said burnerthrough said residues whereby to crack thermally residual sulphate in said residue.

21. Apparatus for performing the method claimed in claim 1, substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.