AU663225B2 - Process for the production of quick lime - Google Patents

Description

AUSTRALIA663225

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT (Original) APPLICATION NUMBER:

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COMPLETE SPECIFICATION LODGED:

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PUBLISHED:

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4' b I: 4 NAME OF APPLICANT: ACTUAL INVENTOR(S): ADDRESS FOR SERVICE: INVENTION TITLE: B J CATALANO PTY LTD FRANK TRASK, III KELVIN LORD AND COMPANY 4 Douro Place West Perth WA 6005 "PROCESS FOR THE PRODUCTION OF QUICK LIME" -p t ASSOCIATED PROVISIONAL APPLICATION NOS: PL 4846 filed 22.09.92 The following statement is a full description of this invention including the best method of performing it known to me/us:d 1 2 2 The present invention relates to a process for the 3 production of quick lime.

4 In particular, the present invention relates to a process preferably for the production of quicklime of a high quality andA 6 having a low silica content from limestone or lime sand.

7 It should be noted that references made hereinafter to 8 limestone are to be read to include lime sand also.

9 Many limestone deposits, in particular limestone deposits o 0 10 of relatively recent geologic origin, for example, deposits 11 laid down during the Miocene era are not considered 12 commercially useful for the production of quicklimne.

13 The unsuitability for commercial use of these deposits J 14 stems from the silica content of the deposits. Silica in 15 the limestone, or lime sand, results in the production of 16 quicklime with a high silica content. The presence of the 17 silica in the quicklime greatly diminishes the value of the 18 end product. Generally, levels of silica in limestone of 19 make the product of low value and. uneconomic, to 20 produce. As will be seen later to maintain an acceptably 0 21 low level of silica in the quicklime, it is necessary to 22 process limestone with a silica content of 2% or less.

23 The present invention provides a process which enables 24 limestone deposits containing silica to be processed to pre-fefb(y produce commercially acceptable quicklime hating a reduced 26 silica content.

27 According to one aspect of the present invention there is 28 provided a process for the production of quicklime 29 comprising the steps of passing particulate limestone 30 through a plurality of s_.cessive electrostatic separation i- I I t t 1 3 2 stages and subsequently heating the limestone to form 3 quicklime.

4 It is envisaged that the electrostatic separation stages may comprise a plurality of electrostatic separation 6 plates, a series of roll separators or a combination of 7 both.

8 The electrostatic separation plates used may be provided in 9 the form of pairs of arcuate metallic plates spaced apart e* S 10 from one another and positioned so as to be inclined 11 slightly away from the vertical. The separation plates may 12 be spaced apart from one another such that the distance 13 between the plates is greater towards the bottom of the 14 plates and les i towards the top of the plates.

Pref ebly -e 15 limestone used in the process of the present invention 16 is supplied to the electrostatic plates in a dry condition 17 and sufficiently particulate to enable much of the silica .0 0 18 mixed in with the limestone to be liberated therefrom.

19 Where required, a, deposit may be pretreated by crushing and 20 drying the limesand prior to separation to facilitate the o 21 separation process.

22 The present invention will now be described, by way of 23 example with reference to the production of quicklime from 24 a deposit of limesand. The figures illustrate, schematically, an electrostatic separation unit used for 26 the separation of silica from lime in limestone.

27 Figure la illustrates the electrostatic unit as positioned 28 for an initial pass of limestone; 29 Figure lb illustrates the electrostatic unit as positioned -Tss 30 for a subsequent pass of the limestone, and S. S S

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55 4 Figure ic' illustrates the electrostatic unit as positicned for the final pass of the limestone therethrough.

The limesand has an initial silica content of 12.3% and could thus not be used to produce quicklime of acceptable quality by known methods.

The lime sand is taken and crushed to a particulate material of approximately '00 microns diameter, or sufficiently to liberate the quartz from the limestone.

The particular processing operations required at this stage are very much dependent on the nature of the deposit.

Thus, where the deposit is particulate in nature, little or no processing beyond a screening to obtain the required size fraction may be required. If, on the other hand, the deposit is lithified to a considerable degree, crushing and screening of the material may be necessary. In each case, the skilled addressee would be able to determine the treatment necessary by observation and analysis of the deposit. The crushed particulate limesand is then dried to remove any moisture present. Generally, the drying process may be carried out between 100 and 160°C, although the temperature is not critical and would in each case be determined by the skilled addressee. Drying may be carried out at all temperatures below the calcination point of the limesand particles.

The dried limesand is then subjected to air classification to remove the fraction smaller than 100 microns, The pre-treatment of the lime sand is thus completed and the material is passed to an electrostatic separation unit.

~~l~r~llRC*Tn*i:-~ rh~rilll C S- 5 2 An electrostatic separation unit 10 is illustrated 3 schematically in Figures la, Ib and Ic as a first 4 embodiment of the present invention. The separation unit 10 comprises a pair of electrostatic separation plates 12 6 and 14. The plates 12 and 14 are arcuate in shape and are 7 arranged with a space 16 provided therebetween. The plates 8 12 and 14 are each connected to a high voltage DC power go 9 supply (not shown) in a manner such that plate 14 is connected to the high voltage-positive polarity and plate 11 12 is connected to earth.

S 12 The plates 12 and 14 are moveable relative to each other 13 and the distance between the plates 12 and 14 along the i 14 length of each plate 12 and 14 may be varied. In use, the plates 12 and 14 are arranged to be inclined slightly away 16 from the vertical.

17 In Figure la, the plates 12 and 14 are arranged for the 18 first pass of the pretreated limestone therethrough.

-19 General1y. the plates 12 and 14 are. arranged such that 20 toward an upper edge 17 thereof the plates 12 and 14 are 21 approximately 65mm apart and towards a lower edge thereof 22 the plates are approximately 100mm apart. The plate 14, 23 being the positive electrode is arranged to slightly 24 overlie plate 12, being the negative electrode. A voltage of approximately 15kv is applied across the plates 12 and 26 14. Pretreated limestone material indicated schematically 27 by the arrow 18 is heated to between 70°C and 90°C and is 28 introduced into the space 16 between the plates 12 and 14 29 in such a manner that the material impinges directly on the plate 12, as indicated by arrow 18. The limestone falls 1 6 2 under the influence of gravity whilst being simultaneously 3 subjected to the voltage applied across the plates 12 and 4 14. Silica is attracted towards the plate 14 being the positive electrode, whereas the limestone falls influenced 6 by gravity adjacent the plate 12 being the negative 7 electrode.

8 Towards the lower edge of the separation unit 10 the 9 limestone material is separated into two streams. A first e* 10 stream 20 is uninfluenced by the voltage across the plates 11 12 and 14 and comprises mainly limestone. A second stream se:" 12 22, influenced by the applied voltage, has migrated Luwards 13 the positive plate 14 being the positive electrode. The 14 second stream 22 comprises mainly silica with some 15 entrained limestone.

16 A splitter 24 is placed directly beneath the unit 10. The 17 splitter 24 is placed so as to enable the streams 20 and 22 18 to be collected separately. Typically, on a first pass 19 with .the plates 12 and.14 positioned as described above the 20 splitter 24 would be placed approximately 10mm from the 21 positive electrode being plate 14 and 90mm from the 22 negative electrode being plate 12.

23 The material collected in stream 20 may then be subjected 24 to a second separation stage as illustrated in Figure lb.

For the second separation stage the relative positions of 26 the plates 12 and 14 may be altered tu move the plates 12 27 and 14 further apart at lower edges thereof. The stream 28 is treated as outlined above for the initial separation 29 stage. Again, the splitter 24 is used to collect two streams of material. The splitter 24 may b3 positioned .I t I I 1 7 2 further away from the plate 14 for (the positive electrode) 3 for the second stage. Typically, the splitter will be 4 from the plate 12 and the plates 12 and 14 will be 105mm apart at their lower idges.

6 Successive separation stages may be carried out on the 7 material. At each successive stage the splitter 24 may be 8 positioned closer to the negative electrode, (as shown in *o 9 Figure Ic) plate 12 so as to ensure that only limestone is 10 collected and all silica is discarded.

11 In eac;h case, the number of passes through the separati>n 12 unit 10 and the conditions used during the process will be 13 determined by the type of material being processed and the 14 desired quality of the final produce.

15 Experimental evidence has shown that limestone with an 16 initial silica content of 12-13% can be upgraded to a 17 product containing 1.2% silica with a material recovery 18 rate of 69%.

.9 Further experimental-evidence has demonstrated that with 20 pretreated limestone material heated to temperature of 21 approximately 130°C and having a silica content of 22 approximately 34% it is possible to produce a final product 23 having less than 2% silica after four passes through the 24 separation unit 10 substantially as described hereinabove.

The feedrate for the material through the plates in these 26 experiments was 82 kg/hr.

27 It is envisaged that the above result s may be improved by 28 using five passes through the separation unit 10. Further, 29 results may improve upon additional division of limestone material into different size granules or size IliC 1 8 2 classification prior to the separation process as described 3 hereinabove.

4 In a second embodiment of the present invention one or more separation stages utilising a roll separator may be 6 incorporated. The roll separator typically comprises a 7 horizontal roller which rotates about its axis at a 8 suitable speed. Located in a manner spaced apart from that 9 roller is a plate.

o 10 The plate and roller may be connected to a high voltage DC 11 power supply in a similar manner as plates 12 and 14 of the 12 firsc embodiment. For example, the plate may be connected 13 to the high voltage-positive polarity whilst the roller is 14 connected to earth.

15 Pretreated and heated limestone material similar, if not 16 the same as that described hereinabove is transported so 17 that it feeds onto the roller. The limestone receives an *4 18 electrical charge as a result of a corona discharge from S 19 the plate and is thereby attracted to the roller.

o 20 This charge decays over time, the rate of decay depending 21 on the surface conductivity of a given particle. During 22 this time, the material adheres to the roller and is thus 23 moved by and follows the rotation of the roller.

24 When, as a result of the decay of the charge, the force attracting the particle to the roller is exceeded by the 26 combined effects of centrifugal force and gravity. The 27 particle separates from the roller and follows a ballistic 28 trajectory.

29 The higher surface conductivity of silica results in a different trajectory from that of the limestone. As such, I 1 9 2 through using a splitter it is again possible to collect 3 material in two streams, one of which contains a 4 progressively higher proportion of limestone for the more passes that are made over the roller separator.

6 Again, separate treatment of coarse and fine fractions of 7 the original pretreated limestone would improve the above 8 process by minimising even further the percentage of silica *g 9 in the product.

10 Limestone exiting the final stage of the electrostatic 11 separation process as described for either embodiment is 12 then converted into quicklime in the final stage of the **so 13 process.

14 The conversion to quicklime is carried out in any one of a 15 number of ways which are well known in the art. For 16 example, a fluidised bed kiln, a rotary kiln, a vertical 17 kiln or retort or a suspension system. Any of these can be 18 used singly as a combined preheater and calciner or two or 19. more can be used in combination where, for example, one or 20 more act as a preheater and one or more act as a calciner.

*g 21 The quicklime produced from the limestone treated in the 22 separation process described above will have a 23 correspondingly low silica content and will hence be a more 24 valuable product.

Thus, the process of the present invention may be used to 26 enhance the value of an otherwise commercially unusable 27 product.

:d It is envisaged that the pretreated limestone may be 29 preheated to a temperature within the range of 50°C and 250 0

C.

1 10 2 Modifications and variations such as would be apparent to a 3 skilled addressee are deemed within the scope of the 4 present invention.

In particular, it is envisaged that the parameters used to 6 govern the operation of the electrostatic separation 7 process will be highly dependent on the characteristics of 8 the quality of the limestone added and the end results 9 required.

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Claims (1)

1. 7. A process according to any one of claims 3 to 6, in 6 which the plates have upper edges that are closer together 7 than their respective lower edges. S 8 8. A process according to any one of claims 3 to 7, in 9 which the plates are movable with respect to each other 10 such that their distance apart may be varied. 11 9. A process according to claim 8, in which the position o: 12 of the plates wlLh respect to each other is varied as the 13 passes are repeated. 14 10. A process according to claim 5, in which the position S* 15 of the splitter is varied for each separate pass such that e. 16 as the number of passes increases the splitter is 17 positioned progressively nearer to the negative electrode 18 plate. 19 11. A process according to claim 1 or 2, in which at least 20 one of the electrostatic separation stages is performed 21 using a roll separator comprising a rotating roller and a 22 pl.te electrode whereby the limestone falls onto the 23 roller, the roller and plate having an electrical potential 24 applied thereacross such that two streams may be collected from the limestone, one stream having proportionally less 26 silica than the other so separated. 27 12. A process for the production of quicklime 28 substantially as hereinbefore described with reference to 29 Figures la, lb and Ic. 13 2 3 4 DATED SEPTEMBER 21 1993 B J CATALJANO PTY LTD 6 By their Patent Attorneys 7 KELVIN LORD AND COMPANY 8 PERTH, WESTERN AUSTRAIIA 9 00 0 46. v 0p