pnuematic conveying in the aluminium industry.pdf

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12Pneumatic Conveying in theAluminum Industry

1 INTRODUCTIONT h e a l u m i n u m i n d u s t ry e m p l o y s p n e u m a t i c co n v e y i n g w i d e l y for i ts mater ia lshand l ing processes. A s with many indus t r ies , there is economy in scale, and soindividual plants tend to be very large. It is still very m uch an expanding industry,and an industry that most large countries in the world l ike to have as part of theirindustrial infrastructure, p art icular ly if cheap p owe r is availab le from hydro-electric sources or from surp lus gas reserves.

The economy of scale is such that alumina is one of the major bulk solidsthat is widely transported around th e world by bulk carrier. A t ports , ship off-loading systems based on p n e u m a t i c co n ve y in g of the mater ia l are c o m m o n l yemployed, such as that depicted in Figure 1.7. Over-land transport is generally byrail vehicles and these often have the capabil i ty of being pressurized to about 30lbf/in" gauge so that the y can be off-loaded by posit ive pressure conveying sys-t e ms in a reasonably short period of t ime .1.1 Systems and ComponentsThe first point to note about a l u m i n a is that it is a very abrasive material. As aconsequence th is mu s t feature prom inent ly in all decis ions made wi th regard to theselection of systems and componen t s .

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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366 Chapter 12

Th is is one of the in du str ies that tends to em ploy f luidized motion convey-ing systems. This type of conv eying sys tem w as in t roduced in section 7 of Chapte r1 and are considered in some detai l in Chapte r 18. Air-assis ted gravity conveyorshave been quite widely used for f i f ty years or more, but the more recent innova-tion of full channel conveyors are gaining wider acceptance for a lumina . A par-ticular advantage of this type of system is that the air r equ i r ements are very lo wand the transport velocity is also very low, and so p r o b l e m s of wear associatedwith a l u m i n a are signif icantly r e d u ce d .This is also an indus t ry where innova to ry pneu m at ic conv ey ing syst ems areemployed. This type of system w as int roduced in section 6 of Chap te r 1 and areconsidered in more detai l in Chapte r 17. Plug fo rming systems based on internalby-pass pipes are probably the most commonly used system. This , once again, isas a consequence of the abras ive nature of the m ate r i a l s conveyed .

This type of system, however, should only be used when actually required,and this is dictated by the grade of the m a t e r i a l . If a mate r i a l has good air re tent ionpropert ies it wil l convey qu i t e naturally in dense phase and at low velocity in aconvent iona l pneu mat ic conve y ing sys tem, and an innovatory sys tem would bequite unnecessary.

With regard to pipel ine feeding devices , the ideal requirement is that thefeeder should have no moving parts , part icular ly if there is a pressure drop acrossth e feeder . Blow tanks, therefore, are wide ly used . If it is necessary to used a ro-tary valve then it wil l h av e to be made of appropriate wear resistant materials, forboth the rotor and casing, and an increase in air leakage with respect to t ime mustbe anticipated for the feeder .

2 MATERIAL GRADEA l u m i n a is another material that comes in a range of grades, and the grades aresuch that the material may be a powder having good air retention propert ies , inwhich case i t may be capable of b e i n g co n v e y e d in dense phase . Al ternat ive ly , if itcomes as a fine granu la r mater ia l with very poor air retention it w i l l probab ly on lybe capable of b e i n g co n v e y e d in di lu te phase in a conve n t iona l pneum at ic convey-ing system . It is a fine div is ion between the two, as was considered in the previouschapter, with regard to the degradat ion of fine granu lar mater ia ls.A l u m i n a in fine powdered form is often referred to as floury a lumina and isgenerally capable of be ing conveyed na tu ra l ly in dense phase and hence at lowvelocity. Fine g ranular alu m ina is of ten referred to as sandy a lu m in a and this isgenerally only capable of be ing conveyed in di lu te phase suspens ion f low. Theconve ying character ist ics for two typ ical grades of a l u m i n a were presented in Fig-ure 9.11 in relat ion to the use of s tepped pipelines for the conveying of diversem aterials . In order to re info rce , at the ou tset , th is impo r tant point of the inf luenceof material grade on pn eu m at ic conveying per formance , these conveying charac-teristics are reproduced here in F igure 12.1.

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Aluminum Industry Materials 367

Conveying LinePressure Drop - l b f / i n 2Conveying

Limit5040

[200 120 100 80

30

20

10030

SolidsLoadingRatioooo

5 0 '

Solids LoadingRatioConveying LinePressure Drop- lbf/in2 /N O

(a)0 40 80 120 160 200

Free Air Flow Rate - ft3/min (b )0 40 80 120 160 200

Free Air Flow Rate - ft3/min

Figure 12.1 Convey ing characteristics for (a) f loury and (b) sandy grades of alumina.A sketch of the two inch n o m i n a l bore pipe l ine through w h i c h these two

materials were conveyed is presented in Figure 12.2 fo r re fe rence . A high pressurebottom discharge blow tank w as used to feed th e ma te r i a l s into th e p ipe l ine .

Figure 12.2 Details of pipe l ine used for the convey ing of the two grades of aluminapresented in figure 12.1.



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368 Chapter 12

As a consequence of the relatively short length of the pipeline, and the highpressure air available for conveying, solids loading ratios of up to 200 wereachieved with the f loury alumina. Values up to about 40 were achieved with thesandy alumina, bu t despite th e h igh pressure a ir available, th e material could n o tbe conveyed in dense phase and at low velocity.2.1 Conveying Air VelocitiesMore detailed conveying data fo r t h i s floury grade of alumina is presented in Fig-ure 12.3, with lines of constant conveying l ine inlet air velocity also superimposed.On this plot a second horizontal axis has been added. This is of conveying line exitair velocity. Since the conveyed material at the end of the pipeline is always atatmospheric pressure, conveying l i ne exit air velocity is directly proportional tofree air flow rate and so both axes apply.

With both conveying l ine in let and exit values of conveying air velocity rep-resented, the magnitude of the expansion of the air through the pipeline can beclearly seen. From points on the 4 5 lbf / in2 pressure drop curve, fo r example, it willbe seen that the conveying air velocity expands by a factor of approximately fourtimes between inlet and outlet. This is due to the fact that absolute values of pres-sure, and temperature, have to be used in all equations associated with th e com-pressible flow of air.

200. 160 120 100Ratioo 40^o50

4 0J30oE 2 0.2'CI10 Conveying LinePressure Drop

- l b f / i n 250 100 , 150 200Free A ir Flow Rate - ftVm in

0 2000 4000 6000 8000Conveying Line Ex i t A ir Velocity - ftVmin

Figure 12.3 Conveying data for floury alumina conveyed through th e pipeline shownin figure 12.2.



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50ooo'40

730

$20_ oI10

Solids Loading RatioConveying

Limit

Conveying Line Inlet A irVelocity - ft/minsure Drop - l b f / i n 2

0 50 100 , 150 200Free A ir Flow Rate - fP /min0 2000 40 00 6000 8000

Conveying Line E\\t A ir Veloci ty - f t j/minFigure 12.4 Conveying data for sandy alumina conveyed through the pipeline shownin f igure 12.2 .

Similar data for the sandy g rade of a l u m i n a is presen ted in Figure 12.4. Withthis mater ial the m i n i m u m c o n v e y i n g air veloci ty was always above 2000 ft/min,and although the m i n i m u m v a l u e of conveying air velocity reduced sl ightly withincrease in air supply pressure , it was only marg inal .

3 LOW PRESSURE CONVEYINGAs ment ioned before , low pressure d i lu te phase conveying data i s genera l ly in-c luded in the conve ying ch arac te r is t ics der ived wi th h igh pressure conv eying fa-c i l it ies , and so th is data i s equ al ly va l id . Care must be exerc ised , however, in en-suring that the approp riat e m in im um convey ing air velocity is used .

Both ca lc ined a lumina and hydrate of alumina have been conveyed throughthe Figure 10.16 pipe l ine of two i nch nomina l bo r e and 110 feet long. The lowpressure conveying charac te r is t ics for the se tw o mater ia ls are presented in Figure12.5. In terms of convey ing capa bi l i ty there i s li t tle dif fe rence between the twomater ia ls . T h e hyd ra t e o f a l u m i n a shows a t e n d e n c y to a p r e ss u re m i n i m u m p o i n tat low air f low rates and so the mate r i a l f low rate in th i s r eg ion is sl ightly lowerthan tha t fo r the ca lc ined a lu m ina .There is also a s l ight d i f fe rence in m i n i m u m c o n v e y i n g air velocities be-tween the two materials. That for the ca lc ined a lumina is about 2300 f t /m in andthat for the hydrate of a l u m i n a is abou t 2500 f t /min .



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370 Chapter 12

ooo7 6_c'f - *uta 4oi3 3fc" c 3|2S 1

0

Conveying L inePressure Drop- Ihf/ in2 S o l i dsLoading

Rat io

(a)40 60 80 100 120Free A ir Flow Rate - f t3/min

0 716.Q7 5oC 3 ilg

3

0(b)

Solids/Loading^ D

I60S40

2 0Hydrate ofA lum ina

40 80Part icle Size - micro n

120 160

Figure 12.6 Cu mu lat ive par t ic le s ize dis t r ibu t ions fo r a lum ina m ate ria ls in figure 12.5.

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T he mean particle size for the ca lc ined a lum ina w as about 66 /mi and thatfo r th e hydrate of a lumina w as about 60 jum. Granular materials , such as these,generally do not have suff ic ient air retention at this mean part icle s ize for them tobe capable of being conveyed in dense phase in a conv ent ional conveying sys tem.It is unl ike ly , therefore , that e i t he r of these ma ter ia ls cou ld be conveyed in densephase , even i f a m uch h ighe r ai r supp ly pressure were a vai lab le . The par t ic le den-sity for th e calcined alumina w as about 2 45 lb / f t j and t ha t for the hydrate of a lu-m i n a was about 150 lb / f tj .

4 HIGH PRESSURE CONVEYI NG

Both the calcined alum ina and the hydrate of a lum ina have been conveyed in h ighpressure conveying facilities and it wi l l be seen that they could not be conveyed indense phase and at low velocity, de spite th e high pressure. Data on a n u m b e r ofother materials, such as a l u m i n u m fluoride, fluorspar and cryolite is also pre-sented.4.1 Calcined AluminaConveying character is t ics for calcined a l u m i n a conveyed through the Figure 7.13pipel ine of th ree inch no m ina l bore are presented in Figure 12.7 . The m inim umvalue of con veying air velocity for the material was a bout 2300 f t /min.

25

20

u

II1 0OJI5

Sol ids Loading Rat io

Conveying Line PressureDro p - Ibf / in

0 100 200 300 400Free Air Flow Rate - ft3 /min

Figure 12.7 Conveying character i s t ics fo r calcined alumina conveyed through th epipel ine shown in f igure 7 .13 .



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372 Chapter 12

It wil l be seen that th e maximum value of solids loading ratio for the cal-cined alumina w as just over 12 in Figure 12.7 and that a similar value was ob-tained for the material in the low pressure test facil i ty in Figure 12.5a. This is dueto the fact that the material could only be conveyed in dilute phase suspensionf low in both cases, and that the pressure gradients for the two pipelines were verysimilar. The Figure 10.16 p i p e l i n e was 110 feet long and the maximum value ofpressure drop was 8 Ibf / in 2 , and the Figure 7.13 pipeline was 310 feet long with amaximum pressure drop of 25 Ib f / in 2 . T h e approximate factors of three in terms o fp ipe l ine length and pressure drop cancel each other out.

F or reference and comparison purposes, a number o f other materials con-veyed through th e Figure 7.13 pipeline were presented in Figures 11 .10 to 11 .12 ,including potassium sulfate and dicalcium phosphate. These show a very widerange of conveying capabilities.4.2 Hydrate of AluminaA sketch of the high pressure pipeline faci l i ty in which the hydrate of alumina wasconveyed is presented in Figure 12.8. It was a two inch nominal bore pipeline, 320f ee t in length and incorporated thirteen 90 bends, having a bend diameter, D, topipe bore, d, ratio of about 24:1. Once again the material was fed into the pipelineby means of a high pressure, top discharge blow tank, having a f lu id iz ing mem-brane.

The conveying characteristics for the hydrate of alumina conveyed throughth is pipeline are presented in F i g u re 12.9.

Pipeline :length = 3 2 0 f tbore = 2 inbends = 13 x 90

Figure 12.8 Sketch of pipel ine used for the high pressure conveying of hydrate ofalumina.

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Aluminum Indus try Ma terials 373

16

12

_ ott,ua 4S

Solids Loading Ratio

Conveying Line Pressuredrop - I b f / i n 2

50 100 150Free Air Flow R a te - ff / m in

200

Figure 12.9 Conveying characteristics for hydrate of a lumina conveyed through th epipeline shown in figure 12.8.As with the ca lc ined a l u m i n a , the hydra te of a l u m i n a co u l d not be conveyed

in dense phase either, despite the avai lab i l i ty of high pressure air . The m a x i m u mvalue o f solids loading rat io w as a b o u t 16 , w h i c h is s i m i l a r to tha t achieved in thelow pressure conveying tr ials reported in Figure 12 .5b , but this is due once againto the c o m m o n a l i t y of pressure grad ien ts be tween the two sets of data. The m i n i -mum value of convey ing air ve loc i ty was about 2500 f t /min once again.

4.3 Aluminum FluorideSimilar data fo r a l u m i n u m f l u o r id e conveyed th rough th e Figure 12.8 pipeline ispresented in Figure 12.10. It w i l l be seen tha t the re is little d i f f e r ence be tween th econveying capabi l i ty o f the a l u m i n u m f luor ide and the hydra te o f a lumina . Lowveloci ty , dense phase conveying of th is mate r ia l was not a possibil i ty in the con-veying system employed , as w i t h the ca lc ined a lumina and hydra te of a l u m i n areported above.

The minimum conveying air velocity for the a l u m i n u m f l u o ri d e was sl ightlyhigher at 2600 ft/min and m ater ia l f low ra tes w ere s l ight ly lower than those for thehydrate of alumina . The bu lk dens i ty of the hydra te of a l u m i n a was about 75 lb/ft3and that for the a l u m i n u m f l u o r id e w as abou t 90 lb /f t3. Such consis tency in theconveying charac te r is t ics for a g r o u p of dif fe rent m a t e r i a l s is u n u s u a l .



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374 Chapter 12

ooo

16

12

_o

Sol ids Loading R at io

Convey ing Line Pressuredrop - l b f / i n 2

50 100Free A ir Flow Rate - f t3/min

150 200

Figure 12.10 Conveying character is t ics fo r a l u m i n u m f luor ide conveyed through th epipeline shown infigure 12.8.4.4 FluorsparA sketch of the high pressure p i p e l i n e faci l i ty in which fluorspar w as conveyed ispresented in Figure 12 .11. I t was a two inch nominal bore pipeline, 2 30 feet inlength and incorporated nine 90 bends, having a bend diameter, D, to pipe bore,d, ratio of about 24:1.

Pipel ine :length = 2 3 0 f tbore = 2 inbends = 9 x 90

Figure 12.11 Details of pipel ine used fo r high pressure conveying of f luorspar.



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T h e fluorspar was f ed in to th e pipeline by means o f a high pressure top dis-charge blow tank, having a f l u id i z in g membrane. The fluorspar had a particle den-sity of about 230 Ib/ft3 and a b u l k density of about 100 Ib/ft3 , which is the highestamong the group of materials considered in this chapter. The mean particle size ofthe material was approximately 66 micron. At this particle size the alumina couldnot be conveyed in dense phase, but the fluorspar had a degree of air retention.T h e conveying characteristics for the f luo r spa r conveyed through th e Figure 12 . 11p ipe l ine are presented in Figure 12 .12 .

From Figure 12.12 it w i l l be seen that th e fluorspar w as able to be conveyedat solids loading ratios of up to about 70. This is very much an intermediate valueof solids loading ratio and it is suggested that it is another case of 'medium phase'conveying. This was referred to in the previous chapter with regard to sodium su l-fate in Figure 11.12 , and in Chapter 10 with regard to pulverized coal in Figure10.25.T h e minimum value o f conveying a ir velocity for the fluorspar w as about1400 f t /min for conveying l ine in le t air pressures above about 2 0 psig. At lowpressure, and for the di lu te phase suspension f low of the fluorspar, th e m i n i m u mconveying air velocity was about 2500 f t /min .

30

20

g 10< 5

.70 60Solids Loading

Ratio

Conveying LinePressure Drop- lbf/in 2

1 1 I

15

10

50 100Free Air Flow Rate - f tVm in

150 200

Figure 12.12 Conveying characteristics for f luorspar conveyed th rough th e pipe l ineshown in figure 12.11.

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376 Chapter 12

In the next chapter, c on ve yin g data for barite , bentonite and cement, eachconveyed through the F igure 1 2 . 1 1 pipel ine, were all capable of being conveyed atsolids loading ratios of well over 100 and at c o n v e y i n g air veloc ities down to 600ft/min. Then in Chapter 14 data for si l ica sand h avin g a mean particle size of ap-proximately 70 micron is presente d and the m ax im um val ue of solids loading ratiois about 25, with a corresponding conveying l ine inlet air velocity of about 2300ft /min. These are typical of the o perating lim its for dense and dilu te phase convey -ing for high pressure conveying in a pipel ine of this length .4.5 CryoliteCryolite is variously referred to as crushed bath and bath mater ia l . It has a meanparticle size typically about 0-1 inch but this depends upon the crushing process.T he material generally has a very w ide particle size distribution and often containsa high proportion of f ines. The material reported here had a top size of/2 inch.One of the pipelines through which this cryolite was conveyed is shown inFigure 12.13 for reference. It was two inch nominal bore, 165 feet in length andcontained eleven 90 bends, each having a D/d ratio of about 6:1. The materialwas fed into the pipe l ine by m e a n s of a high pressure blow tank. Since th e mate r ia lhad such a large mean particle size and contained large l umps , in addit ion to beingvery abrasive, a blow tank was an ideal feeder for the cryolite .Conveying characteristics for this cryolite conveyed through the Figure12.13 pipeline are presented in Figure 12.14. A s expected, th e material could onlybe conveyed in dilute phase suspension flow. Despite the large particles in thematerial, and a particle density of about 190 lb/ft j, the m inim um conveying airvelocity was-about 2800 ft /min. Th is relatively low valu e of pick-up velocity, forsuch a material, is he lped significantly by the fact that the material had a very wideparticle size distributio n and a large proportion of fines.

Pipeline :length = 1 6 5 f tbore = 2 inbends = 1 1 x 90 'D /d = 6

Figure 12.13 Sketch of pipe l ine used for the high pressure conv eying of cryolite.



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Solids Loading Ratio16

12oooI"eST, 4

Convey ing L ine PressureDrop - I b f i ' i n

0 50 100 150 200Free A ir Flow Rate - i tVmin

Figure 12.14 Conveying characterist ics fo r cryol i te conveyed through th e pipel ineshown in f igure 12.13.

A s a consequence of the h i gh conveying air velocities required fo r convey-ing, and the abrasive nature of the material, it is generally recommended that wearresistant pipeline should be specified for conveying pipelines. Either alloy castiron or basalt lined pipe would be suggested. All bends in the pipeline would alsohave to be similarly reinforced, and possibly with alumina ceramic materials forthe greater wear resistance r eq u i r ed .

Straight pipeline is not generally as vulnerable to erosive wear as the bendsin the pipeline, but when large particles have to be conveyed the problem is exac-erbated. In being conveyed through a horizontal p i p e l i n e th e gravitational force onlarge particles is such that they tend to 'skip' along the pipeline and so low angleimpact of the particles against the pipeline occurs on a regular basis. This wearmechanism is considered in detail in Chapter 20.

This cryolite has also been conveyed through the Figure 7.13 pipeline,which is three inch nominal bore and 310 feet long. In order to convey th e cryolitethrough this pipeline, with h i g h pressure air, a free air f low rate of 600 f t Vmi n w asprovided, rather than the 400 f r Ym i n that was used for other materials that havebeen conveyed through this p i p e l i n e and reported here. The conveying characteris-tics are presented inFigure 12.15. With a pick-up velocity of 2800 ft/min, convey-ing was possible with air supply pressures up to 35 psig with 600 f tVmin of freeair available.



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378 Chapter 12

40

ooo30

(2 20oE13|10C S Solids Loading RatioC o n v e y i n g Lin e PressureD r op - Ibt7in2

100 200 300 400Free A ir Flow Rate - ft3 /min

500 600

Figure 12.15 C onve y ing characteristics fo r cryolite conveyed through th e pipe l ineshown in f igure 7.13.

Figure 12 .15 clearly shows th e influence of free a ir f low rate on the convey-ing capability o f dilute phase conveying systems. With only 400 f tVmin themaximum value of air supply pressure that could be used would be about 20 psig,regardless of the fact that air was available at 100 psig. With only 200 ft3/min al-most nothing could be conveyed through the pipeline at all.

The conveying characteristics for calcined alumina conveyed through thissame pipeline were presented earlier inFigure 12.7. With only 400 ftVmin of freeair available fo r conveying, bu t with a lower pick-up velocity o f 2300 f t /min, airsupply pressures up to about 25 psig could be employed. If the two sets of data arecompared it wil l be seen that a maximum of about 27,000 Ib/h of cryolite could beconveyed with a pressure drop of 25 lb f / in 2 , but only about 23,000 Ib/h of calcineda l u m i n a could be conveyed with the same pressure drop.

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