ALUMINA QUALITY REQUIREMENTS FOR IMPROVED CELL FEEDING
Evan W. Andrews and Barry J. Welch
Department of Chemical and Materials Engineering,
University of Auckland, Auckland, New Zealand
Since the inherent problems that smelter cells encounter for stable operation are closely linked with alumina feeding, we have been examining the challenges and obstacles that must be overcome in order to continuously feed cells. This is extending the present feeding technology to its limits. Such an approach has the following advantages:
Notwithstanding these advantages, the continuous feeding system is likely to be more prone to blockages because of the low flow rates involved and the small openings required to deliver these flow rates. Problems may also occur if the alumina is of such poor quality that it fails to discharge out of the hopper in a consistent fashion.
This paper addresses these issues and identifies what alumina quality factors will need further consideration by alumina refineries and alumina smelters, should a system of this type be adopted in the future.
continuous feeding, alumina quality considerations.
ALUMINA QUALITY REQUIREMENTS FOR
IMPROVED CELL FEEDING
Evan W. Andrews and Barry J. Welch
The method by which alumina is added into Hall-Héroult cells has changed markedly over the last 35 years. Manual systems have been replaced with automatic "break and feed" systems, which in the last 10 to 15 years have been superseded by point feeders. Consequently, the amount of alumina and the time interval between successive feed actions has become more continuous. Nowadays between 0.5 and 3 kg of alumina is fed into the cell every two to six minutes.
Whilst point feeding systems have been a big improvement on the large dump "break and feed" systems, the discontinuous addition of discrete quantities of alumina into the cell still causes inefficient alumina dissolution, a limited amount of sludge formation and fluctuations in the concentration of alumina in the electrolyte. Use of low ratio electrolytes (1.0-1.2 or 8 to 13 wt% excess AlF3), with reduced alumina solubility, have compounded this problem with even tighter restrictions being placed on alumina concentration levels (Taylor, 1997).
Development of a continuous alumina feeding system is the next limiting step for improving the alumina dissolution and control of the alumina concentration in Hall-Héroult cells and a recent continuous feeding development, consisting of a tapered cone valve mechanism, is described in U.S Patent 5476574, (Welch et al 1995). This system is based upon varying the vertical placement of a tapered cone within a hole at the base of the feed hopper, to vary the discharge rate of alumina into the electrolyte. The flow rate of alumina at set valve positions requires calibration prior to use and thereafter its accuracy is dependent on the alumina properties. Off-line feeding trials at a smelter established that the flow rate errors, as a result of alumina quality variations, were in the order of +/- 20%, (Wai-Poi et al, 1997).
In order to feed alumina more accurately, we have embarked on a programme to further understand the alumina properties that cause flow variability and also to isolate options for practically overcoming these.
2.0 CONTINUOUS ALUMINA FEEDING SYSTEM
A range of flow devices have been described in the literature for measuring powder flow rates. These were examined, and a variation based on the slot flow meter (Harris et al 1997) was adapted so that measurements could be made in the flow rate range used for aluminium smelters.
The laboratory flow meter is comprised of a chamber and connected sensing system (not shown). With calibration, this can be used to determine the discharge flow rate of alumina at each valve position. No prior knowledge of the feed material is required in order to determine.
the new feeder flow rate. It was able to detect changes in flow rates. An in-built self re-calibration system was also designed.
The flexibility of the continuous alumina feeding system was tested for a range of properties by synthesising mixtures. The accuracy of flow rates and the re-calibration system was within ±6% of actual values. Further details of this system will be disclosed at a later date.
3.0 What alumina quality considerations are considered important for Alumina feeders?
In a recent alumina quality survey initiated by the Australian Alumina Quality Workshop, (Welch, 1993), it was identified that the factors that cause the greatest dissatisfaction for smelters operating with point feeders, are:
- attrition resistance,
- aluminium fluoride consumption
- and dustiness
Segregation and attrition quite understandably rank at the top of the list because they lead to variations in the amount of alumina discharged between successive feed dumps, and thus effect the accuracy to which the concentration of alumina in the electrolyte can be controlled.
Use of high surface area alumina grades in conjunction with low ratio electrolytes has been beneficial, but at the expense of increased aluminium fluoride consumption, (Wai-Poi and Welch, 1994). Aluminium fluoride is depleted from the electrolyte on reaction with moisture contained in the alumina feed according to equation ().
Dustiness is linked with the alumina fines content. Reducing the amount of alumina fines is advantageous for improving the air quality in pot lines as well as improving the flowability of alumina inside hoppers. Excessive fines can lead to problems in discharging alumina out of the feed hoppers, even with "break and feed" systems.
Particle size distribution has always been considered a key property. The presence of excessive fines causes variability in flow. When partially aerated, fine alumina materials flow extremely readily, whereas, when there is no aeration the flow can be extremely inconsistent and poor. The situation for fines is complicated by the fact that most often, smelters are using reacted alumina. As shown in Figure 2, whilst a poor quality alumina can undergo degradation and cause an excessive built-up in fines, there is inevitably an increase in the fines contents compared with the primary alumina. The cause of the build-up of the extra fines is:
- Condensation of vapours and particulates leaving the cell.
- Dust arising from alumina feeding because of the high air flow rate within the hooding of the cell (typically 6000 Nm3/hr).
- Particle breakdown within the dry scrubbers.
- Particle breakdown in the alumina transport system.
Further increases in fines can occur through segregation, aggravating the problem.
Recent studies of impurity distributions by Wedde and Co-workers have shown that there are also fine particulates adhering to the large alumina grains and these can be mechanically detached (Wedde et al, 1998). Thus, these fine particulates, can also cause variability in the fines content. From this, it is evident that the problems of fines are not solely linked with the primary alumina supplied by the refineries.
3.1 How well will the continuous feeder rate against these problems?
Particle property changes, as a result of segregation, attrition and fines content variations, will have minimal effect on the performance of the continuous feeding system because the solids flow meter can be used to re-calibrate the feeder flow rates at each valve position.
If there is any consolidation or rat holing of alumina in the feed hopper, then the performance of the continuous feeder is likely to be compromised because it will no longer be possible to discharge the alumina continuously.
Aluminium fluoride consumption is expected to be reduced with continuous feeding because the slower addition rates will enable better pre-heating of alumina in the feed chutes. Pre-heating alumina will also improve alumina dissolution, (Wai-Poi and Welch, 1994), (Kobbeltvedt et al, 1996).
There is likely to be more pot room dust generated with continuous feeding compared to point feed type systems that direct alumina to the hole in the crust using feed chutes. This is because a continuous stream of alumina is expected to evolve more dust fines than multiple alumina dumps at discrete time intervals.
3.2 What other problems are likely to occur?
Oversized material could also effect the performance of this system because of the small size of the openings to which material must pass at the base of the feed hopper. For a 170 kA cell operating with one outlet point per half-hopper, the outlet aperture will be about 25 mm in diameter and the size range of the feed control apertures would span between say 10 and 25 mm. With higher ampere cells, the size of these openings would be increased in-line with the alumina consumption rate.
Steps have been taken to prevent oversized material from fouling the various components of the feed system and will be discussed at a later date. It would still be desirable for alumina refineries to reduce the amount of oversized material in their final product. Smelters also have the responsibility for reducing the amount of foreign material such as gloves and broom bristles that can end up in the alumina stream.
4.0 SUMMARY OF Key alumina quality factors for continuous feeding
The problems of segregation and attrition resistance are not expected to cause concerns for the continuous feeder so long as alumina can flow out of the feed hopper in a consistent fashion. It is for this reason that flowability is rated the number one concern for continuous feeding in Table 1.
Oversized material is not likely to cause problems if a screen is placed in the feed hopper, however, steps should still be taken by alumina refineries and smelters to minimise the amount of oversized material in the alumina feed stream.
The potential savings in aluminium fluoride by pre-heating alumina, as will be possible with the continuous feed process, have not been quantified. It is expected, however, that the overall result will cause less dissatisfaction than is currently the case for point feeders.
The level of pot room dust is expected to be slightly higher for continuous feeding when compared with point feed type systems that transfer alumina from the hopper to the crust in feed chutes.
Application of a continuous alumina feeding system will result in both environmental and economic benefits to aluminium smelters. In order to realise these benefits, steps will need to be taken by alumina refineries and smelters alike, to ensure that the alumina being supplied to the cell, flows readily and is as free as possible of oversized lumps of foreign material.
Harris, B., Davies, C.E., Davidson, J.F., (1997) "The slot flow meter: a new device for continuous solids flow measurement", Chemical Engineering Science, Vol. 52, No. 24 pp. 4637-4648.
Kobbeltvedt, O., Roleseth, S., Thonstad, J., (1996) "On the mechanisms of alumina dissolution with relevance to point feeding aluminium cells", TMS-AIME Light Metals, pp. 421-427.
Taylor, M.P., (1997) "Challenges in operating and controlling the electrolyte in aluminium smelters", Proc. of the 5th International Conference on Molten Slags 97, Sydney Australia, pp. 659-674.
Wai-Poi, N.S., Andrews, E.W., Welch, B.J., (1997) "Development of a continuous alumina feeding system for aluminium reduction cells", 8th International Congress of ICSOBA, Milan, Italy, Vol. 24 (no. 28), pp. 283-289.
Wai-Poi, N. and Welch, B. J. (1994) "Comparing alumina quality specifications and smelter expectations in cells", TMS-AIME Light Metals, pp. 345-350.
Wedde, G., Bjørnstad, E., Sturm, E., (1998) "Potential impact on electrolysis from removal of impurities from enriched alumina", Proc. 6th Australasian Aluminium Smelting Technology Conference and Workshop, Queenstown, New Zealand.
Welch, B.J, Stretch, D.J., Purdie, J.M., (1995) "Continuous alumina feeder", U.S Patent Number 5,476,574, December 19.
Welch, B.J. (1993) "Alumina quality survey" Proc. 3rd International Alumina Quality Workshop, Hunter Valley, NSW, pp 158-66.