THE EFFECTS OF FLOCCULANTS ON THICKENER SCALING
Peter V. Avotins, Larry L. Laviolette, Emiliano F. Repetto
Cytec Industries Inc.
Araujo M. Eli
Settler life varies from a few months to over a year depending on bauxite and liquor compositions as well as process conditions. Flocculant use is integral to settler operation but it's effects on scaling have not been studied. Over the past few years, refineries have started to use hydroxamated polyacrylamides as flocculants in their settlers. It has been found that hydroxamated polyacrylamides can provide a reduction in the volume of scale formed and a change in scale composition. This scale is usually easier to remove from settler surfaces. The net result is that these refineries run longer thickener campaigns.
THE EFFECTS OF FLOCCULANTS ON THICKENER SCALING
Peter V. Avotins, Larry L. Laviolette, Emiliano F. Repetto and Araujo M. Eli
Most liquor streams in the Bayer process are supersaturated with respect to one or more components. As these solutions encounter changes in temperature, solids content and other process conditions, scales of various composition are formed on piping and other reactor surfaces. These have an adverse effect by reducing the heat transfer coefficients significantly and by creating blockages in pipes and valves. Refineries must remove the scales from this equipment at considerable expense in labor and down time.
The characterization of the scales and methods of their removal have received much attention in the literature (Zhong-Lin et. al., 1995), (O'Neill, 1986), (Mueller-Steinhagen, 1994), (Delgado et. al., 1992). Most of these studies address the scales formed in digestion, particularly on surfaces of heat exchangers. However, scaling in thickeners that separate mud from aluminate liquors is also a significant problem.
Thickener scales can build up on the rake, thickener walls and launders. The higher load and friction between the rake and thickener sides or floor increase the torque in gearbox drives. Pieces of scale may break off the thickener walls and cause the rakes to jam. This may shut down the thickener for descaling and repair. These factors have an effect of shortening thickener campaigns. The duration of thickener campaigns varies from refinery to refinery depending on factors such as thickener and rake design, liquor composition, slurry rheology and other process conditions. We have found that the choice of flocculant may also affect the scale formation in thickeners. The use of hydroxamated polyacrylamides changes the scales in a favorable direction so as to increase the duration of thickener campaigns and to facilitate easier descaling.
Scales occur on the rake arms, walls, launders and most surfaces exposed to the liquors in the thickener. They are classified as growth scales or settled scales (Roach and Cornell, 1985). Growth scales occur mostly on the thickener walls and launders. They are generally hard scales, consisting essentially of gibbsite with minor amounts of hematite and other impurities. The settled scales may form from mud slurry that has deposited mainly on the rake arms, and it is held together by precipitates from aluminate liquor. Settled scales also form along thickener walls and edges of launders in areas of relatively stagnant liquor flow. These scales are less dense and softer than growth scales seen in the same locations.
The effect of hydroxamated flocculants versus conventional polyacrylate/starch flocculants is to reduce the rate of scaling of both types of thickener scale. In this paper, we present some observations of thickener scaling where hydroxamated polymers are used as flocculants. These refineries are Alcan in Brasil and Alcoa in Pinjarra in Western Australia.
2.0 ALCAN, BRASIL
When Alcan, Brasil refinery started using SUPERFLOC® HX-200, a hydroxamated polyacrylamide flocculant, several positive responses were seen. The plant throughput increased to full capacity, overflow solids were lowered, and the plant obtained better overflow filtration. In addition, a lower proportion of Catuagasses bauxite could be used in the ore blend to the refinery.
The normal thickener campaigns at the refinery averaged about 5 months for each cycle of operation. After 7 months of operation with HX-200, the thickener was emptied for descaling. The scales on the walls and bottom of the thickener were much thinner than those formed during pre-HX campaigns. The scales on the walls were more compact and rock-hard. Photographs of the scales formed in the two periods are shown in Figures 1, 2, 3 and 4. The photograph in Figure 1 compares thickeners with of growth scales taken during pre and post HX-200 addition campaigns. The samples are 25 cm and 2.5 cm in thickness, respectively. They were taken from about the same location along the thickener wall. Figure 2 shows scale from the HX-200 campaign. It is about 2 cm in thickness and it was easily removed from the wall. Figures 3 and 4 show much thicker scales observed on the floor and walls during the pre-HX campaign.
Samples of scales were analyzed by ICP and the results are shown in Table 1.
The scales formed under previous operating conditions contained higher amounts of iron, silica, and soda. X-ray diffraction patterns identified both samples having gibbsite as the major and hematite as the minor constituents.
The comparative rates of scale growth are 0.2 cm/month during HX-200 use and 2.2 cm/month in pre-HX periods. A comparison of thickener operations during the two periods is shown below:
3.0 ALCOA, PINJARRA
A rapid buildup of settled scale was observed at the Pinjarra refinery during the first part of 1997. In some cases the scale isolated the thickener from the overflow launders and the liquor level raised to the top of the thickeners. The measurement of the liquor level in the thickener is an indirect measure of the scaling rate. Other observations of scaling at Pinjarra include the measurement of scale height in the launders/walls and the use of steel coupons that are inserted in appropriate thickener locations.
The coupons are L shaped pieces of steel about 30 cm in length along the stem and 5 cm along the base of the L. The coupon is 5 cm wide. It is suspended from the top of the thickener and immersed to a depth of about 24 cm. The L faces the direction of liquor flow in the thickener. These coupons stay in service for 1 week. Considerable scale is built up within this time period. It is possible to estimate the growth rate of both settled and growth scales.
A plant test of HX-400, a hydroxamated polyacrylamide flocculant, was started on thickener F-11 in late September 1997. The scaling rates as measured by the % liquor level in the thickener are shown in Figure 5. In this graph, a 100% level indicates liquor at the top of the thickener. As shown in Figure 5, a campaign with plant flocculant was started in mid-May 1997. By August, the level had risen from 20% to 70%. The thickener was taken out of service for descaling. It was returned to duty in early September. HX-400 use started on Oct. 31 and it ran until mid-December. At that point the thickener was shut down due to spikes in the torque. It was found that one of the scrapers on the rake was catching on the launder. After repairs, F-11 was returned to service in early January and it continued to run until June. There was no significant increase in the liquor level during the time of using HXPAM on F-11.
A photograph of some coupons, to measure the scaling rate in F-11, are shown in Figure 6. The coupon on top was in service during the week prior to the start-up with HX-400. Significant scale build-up is seen. The L contains a mound of scale about 4 cm high. The stem of the L also shows considerable scaling. This scale is about 80 mm thick. The coupon on the bottom was removed after the first week of operation on HX-400. It has a small mound of scale, less than 1 cm high. Virtually no scaling is seen on the stem of the L.
4.0 SCALING RESPONSE TO THE USE OF HYDROXAMATED POLYACRYLAMIDES
Since many factors impact scale formation in the thickeners, it is difficult to pinpoint the mechanisms by which HXPAM's allow for the favorable scaling response. One reason may be that overflow suspended solids levels are reduced significantly when comparing thickener operation with HXPAM's versus polyacrylate. In nearly all of the refineries that switched to using HXPAM, the overflow solids content has been reduced by 30% or more. In returning to results from our recent work at Pinjarra, Figure 7 shows the clarities before and after switching over to use of HX-400. The clarities were lowered from about 0.18 g/l to about 0.07 g/l.
The lower solids in the overflow indicate fewer suspended particles in the bulk liquor above the mud bed. This behavior is readily apparent when comparing laboratory-settling tests done in graduated cylinders with hydroxamated polyacrylamides versus polyacrylates. The HXPAM treated slurries settle with a distinct flocc line and relatively clear liquor above the settled mud. On the other hand, tests with polyacrylate give a diffuse interface with no distinct settling line. It is often difficult to gauge the settling rate.
These factors lead us to believe that the bulk of green liquor over the mud bed contains much fewer solids particles when HXPAM is used as the primary flocculant. An examination of the settled scale indicates a structure of mud particles and mud floccs surrounded and penetrated by gibbsite. A higher amount of solids that entrap the liquor may lead to higher rates of scaling, particularly in the more quiescent locations within the thickener.
Another factor in scale formation is the rate of gibbsite deposition on the surfaces of the vessel. Some laboratory experiments were conducted to determine whether HXPAM's have any contribution to the deposition rate. In these experiments, samples of aluminate liquor were agitated for several hours under controlled conditions. Steel coupons were inserted in the reactors. The amount of precipitate was recorded as a weight gain on the coupons upon completion of the experiment. HXPAM flocculant was added to some of the test reactors. The results are shown in Figure 8. The percent weight gain was lower by about 50% in the test where we added 10 mg/l of HX-400. While this experiment may not reflect the actual process conditions, the results do show a substantial reduction in the amount of aluminate precipitation by HX-400. This property may be significant in reduction of thickener scaling by use of hydroxamated polyacrylamides.
Since the invention of hydroxamated polyacrylamides as thickener flocculants for the Bayer process (Spitzer et. al., 1988), a considerable amount of development work has been completed. (Ryles et. al., 1996) At present 18 of the 30 plus alumina refineries worldwide utilize HX polymers in their clarification circuits . As a result of the additional benefits in reducing Bayer process scales , which will lead to significantly longer thickener campaigns, it is anticipated that more refineries will make use of this technology.
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