INVESTIGATION OF THE AGITATION EFFECT ON SCALE GROWTH IN MIXING TANKS

M. Davoody1, L. Graham1, B. Nguyen1 and J. Wu1*

1 CSIRO Fluids Engineering Laboratory, Clayton, Australia

Corresponding author: Jie.Wu@csiro.au

ABSTRACT

A novel approach is proposed for the numerical evaluation of scale thickness and its distribution in a mixing tank. The methodology applied in the current study consists of two major phases: identifying an accelerated process to grow scale under controlled conditions, and physical analysis of the grown scale on the tank wall for different tank design options. For the former, the selected chemical system was based on CaO where scale forms in a reasonable time. The subsequent analysis of scale growth was achieved in a purpose-built tank that could be disassembled and measured with a coordinate measuring machine (CMM) thus giving the distribution of the scale thickness over the tank walls.

This approach was used as a qualitative and quantitative study examining the effects of the flow velocity, and baffle configuration on the pattern of the scale formed in mixing tanks. In conventional baffled tanks it has been observed that the overall mass of scale declined with an increase in the impeller speed. At the same time, results indicate when close to the liquid surface, the average scale thickness at higher impeller speeds increases, implying that the build-up of the scale at the near-surface zones becomes thicker as the speed increases.

An unbaffled tank exhibited much better scaling performance when compared to the baffled case, which can be attributed to the increased flow velocity near the walls in the absence of baffles. It is hoped that results obtained by this approach can be used to inform the design or retrofitting of mixing tanks in the alumina industry to minimise the scale growth.

1. INTRODUCTION

Scale build-up in alumina Bayer processing costs the industry millions of dollars in the form of increased capital expenditure, reduced capacity and production losses during de-scaling operations. It is a major issue in alumina refineries, particularly in agitated tanks. It has been reported that in alumina refining using the Bayer process, scaling causes restrictions in liquor flow in vessels and pipes, decreases in heat exchanger efficiency due to increased heat transfer resistance, and increases in energy consumption (Barnes et al., 1999).

While the majority of the available literature focuses on the application of chemical anti-scalants, there has been much less work reported on preventing scale formation through optimising the design of agitated tanks. This is mostly due to the lack of quantitative information on the scale growth behaviour on the tank walls under controlled conditions for the various hydrodynamic design options.

Scale is a significant issue in many minerals industry processing (Figure 1). It leads to

significant costs in performance reduction as well as downtime and cleaning of scaled components.

The present work aims at introducing a novel and reliable approach to grow, study, and quantify scale thickness and distribution in a stirred vessel. The effect of the configuration of the tank on the scale distribution is then investigated, particularly the presence or absence of baffles.

Figure 1. Example of industrial scale in a tank

2. EXPERIMENTAL METHOD

The stainless steel tank used in this work was 10 cm (T) in diameter. Four equally spaced baffles with the width of 1 cm (0.1T) were attached to the internal wall along the entire depth of the tank to minimise vortex effects. The agitation was provided by a Lightnin A310 impeller attached to a shaft that was placed on the vertical axis of the tank and driven by a motor. Impeller clearance from the bottom of the tank was 30 mm. The Lightnin A310 impeller is an axial-flow impeller widely used in the mineral processing industry. Of note is that it is the impeller of choice in several mineral processing operations that have high maintenance costs associated with scale removal (for example, precipitation and neutralization tanks). Thus, the A310 impeller was used to provide agitation in the reactor used in this study as a representative of full-scale practice.

The tank was designed and fabricated in such a way that it could be disassembled into nine segments including a base, four walls, and four baffles (Figure 2).

The baffles could be altered to change the tank to an unbaffled tank of the same diameter. The tank has been engineered to be leak-proof as it was essential to ensure the solution inside would not leak into the water bath surrounding the tank during experiments conducted at high temperature.

(a) (b)

Figure 2. (a) The Disassembled tank fabricated at CSIRO's workshop (b) the whole tank

The scale growth on the walls were physically scanned using a Coordinate Measuring Machine (CMM) after a thin protective coating was applied to protect the scale during measurements. Before the main runs, the surface profile of one of the plain reactor wall segments was documented using a Sheffield Discovery II CMM with a measurement accuracy of ±1 μm and a system accuracy of ±6 μm. Once the reference coordinate values were recorded and stored, the tank was

reassembled and placed into the water bath for the scale growth runs. Once scale formed, the reactor was disassembled, scale was coated with spray paint, and the profile of the coated scale on the wall segment was measured using the CMM. The coordinate values of the scaled wall segment were compared against the original ones, based on which the thickness and the distribution of the scale was determined. Figure 3a shows the wall segment arrangement on the CMM, and Figure 3b provides a closer view of the wall segment during the scan. Further details are provided elsewhere (Davoody et al., 2017).

(a) (b)

Figure 3. (a) Wall segment arrangement under CMM (A: CMM controller, B: Vice, C: Spray painted wall segment and D: CMM probe). (b) Close-up view of the setup during the scanning operation

3. RESULTS AND DISCUSSION

Scale test results

Figure 4 illustrates the typical pattern of grown scale on the walls of the reactor after 60 minutes of operation. For quantitative measurement, the tank was disassembled after each run, and each segment was separately coated and individually analysed.

Figure 4. The grown scales on the walls of the mixing tank for the baffled case

Figure 5 shows a visual comparison of the scale distribution for a wall segment, with CMM results, presented as 3D surface plot as generated by Paraview 5.3.0 software. It is evident that CMM can read and map the scale thickness throughout the wall surface. In this Figure, Max, Average, and Sum of scale refer to the highest value of scale thickness in a wall segment, the average scale thickness value based on the total number of readings on a wall segment (12600), and sum of the total scale thickness values recorded by the CMM on a given wall segment, respectively. The scale-map in Figure 5 shows that scale formation is dominant in the top region of the wall segment near the liquid surface and insignificant at the tank bottom and close to the impeller region.

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(b)

Figure 5. (a) Scale distribution on the surface of a wall segment of a baffled tank, (b) the quantitative map

Experiments were repeated in an unbaffled tank under similar operating conditions, and results are shown in Figure 6.

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Figure 6. (a) Scale distribution on the surface of a wall segment of an unbaffled tank, (b) the quantitative map

It is clear that the scale present in the unbaffled tank is thinner and sparser compared to that found in the baffled tank.

By comparing the average values in Figures 5 and 6, it can be estimated that scale formation on a wall of a mixing tank may be reduced up to 46% simply by operating in unbaffled mode.

To further analyse the scale profile on the tank’s surface, a term was defined as Average Scale Thickness (AST)Y that determines the average values of scale thickness in every 10 mm of the tank height (in the Y direction). Figure 7 presents the (AST)Y values as a function of tank height for segment 1 of the baffled case.

Figure 7. (AST)Y values as a function of liquid height for Segment 1

The (AST)Y numbers in Figure 7 indicate that the scales have relatively low average thickness values up to liquid height of 60 mm. In this range, the strong turbulence generated by the impeller prevents/mitigates the scale growth. Above this region, the (AST)Y values increase significantly and reach a maximum value of 0.618 at the level of 140 mm.

The variability between tank segments is shown in Figure 8.

Figure 8. Scale thickness and distribution on four wall segments of a mixing tank

Figure 9 provides a comparison between the (AST)Y values for all four segments at different heights from the data presented in Figure 8. It is clear that all segments consistently exhibit the highest accumulation of scales on their top regions where the flow is slower in motion.

Figure 9. AST values as a function of the tank height for all segments

The maps depicted in Figure 8 also suggest that scale is more dominant on the left hand side of each wall segment when compared to that on the right hand side.

Considering the clock-wise rotation of the impeller, it may be commented that scale build-up is strongest in the left hand top corner of each panel, on the downstream side of each baffle, which forms a more quiescent dead zone.

The images in Figure 8 reveal another important aspect to the scale growth pattern. It is very clear that grown scale on all four segments have exhibited different patterns of growth and distribution. Although there is consistency in distinguishable regions (for example, impeller region vs stagnant region), the scale distribution is nevertheless to some extent random and chaotic.

Industrial scale

Results from a scale test using a real chemistry system from the industry are shown in Figure 10 for both baffled and unbaffled cases. The experiments in both tanks were conducted under similar impeller speed. In both cases, it was observed that the bottom region of the reactor was almost free of scale, whereas the scale formation on the top section was quite

noticeable. The same generic behaviour of scale formation is shown as in the test work presented earlier with the unbaffled tank showing less scale.

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(b)

Figure 10. Photographs of scale from baffled (a) and unbaffled (b) tanks. Industrial material from operating plant

4. CONCLUSION

A qualitative and quantitative study on the scale formation in mixing tanks was conducted. A purpose-built tank that could be disassembled has been demonstrated to provide quantitative scale growth distributions when wall segments are scanned using a CMM. The CMM readings when presented as 3-D graphs of scale thickness and distribution, enable a quantitative comparison of scale growth for two tank configurations, with and without baffles. It was observed that the bottom region of the reactor was almost free of scale, whereas the scale formation at the top part was quite noticeable. It was also noticed that the overall mass of scale would reduce

significantly by operating the tank in unbaffled configuration. Similar results were obtained from tests of an industrial sample.

5. REFERENCES

Barnes, M.C., Addai-Mensah, J., Gerson, A.R., 1999. The mechanism of the sodalite-to-cancrinite phase transformation in synthetic spent Bayer liquor. Microporous and Mesoporous Materials 31, 287-302. Davoody, M., Graham, L.J.W., Wu, J., Youn, I., Raman, A.A.A., Parthasarathy, R., 2017. A Novel Approach To Quantify Scale Thickness and Distribution in Stirred Vessels. Industrial & Engineering Chemistry Research 56, 14582-14591.

6. ACKNOWLEDGEMENTS

One of the authors (M.D.) gratefully acknowledges the support of the Australian Government Research Training Program (RTP) Scholarship through RMIT University. Appreciation is also extended to Mr. Dean Harris, Mr. Greg Short, and the CSIRO Clayton workshop.