Heat Exchanger World magazine April 2024 | Page 36

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Materials

Figure 1 . HPAL for laterite nickel employing direct heat transfer .

condensers , which will result in larger equipment and more acid required for the HPAL process .

Benefits of indirect heating There has always been a drive to apply indirect heating in HPAL plants because of the benefits of indirect heating in comparison with direct heating :

• Increase autoclave production capacity ;

• Reduced acid consumption ;

• Reduced neutralising agent consumption ;

• Recovery of demineralised condensate and process condensate .

Challenges of indirect heating However , while indirect heating methods can be effective in some situations , they come with certain issues and challenges in the context of HPAL :

• Heat Transfer Inefficiency : Indirect heating methods often result in lower heat transfer efficiency compared to direct steam injection . The heat must pass through heat exchangers , leading to potential losses and slower heat transfer rates . This inefficiency can impact the overall performance of the leaching process .

• Temperature Control Challenges : Achieving precise temperature control is more challenging with indirect heating . The heat transfer rate is influenced by factors such as the design of the heat exchangers , the flow rate of the heating medium , and the nature of the slurry . This can lead to difficulties in maintaining the optimal temperature for the leaching reaction .

• Equipment Complexity and Maintenance : Indirect heating systems can introduce additional complexity to the overall process design , including the incorporation of heat exchangers , pumps , and associated control systems . This complexity may result in higher maintenance requirements and increased risk of equipment fouling or scaling .

• Potential for Corrosion and Fouling : The use of indirect heating methods may involve the circulation of corrosive slurries through heat exchangers , which can lead to corrosion over time . Additionally , the slurry may contain impurities that could contribute to fouling or scaling on heat transfer surfaces , reducing efficiency .

Application of KLAREN Technology in HPAL With the KLAREN self-cleaning heat exchanger technology , most of these issues and challenges can be overcome . In addition to the mitigation of fouling effects , several other significant benefits for slurry heating are afforded by the mode of operation of the self-cleaning fluidized bed heat exchangers . Increased turbulence at the slurry / tube interface is imparted by the action of the fluidized particles , resulting in very high heat transfers at relatively low liquid velocities . Furthermore , given the thixotropic nature of most process slurries , the action of the fluidized particles imparts additional shear to the process slurry , resulting in shear thinning effects with a consequent reduction in pressure drop and enhancement of heat transfer across the heat exchanger . The benefits provided by the self-cleaning fluidized bed heat exchanger make it a good option for the indirect heating of laterite nickel slurry for High-Pressure- Acid-Leach ( HPAL ) plants . Figure 2 shows a typical flow diagram for a HPAL plant utilizing KLAREN self-cleaning heat exchangers . This figure shows only two heat exchanger shells in series . One shell contains both preheaters , while a second shell contains the final heater . The advantages of using indirect heat transfer by employing heat exchangers is that the fresh slurry feed is not diluted with the condensates in the various mixing condensers . This then results in a higher utilization of high-pressure autoclaves allowing an increased nickel and cobalt throughput per cubic metre of autoclave volume .

36 Heat Exchanger World April 2024 www . heat-exchanger-world . com