Materials
Innovative solution to indirect heating in High-Pressure-Acid-Leach Process ( HPAL )
The following article explores the application of KLAREN self-cleaning fluidized bed heat exchangers as an ideal solution for indirect heating of laterite nickel slurry in High-Pressure-Acid-Leach ( HPAL ) plants .
Text & images by Taprogge GmbH
Self-cleaning fluidized bed heat exchangers are able to solve severe fouling problem in indirect heat transfer due to the circulation of cleaning particles through the tubes of vertical shell and tube exchangers . Moreover , self-cleaning fluidized bed heat exchangers perform with constant heat transfer at low liquid velocities , which makes new designs of shell and tube configurations possible . Furthermore , self-cleaning fluidized bed heat exchangers show a strong shear thinning effect in non- Newtonian liquids . All the above characteristics make the self-cleaning fluidized bed heat exchanger an ideal tool for the indirect heating of laterite nickel slurry in High-Pressure-Acid-Leach ( HPAL ) plants .
Background HPAL Nickel and cobalt are often found together in ores . Among other processing methods , the High-Pressure- Acid-Leach ( HPAL ) process is a hydrometallurgical technique for the extraction of valuable metals , particularly nickel and cobalt , from laterite ores . Laterite ores are a type of ore that is rich in iron and low in sulphur , but also contains significant amounts of nickel and cobalt . The HPAL process addresses the challenges posed by low nickel content and high impurity levels in these ores and has as a major benefit the ability to quickly leach nickel and cobalt from laterite ores . The HPAL process involves subjecting laterite ores to high-pressure acidic leaching , wherein sulfuric acid reacts with the ore under elevated temperature ( roughly 255 ° C ) and pressure conditions ( roughly 50 bar or 725 psi ). The HPAL process works as follows : ore is mined and crushed to create a fine material . This fine material is mixed with water to create a slurry which is then preheated . The hot slurry is heated from approximately 65 ° C to a temperature of 230 ° C and pumped into an autoclave where acid is added . The slurry and acid then react as they flow through several compartments within the autoclave ( temperature increases to 260 ° C ). Upon leaving the high pressure and temperature atmosphere of the autoclave the slurry must be returned to atmospheric conditions . This is accomplished through two or more flash stages . Once the slurry is at atmospheric conditions it is washed and separated at which point the nickel and cobalt can be recovered from the liquid fraction . In most plants direct heating by steam or vapor injection , is used to heat the slurry . Figure 1 shows a flow diagram of a HPAL plant heating cold incoming slurry and recovering heat from the hot discharged slurry , while applying direct contact heat transfer by mixing the cold slurry with the flash vapor produced from the hot slurry in spray-towers / mixing condenser and using high pressure steam in a spray-tower / mixing condenser as the final heating step . The disadvantages of using direct heat transfer by employing direct steam injection is that the fresh slurry feed is diluted with the condensates in the various mixing www . heat-exchanger-world . com Heat Exchanger World April 2024
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