Mechanisms of acid mist formation in electrowinning

Al Shakarji, Reza (2012) Mechanisms of acid mist formation in electrowinning. PhD thesis, James Cook University.

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Abstract

Acid mist is generated during electrowinning in the final stage of hydrometallurgical production processes of metals including copper, zinc and nickel. In electrowinning, oxygen bubbles are formed on the anode. Burst of these bubbles at the solution/air interface produces fine acid-containing liquid droplets which become airborne and disperse throughout the workplace. These droplets are known as acid mist. Acid mist poses a serious health hazard to the workers. It also results in severe structural and equipment corrosion, costing the industry millions of dollars every year.

Prior to this study, little quantitative information was available on acid mist generation and the process parameters that affect its amount. Moreover, while it was well acknowledged that the amount of acid mist was related to the size of the electrolytically generated bubbles, no systematic measurements had been made to quantify bubble size and its relationship with materials and process variables. The present work investigates the parameters that affect acid mist amount and bubble size, acid mist-bubble size correlation, acid mist generation mechanisms, and the effectiveness of typical acid mist suppression techniques.

Five variables are tested for their influence on the amount of acid mist generated, namely, solution temperature, FC-1100 (a surfactant widely used in copper electrowinning to suppress acid mist), electrical current density, solution acidity, and anode age. Among them, solution temperature and FC-1100 show significant effect. More than 90% of the variations in the acid mist generation can be explained by changes made in the two parameters and their interaction. To a lesser extent, electrical current density and solution acidity also affect the total amount of acid mist generated. Anode's age and most of the 3, 4, and 5-way parameter interactions show negligible influence on the amount of acid mist. Overall, acid mist increases with temperature and current density. In contrast, increasing the viscosity of the electrolyte solution decreases the amount of acid mist.

A technique is developed for the size measurement of oxygen bubbles formed on the anode in copper electrowinning. High resolution images of the bubbles are captured by utilizing a high speed camera, super bright LEDs, and a special viewing chamber. Regardless of the operating conditions, bubbles are found to be generated in a wide size distribution ranging from 20 μm to more than 400 µm in diameter with an overall average diameter of about 53 µm. Statistical analyses on the measurement results show that addition of FC-1100 and solution temperature are the two most influential test parameters on the bubble size followed by the age of the anode. In contrast, current density and solution acidity have negligible effect on the bubble sizes.

In the absence of surfactants, the amount of generated acid mist decreases as the average bubble size increases. In contrast, in the presence of a surfactant such as FC- 1100, acid mist shows little correlation with the average bubble size. The significant change in the bubble burst mechanism, due to the presence of surfactant molecules in the solution/air interface, is believed to be the reason for the suppression of the effect of bubble size on acid mist.

Theoretical assessment of the source of acid mist suggests that acid mist is formed almost entirely from airborne jet drops and not film drops as generally understood. Jet drops are created from the disintegration of a liquid jet that is formed after the collapse of the bubble cavity. The diameter of the jet drops, generated during copper electrowinning, is estimated to range from about 0.1 µm up to 54 µm. The maximum acid mist concentration, for a given air speed above the cell, is shown (theoretically and experimentally) to occur not at the solution/air interface but rather at a certain height above the solution line.

Floating plastic objects, such as small balls and beads, are widely used in industry to supress acid mist. The two important factors that influence the performance of such floating barriers are shown to be their coverage of the solution's surface area and their height above the solution line. When comparing single layered floating barriers, spherical shaped barriers reduce acid mist the most due to their higher buoyancy and higher coverage of the solution surface. In general, barriers made of high density polyethylene are 3.4% more effective in reducing acid mist than those made of polypropylene due to their lower density which enables them to be more buoyant in the solution and intercept acid mist more effectively.

Acid mist is reduced noticeably with the addition of FC-1100 to the solution up to 30 ppm. Further addition of FC-1100 makes much less difference in the suppression of acid mist. When FC-1100 is used in combination with floating barriers (such as spheres), the reduction in acid mist further improves by an average of 29% in comparison to the use of FC-1100 alone.

Item ID: 27530
Item Type: Thesis (PhD)
Keywords: acid mists, acidic mists, aerosols, bubble sizes, bubble sizing, copper processing, copper electrowinning, current density, elasticity, electroextraction, electrowinning, FC-1100, floating barriers, health aspects, image analysis, jet drops, oxygen evolution, surface tension, surfactants, viscosity
Additional Information:

Appendix A (data) is not available through this repository.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Al Shakarji, Reza, He, Yinghe, and Gregory, Simon (2011) Statistical analysis of the effect of operating parameters on acid mist generation in copper electrowinning. Hydrometallurgy, 106 (1-2). pp. 113-118.

Chapter 4: Al Shakarji, Reza, He, Yinghe, and Gregory, Simon (2010) Measurement of bubble size distribution in copper electrowinning process by image analysis. Proceedings of Copper 2010: Electrowinning and -refining Copper 2010. , 6 - 10 June 2010, Hamburg, Germany.

Chapter 5: Al Shakarji, Reza, He, Yinghe, and Gregory, Simon (2011) The sizing of oxygen bubbles in copper electrowinning. Hydrometallurgy, 109 (1-2). pp. 168-174.

Chapter 6: Al Shakarji, Reza, He, Yinghe, and Gregory, Simon (2012) Acid mist and bubble size correlation in copper electrowinning. Hydrometallurgy, 113-114 . pp. 39-41.

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Date Deposited: 02 Jul 2013 05:18
FoR Codes: 09 ENGINEERING > 0904 Chemical Engineering > 090408 Rheology @ 50%
09 ENGINEERING > 0904 Chemical Engineering > 090406 Powder and Particle Technology @ 50%
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