Reactive crystallization of nickel hydroxy-carbonate in fluidized-bed reactor: Fines production and column design

doi:10.1016/j.ces.2005.08.038

Chemical Engineering Science

Volume 61, Issue 5, March 2006, Pages 1377-1385

https://doi.org/10.1016/j.ces.2005.08.038 Get rights and content

Abstract

This study describes experiments carried out in a fluidized bed reactor (FBR) to observe the active mechanisms occurring during the precipitation of nickel hydroxy-carbonate. The fluidized bed process presented in this paper is suitable for recovery of metal ions from solutions in the form of solid coated onto a silica sand surface. The solution containing the metal ion was pumped vertically upwards into the bed and the precipitating agent was injected horizontally into the bed. The solid material was deposited on the surface of the fluidized silica sand. During the process, particle enlargement takes place by heterogeneous nucleation and/or aggregation. In this work a new approach is presented which is able to control the supersaturation levels in the FBR by multiplying the feed points (FP). The particular focus is to describe the relationship between fines, metal removal in the reactor system and chemical performance of the FBR. The influence of change in the number of feeding points in the process was investigated by monitoring fines produced during the process by particle size distribution (PSD), atomic absorption spectroscopy (AAS) and scanning electron microscopy (SEM). The concentration of fines was found to correlate with the degree of supersaturation, and excess fines production was reduced by increasing the number of feed points.

Introduction

Precipitation (also called reactive crystallization) is one of the oldest chemical engineering operations to have been exploited commercially. The major problem often encountered in this process lies in relating the effect of processing conditions to reactor performance and product characteristics (precipitate morphology, purity and particle size distribution). This understanding will enable the optimization and control of precipitation processes in the mineral processing industry as well as in treatment of effluent streams.

This study concerns the removal of nickel by crystallization from a wastewater stream in the form of nickel hydroxy-carbonate. The appropriate reactor technology here is the fluidized bed reactor. This reactor has a particular application to dilute streams, i.e., hydrometallurgical process effluents or acid mine drainage and the potential to improve metal removal or recovery at lower cost. The fluidized bed reactor (FBR) technology has been developed for softening of drinking water (Graveland et al., 1983), as well as for removal of phosphates from wastewater (Seckler, 1994, Battistoni et al., 2001). This technique has been extended to the removal of heavy metals (Wilms and van Dijk, 1988, Zhou et al., 1999, Guillard and Lewis, 2001; Chia et al., 2004; Swartbooi and Lewis, 2005) using carbonate as a precipitating agent and of fluoride (Aldaco et al., 2005) by using lime. However, there is little understanding of the mechanisms occurring in the reactor. To the best of our knowledge, only two papers in the past have attempted to describe the active mechanisms in a fluidized bed. Seckler, 1996a, Seckler, 1996b, Seckler and vanLeeuwen, 1996c proposed a theoretical model for fine particle aggregation with sand grains and later Battistoni et al. (2001) referring to the work of Seckler and co-workers. Thus, understanding of the mechanisms involved in the fluidized bed is not well described.

The overall objective of this research is to understand the mechanisms leading to the production of fines and thus to be able to present their formation or to promote aggregation of fines. In order to achieve this purpose, two types of information are required: the control of the distribution of the supersaturation throughout the bed and the velocity flow rate which influences the collisions in the whole bed. The study aims, therefore, to demonstrate correlations between the operational conditions and the removal efficiency as well as the active mechanisms. To investigate the influence of these factors, an experimental study with the conditions given in Table 1 was carried out at constant ${CO}_{3} / Ni$ ratio and flow rates, but changing the local supersaturation by changing the number of feed points.

Section snippets

Experimental section

Experiments were carried out at laboratory temperature, $24 \pm 2^{\circ} C$ . In order to minimize carbon dioxide uptake from the atmosphere by liquor and evaporative losses, the column reactor was sealed at the top. All the water used was RO (reverse osmosis) water which was distilled and de-ionized in order to avoid the effect of impurities. Prior to each run the vessel and the stirrer surfaces were thoroughly cleaned with acid, ethanol and finally RO water. If the recycle flow is very slow, liquid will be

Results and discussion

The process efficiency was characterized by the level of fines production in the bed. Fines production was used to establish which fraction of the nickel influent was present as free particles and which fraction was coated on the silica sand. In the FBR, adequate mixing results in a uniform concentration in every part of the bed. Considering the FBR to be a plug-flow reactor, the concentrations of reactants and products change progressively as the materials pass through the reactor. There is no

Conclusion

In this study, experiments using a fluidized bed reactor with added silica seeds were performed, to observe the crystallization of nickel hydroxy-carbonate crystals onto a silica sand surface. The concentration of fines was found to correlate with the degree of supersaturation. Two different configurations were investigated: a system called 2FP i.e., with two feed points and a system called 6FP i.e., with six feed points. Based on the findings obtained in this study, it can be concluded:

(1)
The

Acknowledgements

The authors acknowledge the financial support of the National Research Foundation (NRF) and of the University Research Commission (URC) at the University of Cape Town (UCT). They would also like to express their appreciation to Miranda Waldron for her kind help in the characterization of SEM, EDAX and also to Thabang Pikane and Sagwadi Ngcamu for their kind help with the experiment.

References (15)

R. Aldaco et al.
Fluidized bed reactor for fluoride removal
Chemical Engineering Journal
(2005)
P. Battistoni et al.
Phosphorus removal from a real anaerobic supernatant by struvite crystallization
Water Research
(2001)
M.M. Seckler et al.
Calcium phosphate precipitation in a fluidized bed in relation to process conditions: black box approach
Water Research
(1996)
M.M. Seckler et al.
Phosphate removal in a fluidized bed—I. Identification of physical processes
Water Research
(1996)
M.M. Seckler et al.
Phosphate removal in a fluidized bed—II. Process optimization
Water Research
(1996)
P. Zhou et al.
Heavy metal removal from wastewater in fluidized bed reactor
Water Research
(1999)
L. Chia-I et al.
Removal of Cu(II) from aqueous solution in a fluidized-bed reactor
Chemosphere
(2004)

There are more references available in the full text version of this article.

Cited by (42)

Investigation on minimum fluidization velocity in a modified Geldart's diagram
2023, Chemical Engineering Journal
Fine or ultrafine powders (Geldart Group C) have attracted increasing attention in both industrial and academic scopes due to their very small primary particle size and large specific surface area. These Group C particles are usually regarded as cohesive and non-fluidizable because of strong interparticle forces. With the application of nanoparticle modulation technique, a new type of Group C⁺ particles with reduced cohesiveness has been proved to be able to fluidize. The minimum fluidization velocity (U_mf) is one of the most important parameters associated with a fluidized bed system, and it is generally defined as the superficial gas velocity at which the drag force of the upward moving gas counterbalances the particle weight in the bed. However, the incipient fluidization phenomenon for Group C⁺ particles showed considerable discrepancies with this definition because of their non-negligible cohesive forces. A number of U_mf values available in literatures for Group C⁺, A, and B particles were collected and reviewed in this study to provide a comprehensive scope on the U_mf by particle properties. More importantly, the effects of particle cohesion and gravity on the U_mf were qualitatively analyzed. The following conclusions were highlighted: the U_mf for Group C⁺ particles was controlled by the particle cohesive force, and exponentially increased with the cohesion index (σ*); while the U_mf for Group A and B particles was governed by the gravitational forces, and showed a power function with their gravitational forces, not involved in the interparticle forces. This study endeavors to advance the understanding of the U_mf in the fluidization of each group powders, nonetheless, more studies are still needed to enhance and deepen the current knowledge of the fluidization processes.
Nickel isotope fractionation during precipitation of Ni secondary minerals and synchrotron-based analysis of the precipitates
2022, Geochimica et Cosmochimica Acta
The determination of fractionation factors associated with important biogeochemical processes controlling Ni availability in the environment is necessary to confidently use Ni isotope signatures as tracers in environmental studies. In this paper we present experimental results on Ni isotope fractionation during the precipitation of Ni secondary minerals (Ni hydroxide, Ni hydroxycarbonate and Ni sulfide minerals) that can control Ni fate and mobility in settings where high dissolved Ni(II) concentrations may pose a serious threat to the environment. Results show a preferential partition of lighter isotopes into the solid phase with associated fractionation factors ε of −0.40‰ ± 0.04‰, −0.50‰ ± 0.02‰, and −0.73‰ ± 0.08‰ relative to the hydroxide, carbonate and sulfide systems, respectively. Early kinetically induced isotope fractionation, followed by possible re-equilibration with the reacting solutions was suggested for the three investigated systems. Distortions of Ni-O bonds in the octahedra constituting the structures of Ni hydroxide and hydroxycarbonate minerals could have also contributed to the isotopic fractionation measured in these systems. In contrast, for Ni sulfide system, the magnitude of Ni isotope fractionation seemed to be determined by water and sulfide ligands exchange in solution prior to mineral nucleation. Although there is insufficient evidence to determine whether complete equilibrium occurred in the three studied systems, the fractionation factors reported in this study can provide useful indicators of Ni isotope fractionation associated with fast precipitation of secondary minerals involved in the sequestration of Ni from contaminated environments. These findings also contribute to the characterization of Ni isotope systematics which is still in the early stages of development.
A review on fluidization of Geldart Group C powders through nanoparticle modulation
2021, Powder Technology
Geldart Group C powders with large specific surface area have been applied in a variety of processes. Gas-solid fluidization is a popular technique for processing these particles. However, most Group C particles are not fluidizable due to strong interparticle forces. In recent decades, one effective technique assisting their fluidization has been the addition of small amounts of nanoparticles to reduce their interparticle forces. This paper reviews both theoretical and experimental studies on the gas fluidization of Group C particles, including nanoparticle modulation techniques, surface morphology, and modelling of interparticle forces. More importantly, the fluidization of these coated Group C particles (the so-called Group C⁺ particles) have been comprehensively discussed. New parameters were proposed to analyze the effects of different forces on their fluidization. The performance of fluidized bed reactors using Group C⁺ catalysts is also reviewed, albeit more works are still needed before the extended applications in chemical engineering.
Recovery of Cu(II) from aqueous solution by induced crystallization in a long-term operation
2018, Journal of Environmental Sciences (China)
Citation Excerpt :
Overall the particle size distribution was shift towards larger particle size over time, which also indicated that the copper precipitates could keep on coating on the surface of seed grains at least for over 150 days even the experimental conditions changed, and the induced crystallization in the FBR was favorable to recovery of heavy metals from wastewater. Previous studies investigating the removal of heavy metals in a similar reactor, with carbonate addition, found that deposition of the initial metal precipitate layer was inefficient, and the growth of the metal salt precipitating onto the seed surface was the main factor responsible for removal of the formed metal precipitate from suspension (Costodes and Lewis, 2006; Guillard and Lewis, 2001; Zhou et al., 1999). These results are consistent with the conclusion previously reported by Hille for copper sulfide precipitation in a fluidized bed reactor, using controlled sulfide addition, which was ascribed to the low wetting angle of the quartz surface (Mokone et al., 2012; van Hille et al., 2005).
The feasibility of copper recovery by induced crystallization in a long period (174 days) was investigated in a seeded fluidized bed reactor (FBR). The process was divided into 3 periods according to different influent conditions, and the period III was separated into III-a and III-b due to the adjustment of the molar ratio of CO₃^2 − concentration to copper concentration ([C_T]/[Cu^2 +]). The removal efficiency could exceed 95% and the average effluent copper concentration decreased to 3.0 mg/L. The mean particle size of seed grains with copper crystals coating on, raised to 0.36 mm from initial 0.18 mm. During period III-a, the supersaturation exceeded 2.88 × 10⁴, the removal efficiency decreased to 60%–80% and the particle size dropped to 0.30 mm, due to the generation of fines by homogeneous crystallization and seeds breaking. And the morphology of the crystals on the seed grains changed from rod-like to spherical which lead to the particle size decreasing. In period III-b, the supersaturation was modified by adjusting the molar ratio of [C_T]/[Cu^2 +] to 1.2 from 2. The efficiency was back to 95% and the mean particle size grew to 0.36 mm at the end of III-b, the crystals coating on the seeds turned back to rod-like products of good stability. This study illustrated that the copper salt crystal could keep on growing on the seed grains for over 150 days, the feasibility and controllability of copper recovery by induced crystallization process in FBR were satisfactory, even under the dramatic changes in influent conditions.
Removal of calcium hardness from solution by fluidized-bed homogeneous crystallization (FBHC) process
2017, Journal of the Taiwan Institute of Chemical Engineers
Citation Excerpt :
In this work, the bed height is varied to test the effect of the static height of the homogeneous calcium carbonate seed on the efficiency of hardness removal. The amount of fluidized particles determines the reactive surface for adsorption and nucleation, by which either the local supersaturation is reduced or the formation of fines is reduced [23]. A high static bed promotes solid hold-up, which can reduce interphase mixing.
Calcium is one of the divalent ions contributing to the hardness level of the water. This work describes the removal of calcium ions from aqueous solution using carbonate salts as precipitants and the recovery of homogeneous calcium carbonate crystals via a fluidized-bed homogeneous crystallization (FBHC) process without a heterogeneous seed material. The considered parameters were effluent pH, initial molar ratio of carbonate salt to Ca, up-flow velocity, and cross-section loading. The removal efficiency of Ca hardness reached 95% at the optimal pH of 10–11 and the corresponding crystallization ratio was 88% for initial concentrations of Ca of 50–330 ppm. The FBHC process was effective with a cross-section loading of calcium in the water of up to 4.5 kg/m²/h. The efficiency of Ca immobilization as the crystal grew on the fluidized pellets was greatly improved by adjusting the degree of supersaturation in the range 2–3, resulting in the crystallization ratio (CR) and total removal of Ca (TR) of 88% and 92%, respectively. XRD analysis revealed that the formed crystals comprised two calcium carbonate (CaCO₃) phases—calcite and aragonite. SEM images of the surface morphology revealed that calcium carbonate particles (around 1–2 mm) were formed by the aggregation of fine crystals (around 5 µm).
Effect of power ultrasound on crystallization characteristics of magnesium ammonium phosphate
2017, Ultrasonics Sonochemistry
Citation Excerpt :
It was found that the crystals formed in this area were homogeneous in both size and shape, and the surface of the crystal is flat [26]. MZW is usually presented by testing parameters such as temperature, pH value, and ion concentrations etc. [27,28] In light of that MAP is sparingly soluble in water solution, so the effect of temperature on the solubility of MAP is very limited [29]. As a result, the pH value was finally chosen to represent the effect of power ultrasound on the metastable zone of MAP.
Magnesium ammonium phosphate (MAP) crystallization could be utilized for the recovery of phosphorus from wastewater. However, the effectiveness of the recovery is largely determined by the crystallization process, which is very hard to be directly observed. As a result, a specific ultrasonic device was designed to investigate the crystallization characteristics of MAP under various ultrasonic conditions. The results demonstrated that the metastable zone width (MZW) narrowed along with the rising of the ultrasonic power. Similarly, for the 6 mM MAP solution, with the ultrasonic power gradually enhanced from 0 W to 400 W, the induction time was shortened from 340 s to 38 s. Meanwhile, the crystallization rate was accelerated till the power reached 350 W, and then remained a constant value. It can be observed from the scanning electron microscopy (SEM), the MAP crystal became bigger in size as well as the crystal size distribution (CSD) became broad and uneven, with the increase of ultrasonic power. The results indicate that the crystallization process enhanced by power ultrasound could be used as an effective method to eliminate and recover the phosphorus from wastewater.

View all citing articles on Scopus

View full text

Reactive crystallization of nickel hydroxy-carbonate in fluidized-bed reactor: Fines production and column design

Abstract

Introduction

Section snippets

Experimental section

Results and discussion

Conclusion

Acknowledgements

Chemical Engineering Journal

Water Research

Water Research

Water Research

Water Research

Water Research

Removal of Cu(II) from aqueous solution in a fluidized-bed reactor

Chemosphere