The effect of a morphology modifier on the precipitation behaviour of nickel powder

The effect of a morphology modifier on the precipitation process of nickel powder was investigated in order to develop an understanding of its mechanism of action. Experiments were conducted on a pilot-plant scale using a 75-L autoclave with modifier dosages in the range of 0.25-5 vol%. Samples were collected from each successive batch reduction within a cycle and the powder was separated from the liquor before being washed and dried for subsequent analysis.The active particle rate processes were identified by transforming the particle size distribution (PSD) data into moments and from the change in surface area as measured by the BET method. Scanning Electron Microscopy (SEM) micrographs of the powder were used to observe the powder morphology and to validate the proposed particle rate processes and mechanism of action of the modifier.Evolution of the first moment (m0) and third moment (m3), equivalent to the total number of particles and volume, respectively, indicated that growth and aggregation were the major particle rate processes at a modifier dosage of 0.25 vol%. Breakage became apparent at dosage levels above 0.25 vol%. A decrease in BET surface area was noted in each cycle, indicating the presence of aggregation. The magnitude of decrease in the surface area indicated that the extent of aggregation decreased with increasing modifier dosage.SEM micrographs revealed that the powder was compact and aggregated at modifier dosages between 0.25 and 3 vol% and that loose porous powder was produced at 5 vol%. The modifier was found to inhibit growth, resulting in the formation of weaker agglomerate bridges leading to shear-induced breakage. This led to an increase in the surface area available for reduction. However, the effect of increased surface area in promoting reduction was outweighed by growth inhibition above a modifier dosage of 1 vol%. Thus, the number of attainable batch reductions increased when the modifier dosage was increased from 0.25 to 1 vol% and decreased with further increases in modifier dosage.     

The influence of iron on the precipitation behaviour of nickel powder

The effect of iron on the precipitation behaviour of nickel powder was investigated. Reduction experiments were conducted on a 0.5 L laboratory autoclave fitted with a Teflon reaction beaker and a double impeller configuration consisting of an upper axial impeller and lower Rushton turbine. Reduction was conducted in the presence of a morphology modifier at a temperature between 180 and and 2800 kPa pressure using a synthetic nickel ammine sulphate solution (, free NH₃:Ni²⁺ and (NH₄)₂SO₄:Ni²⁺ molar ratios of 2.0-2.1 and 2.2, respectively). Nickel seed was used to initiate reduction and iron was added to the reduction solution as ferrous sulphate solution (acidified to pH 2.5 to prevent oxidation) to give a reduction solution with Fe²⁺ concentration of 6, 20 and 200 mg/L. The effect of iron was investigated by studying the evolution of the moments, volume or mass moment mean D(4.3), number based mean size , nickel depletion rate and population balance in the absence of sampling between batch reductions. Iron was found to act as a growth promoter and nucleation agent through reversible adsorption and hydrolysis on the surface of the seed particles. Growth was preferentially favoured over nucleation up to a Fe²⁺ concentration of 6 mg/L, thereafter the extent of nucleation increased with increasing Fe²⁺ concentration up to 200 mg/L. Nucleation and growth promotion in the presence of high shear rates gave rise to rapid aggregation, which ceased at a critical size of approximately and in the presence of iron and without. However, the sharp increase in the D(4.3) towards the end of the cycle and the general decrease in surface area shows that aggregation of larger particles plays a major role in size enlargement. Comparison of the scanning electron microscopy (SEM) micrographs of the powder with undesirable morphology produced in industrial practice and that produced in the laboratory in the presence of iron showed that iron was one of the factors responsible for the production of powder with undesirable morphology. Based on these laboratory scale experiments, iron levels in reduction solutions should not exceed 6 mg/L for effective control of particle morphology.     

Population balance modeling. Promise for the future

Population balance modeling has received an unprecedented amount of attention during the past few years from both academic and industrial quarters because of its applicability to a wide variety of particulate processes. In this article, a fresh look is taken of the basic issues of the application of population balances towards strengthening the approach as well as widening the scope of their applications with regard to formulation, computational methods for solution, inverse problems, control of particle populations and stochastic modeling.     

Population balances for particulate processes—a volume approach

Population balance models have been used in chemical engineering since the 1960s and have evolved to become the most important tools for design and control of particulate processes. In this paper we show that the intrinsic particle parameter that determines changes in the process and should thus be included in the population balance is the particle volume. The basic population that is modeled should be the mass distribution, or the volume distribution if the density is constant. The population balance thus describes the change of the volume distribution of volume with time. Furthermore, we suggest that the “birth” and “death” terms that are often used to describe discrete events in particulate processes can almost always be replaced by a rate of change term.To design and control existing and future processes, a multi-dimensional population balance model is required. We propose a volume-based model in which the particle properties that are modeled are the volumes of solid, liquid, and air, respectively. In the most general case the model will consist of a properties vector and a distribution tensor. Depending on the complexity of the process, one or more of the properties may be omitted from the model. This is shown in three examples of increasing complexity: comminution, sintering, and granulation.     

Application of in situ adaptive tabulation to CFD simulation of nano-particle formation by reactive precipitation

Reactive precipitation involves four fundamental processes: mixing-limited reaction, nucleation, growth, and aggregation. A novel algorithm, in situ adaptive tabulation (ISAT), has been implemented in a code for micromixing simulations, which is often applied together with computational fluid dynamics (CFD), using full probability density function (PDF) methods to incorporate these fundamental processes in the formation of nano-particles by reactive precipitation in a plug-flow reactor. The quadrature method of moments is applied to solve population balance equations for turbulent aggregation of the growing particles. The various performance issues (error control, accuracy, number of records, speed-up) for ISAT are discussed. Based on a large number of simulations, an error tolerance of 10⁻⁴-10⁻⁵ is found to be satisfactory for carrying out time-evolving full PDF simulations of nano-particle formation by reactive precipitation. Our results show that CFD simulation of reactive precipitation requires a much smaller computational effort when the ISAT algorithm is implemented than when direct integration is used. Finally, the effects of initial species concentrations, micromixing time, and turbulent shear rate on the reactive precipitation of barium sulfate are studied.     

Effect of crystallisation on the reaction kinetics of nickel reduction by hydrogen

The reaction kinetics of precipitation from aqueous solution is not only a function of the concentration of reagents but also depends on the properties of the solid particles formed in the process. These property changes arise from the increasing influence of surface properties in comparison to volumetric bulk-properties as the particle size decreases. The ratio α of the active surface area to the actual surface area of the particles in the system is used in this work to evaluate the reaction activity of the particles. The investigation of the reaction kinetics of nickel reduction without sampling during the process of the reaction was successfully carried out in the experiment. The overall kinetics of nickel reduction have been suggested, where the constants relate to the main processes of nickel precipitation from the solution. The significant agglomeration reduces the deactivation of the nickel particles in the precipitation process, while breakage and crystal growth decrease the activation. The activations of dry and wet seeds are insignificantly different.     

Method of moments with interpolative closure

The article summarizes the principal details of a method of moments with interpolative closure. This is a mathematically rigorous yet numerically economical approach to particle dynamics, describing time evolution of a particle ensemble undergoing simultaneous nucleation, coagulation, and surface growth. The method was introduced some time ago and since then has undergone further development as well as extensive testing in reactive flow simulations of practical systems. These results, scattered over quite diverse literature, are presented here in a unified form, focussing on logical development rather than on chronological order. In addition, the validity of the numerical approach is addressed on rigorous mathematical grounds. Also discussed are method shortcomings along with possible directions to their resolution.     

On the stochastic simulation of particulate systems

The stochastic chemical kinetics approach provides one method of formulating the stochastic crystallization population balance equation (PBE). In this formulation, crystal nucleation and growth are modelled as sequential additions of solubilized ions or molecules (units) to either other units or an assembly of any number of units. Monte Carlo methods provide one means of solving this problem. In this paper, we assess the limitations of such methods by both (1) simulating models for isothermal and nonisothermal size-independent nucleation, growth and agglomeration; and (2) performing parameter estimation using these models. We also derive the macroscopic (deterministic) PBE from the stochastic formulation, and compare the numerical solutions of the stochastic and deterministic PBEs. The results demonstrate that even as we approach the thermodynamic limit, in which the deterministic model becomes valid, stochastic simulation provides a general, flexible solution technique for examining many possible mechanisms. Thus the stochastic simulation permits the user to focus more on modelling issues as opposed to solution techniques.     

Experimental study by online measurement of the precipitation of nickel hydroxide:Effects of operating conditions

The objective of this work is using the online measurement method to study the process of precipitation of nickel hydroxide in a single-feed semi-batch stirred reactor with an internal diameter of D=240 mm. The effects of impeller speed, impeller type, impeller diameter and feed location on the mean particle size d43 and particle size distribution (PSD) were investigated. d43 and PSD were measured online using a Malvern Insitec Liquid Pro-cess Sizer every 20 s. It was found that d43 varied between 13μm and 26μm under different operating conditions, and it decreased with increasing impel er diameter. When feeding at the off-bottom distance of D/2 under lower impeller speeds, d43 was significantly smaller than that at D/3. PSDs were slightly influenced by operating conditions.     

Solution of PBE by MOM in finite size domains

A new approach to solve PBE (Population Balance Equations), FCMOM (Finite size domain Complete set of trial functions Method Of Moments), is presented. The solution of the PBE is sought, instead of the [0,∞] range, in the finite interval between the minimum and maximum particle size; their evolution is tracked imposing moving boundaries conditions. After reformulating the PBE in the standard interval [-1,1], the size distribution function is represented as a series expansion by a complete system of orthonormal functions. Moments evolution equations are developed from the PBE in the interval [-1,1]. The FCMOM is implemented through an efficient algorithm and provides the solution of the PBE both in terms of the moments and in terms of the size distribution function. The FCMOM was validated with applications to particle growth (constant, linear, diffusion-controlled), simultaneous particle growth and nucleation, particle dissolution, particle aggregation (constant, sum, product, Brownian kernels) and simultaneous particle aggregation and growth.     

Population balance modelling of protein precipitates exhibiting turbulent flow through a circular pipe

The breakage of liquid-liquid, solid-liquid and solid-gas dispersions occurs in many industrial processes during the transport of particulate materials. In this work, breakage of whey protein precipitates passing through a capillary pipe is examined and an experimentally derived breakage frequency is applied to construct a suitable population balance model to characterize the breakage process. It has been shown that the breakage frequency of precipitate particles is highly dependent on their shear history and on the turbulent energy dissipation rate in the pipe. The population balance equation (PBE) uses a volume density based discrete method which is adapted from mass density based discretization. In addition to comparing the model with experimental data, predicted results at different velocities are presented. It was found that the population balance breakage model provides satisfactory results in terms of predicting particle size distributions for such processes.     

Experimental characterization and population balance modelling of the dense silica suspensions aggregation process

Concentrated suspensions of nanoparticles subjected to transport or shear forces are commonly encountered in many processes where particles are likely to undergo processes of aggregation and fragmentation under physico-chemical interactions and hydrodynamic forces. This study is focused on the analysis of the behavior of colloidal silica in dense suspensions subjected to hydrodynamic forces in conditions of destabilization.A colloidal silica suspension of particles with an initial size of about 80 nm was used. The silica suspension concentration was varied between 3% and 20% of weight. The phenomenon of aggregation was observed in the absence of any other process such as precipitation and the destabilization of the colloidal suspensions was obtained by adding sodium chloride salt.The experiments were performed in a batch agitated vessel. The evolution of the particle size distributions versus time during the process of aggregation was particularly followed on-line by acoustic spectroscopy in dense conditions. Samples were also analyzed after an appropriate dilution by laser diffraction. The results show the different stages of the silica aggregation process whose kinetic rates depend either on physico-chemical parameters or on hydrodynamic conditions. Then, the study is completed by a numerical study based on the population balance approach. By the fixed pivot technique of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51 (8), 1311-1332], the hypothesis on the mechanisms of the aggregation and breakage processes were justified. Finally, it allows a better understanding of the mechanisms of the aggregation process under flowing conditions.     

催化剂种类对液相氢还原法制备超细镍粉的影响

以硫酸镍为原料,研究不同催化剂对液相氢还原法制备超细镍粉的作用效果,重点研究了催化剂用量对还原率的影响。结果表明,添加催化剂可以促进液相氢还原反应的进行,缩短反应时间,提高单位时间内氢氧化 镍浆液还原成金属镍粉的还原率;活性镍粉、PdCl₂、RuCl₃及蒽醌4种催化剂对比实验结果表明,使用质量浓度为 10 mg/L的PdCl₂作为催化剂配料,可以使硫酸镍液相氢还原反应制备超细镍粉在最短时间内到达最高的还原率。     

Actinides oxalate precipitation in emulsion modeling: From the drop scale to the industrial process

An original process of actinides coprecipitation based on pulsed flow column is studied. The novelty of this process lies in the confinement of the aqueous reagents in separated droplets, dispersed in an inert organic phase (W/O emulsion). Precipitation occurs inside drops when they coalesce. Besides the implementation of well-known technologies of the nuclear industry, this precipitation in emulsion process is particularly convenient for the control of supersaturation, and ensures the sticky precipitates’ confinement within drops, thereby limiting the fouling risk and its adverse consequences on productivity and safety.     

Recovery of nickel powder from copper bleed electrolyte of an Indian copper smelter by electrolysis

When nickel concentration increases in the copper sulphate electrolyte during electrolysis, it starts electrodepositing on the copper cathode thereby affecting the purity of the copper. In order to produce high quality copper cathodes with less than 1 ppm Ni, it became necessary to bleed-off large volumes of foul electrolyte contaminated with nickel and other impurities. The study reported in this paper was part of the effort aimed at devising a cost effective and an ecofriendly method for the production of value added powders from a waste stream, for P/M application. A part of copper salts and regenerated acid was used back into the system. As discussed in our paper on copper recovery from copper bleed stream (CBS), a process involving decopperisation and crystallisation-solvent extraction (SX) separation-electrowinning (EW) has been attempted as an alternative to the conventional process. Optimum conditions for nickel recovery from this type of solution have been investigated through a series of experiments carried out in a rectangular electrolytic bath with SS as cathode and Pb-Sb as anode. A quantitative and selective recovery was found for nickel deposition under suitable conditions. The purity of the electrolytic nickel powders so produced was found to be 99.89%. The compact density of the annealed nickel powder was 7.72 g/cc. Other properties of the nickel powders such as flow-ability, particle size, etc. were also evaluated to assess its suitability for its use in P/M applications.     

Operation of semi-batch emulsion polymerisation reactors: Modelling, validation and effect of operating conditions

A detailed dynamic model was developed for a styrene emulsion polymerisation semi-batch reactor to predict the evolution of the product particle size distribution (PSD) and molecular weight distribution (MWD) over the entire range of monomer conversion. A system exhibiting zero-one kinetics was employed, with the model comprising a set of rigorously developed population balance equations to predict monomer conversion, PSD and MWD. The modelling equations included diffusion-controlled kinetics at high monomer conversion where the transition from the zero-one regime to a pseudo-bulk regime occurs. The model predictions were found to be in good agreement with experimental results. Both particle growth and the PSD were found to be strongly affected by the monomer feedrate. Reactor temperature had a major influence on the MWD which was, however, insensitive to changes in the monomer feedrate. These findings were confirmed experimentally. As a result, it seems reasonable to propose that the use of the monomer feedrate to control the PSD and the reactor temperature to control the MWD are appropriate in practical situations. Consequently, an optimal monomer feed trajectory was developed off-line (using the validated reactor simulation) and verified experimentally by producing a polymer with specific PSD characteristics.     

On the influence of mixing on crystal precipitation processes—application of the segregated feed model

The segregated feed model (SFM), a compartmental mixing model, is used to predict the influence of mixing on crystal precipitation. In this method, the population balance is solved simultaneously with the mass balances using crystallisation kinetic, solubility and computational fluid dynamics (CFD) mixing data. Mean properties are calculated for the three different zones of the reactor (two feed zones and bulk zone). It is predicted that during continuous operation, the product particle size exhibits oscillating behaviour before reaching steady state after about ten residence times. In contrast, the second moment (surface area) sharply increases during the first residence time and remains constant thereafter.Different mixing conditions are modelled by varying the mesomixing and micromixing times, which can be regarded as convective and diffusive exchange parameters between the compartments of the reactor. The overall nucleation rate is found to strongly depend on the mixing conditions, as it depends in a highly non-linear manner on the level of supersaturation. In consequence, the nucleation rate varies over three orders of magnitude between ‘good’ and ‘poor’ mixing conditions. Using the SFM, the effect of different feed points, feed rates, feed tube diameters, energy dissipation rates, impeller types and vessel sizes on the nucleation rate and the particle size during crystal precipitation is illuminated. Predictions of the model compare favourably with batch and continuous experimental data for calcium oxalate.     

液相还原法制备纳米镍粉

采用液相还原法在搅拌反应器内进行了纳米镍粉的制备研究。系统考察了反应物浓度、反应物配比（N₂H₄：NiSO₄）、引发剂浓度、氢氧化钠浓度、反应温度、搅拌速度等操作条件对镍粉物性的影响规律,获得了较优操作条件：NiSO₄为0.8 mol·L^-1,反应物配比为6：1,NaBH₄浓度为0.01 mol·L^-1,NaOH浓度为1 mol·L^-1,反应温度80℃,搅拌速度为2400 r·min^-1,在较优条件下制备出常温下稳定性良好、平均粒径107 nm的面心立方结构的纳米镍粉。     

A population balance framework for nucleation, growth, and aggregation

Nucleation, growth, and aggregation for particulate systems are explored by distribution kinetics and population balances to build a new framework for understanding a range of natural and manufacturing phenomena. Nucleation is assumed to follow classical homogeneous theory or to be caused by heterogeneous nuclei added to the solution. Growth due to monomer addition from solution to clusters, and aggregation between clusters are both represented by integrals of the cluster distribution. When growth and aggregation rate coefficients are independent of cluster size, the population balance equations are readily solved by the moment method. Equations for steady-state well-mixed flow and unsteady-state closed (batch) vessels have relatively straightforward solutions. By incorporating solute (monomer) depletion, the results afford reasonable behavior for the cluster number and mass concentration. The monomer addition terms are shown to be consistent with (and a generalization of) conventional differential growth and growth dispersion expressions.