ArticleComparative experimental study on reactive crystallization of Ni(OH)2 in an airlift-loop reactor and a stirred reactor☆
Graphical Abstract
The size of Ni(OH)2 crystallites decides the tap density of Ni(OH)2 particles.
Introduction
As an important unit operation, crystallization is widely used in the production of basic chemicals, pharmaceuticals, pigments and ceramic powders [1], [2], [3], [4]. By now, diversified stirred tanks prevail as crystallizers owing to their good mixing performance and technical maturity [5]. However, secondary nucleation and crystal attrition are unavoidable due to the existence of mechanical agitation by impellers [6]. As for an airlift-loop reactor (ALR), there are no moving mechanical parts in the reactor, and the shear rate is moderate and relatively uniform throughout the reactor volume. Therefore, it is widely used in fermentation, bio-fuel production and wastewater bio-treatment, to take the advantage of low extent of damage to microbes [7], [8]. In view of these excellent properties, ALR has been considered as a new prospective crystallizer [6], [9], [10], [11], [12], [13].
To date, there are only a few papers on the crystallization in ALRs. Soare et al. [13] studied the secondary nucleation in an airlift crystallizer and found it was flexible in controlling the size of crystalline products by manipulating seed load and air flow rate. Rigopoulos and Jones [10] proposed a model for simulating the semi-batch agglomerative gas–liquid precipitation of CaCO3 by CO2 absorption into lime in an airlift reactor. By comparison with the crystallization of l-ascorbic acid in a stirred crystallizer, Lakerveld et al. [6] found that an air-lift crystallizer can suppress secondary nucleation at a higher super-saturation. As for the study on liquid–liquid reactive crystallization in an ALR, few reports have been found in the literature. Therefore, the objective of this work is to study the process of reactive crystallization in an airlift-loop reactor.
As an important crystalline material for advanced energy conversion and storage, nickel hydroxide (Ni(OH)2) is extensively used as cathode material in rechargeable alkaline batteries [14], [15], [16], [17], [18], [19], [20], [21], [22]. There have been many reports exploring the preparation and crystallizing mechanism of Ni(OH)2 in stirred tanks. E et al. [22] explored the precipitation of nickel hydroxide in a stirred tank by online measurement and got useful data on preparing Ni(OH)2 with small size and small size span. Peng and Shen [23] came up with the template growth mechanism of spherical Ni(OH)2 in a continuous stirred-tank reactor (CSTR). Borho [24] explored the crystallite aggregation and simulated the process of aggregation in a stirred column. Shen et al. [25] investigated the microstructures of spherical Ni(OH)2 particles and found that spherical particles were more stable during the charge/discharge cycles compared with the conventional non-spherical ones. Cai [26] studied the production of spherical Ni(OH)2 using an oscillatory baffled crystallizer and indicated that tap density of Ni(OH)2 had great influence on electronic properties. Bigger tap density means bigger capacity per volume of Ni(OH)2 particles, which makes the electronic properties better. Watanabe and Kikuoka [27] proposed that the nickel hydroxide of small size showed better charge–discharge behavior due to its faster proton diffusion. On the whole, the experimental data and theoretical analyses on preparation of Ni(OH)2 in stirred tanks are abundant and the electro-chemical properties of Ni(OH)2 particles with spherical shape, small size and big tap density are preferred. In light of this knowledge basis, the precipitation of Ni(OH)2 is chosen as the model reaction to study the reactive crystallization in an ALR.
In this paper, the conditions for preparing sphere-like Ni(OH)2 in an ALR are explored first by the orthogonal experiment. Then, the Ni(OH)2 particles are analyzed by the scanning electron microscope (SEM), X-ray diffraction (XRD), laser particle analyzer, tap densitometer and optical microscope. The temporal evolution of size and tap density of Ni(OH)2 particles is characterized by comparison with those prepared in a stirred tank. On this basis, the evolution process of Ni(OH)2 particles in the ALR is elaborated.
Section snippets
Setup and procedure
The experiment was conducted in two reactors, i.e., an ALR and a stirred tank. A schematic diagram of the ALR is shown in Fig. 1. The reactor is composed of an outer cylinder of diameter D1 = 200 mm and height of H1 = 400 mm, a draft tube of diameter D2 = 120 mm and height of H2 = 300 mm, and an air distributor. The clearance between the reactor bottom and the draft tube is 50 mm. As for the stirred tank, it is a cylinder with the same diameter of the ALR and an up-pumping 45° six-pitched-blade turbine
Conditions for preparing spherical particles in ALR
Ni(OH)2 particles with sphere-like morphology have been proved to present better electronic properties, such as higher storage [22]. Compared with other samples, the ones prepared in T6 (abbreviation of Test 6) have the highest sphericity, as shown in Fig. 2. Therefore, the conditions of T6 are selected for studying the process of reactive crystallization.
Factors influencing tap density
Though the morphology of Ni(OH)2 prepared in T6 is spherical, the tap density (0.2233 g·cm− 3) is too small, while the reported tap density of
Conclusions
The growth process of Ni(OH)2 particles in an ALR is studied in this work by analyzing microstructure, shape, size and tap density of particles. Comparing with the experimental results in a stirred tank, the following conclusions can be drawn from this work.
- (1)
The growth of tap density mainly depends on the size of crystallites. The bigger the size of crystallites is, the bigger the tap density is.
- (2)
Mixing in the reaction crystallization plays a key role in the quality of the products. With the
Nomenclatures
- CNiSO4
concentration of NiSO4, mol·L− 1
- D1
diameter of outer cylinder in an ALR, mm
- D2
diameter of draft tube in an ALR, mm
- H1
height of outer cylinder in an ALR, mm
- H2
height of draft tube in an ALR, mm
- qg
gas flow rate, L·min− 1
- T
temperature, °C
- t
reaction time, h
- VNH3
volume of ammonium hydroxide per liter alkaline liquor, ml
- VNaOH
volume of NaOH solution, L
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Supported by the National Key Research and Development Program (2016YFB0301701), the National Natural Science Foundation of China (21406236, 91434126), the Major National Scientific Instrument Development Project (21427814) and Jiangsu National Synergetic Innovation Center for Advanced Materials.