Original Research Paper
Modeling and kinetics study of microwave heat drying of low grade manganese ore

https://doi.org/10.1016/j.apt.2020.05.013Get rights and content

Abstract

During the industrial-scale smelting process of manganese ore, blast may occur due to the high moisture content of the ore, and drying pretreatment of the manganese ore is needed in the aspects of safety. In the present paper, microwave drying experiments were conducted under different particle size distributions and different microwave power conditions to explore the basic theory of microwave drying characteristics and kinetics of manganese ore, and the experimental data were fitted and analyzed by using thin-layer drying dynamics model. Results indicated that with the increase of the particle size and the microwave power of the manganese ore, the microwave drying rate increased, and accompanied with a promoting on the drying efficiency. For describing the microwave drying process of manganese ore, diffusion approach model was adopted among the commonly used thin-layer drying kinetic models. Based on Fick's second law, it can be seen that the surface diffusion coefficient increased from 4.27 × 10−13 m2/s to 8.24 × 10−8 m2/s with the increase of particle size from a particle size range of 0.012–0.095 mm to a particle size range of 4.2–5.0 mm. Clearly, the particle size has a significant influence on microwave drying efficiency.

Introduction

Manganese is a key strategic reserve in the national economy, and has wide applications in the steel industry as a desulfurizing/deoxidizing agent in molten steel or an alloying element to prepare multifunctional alloys with other metals, etc. [1], [2]. An appropriate amount of manganese can effectively improve the strength of steel, eliminating the effect of sulfur and oxygen on the thermal brittleness of steel, improving the thermal processing performance of steel, while slightly reducing the toughness of steel. Manganese ores are also used in non-ferrous metal, batteries, agriculture and other sectors [1], [2], [3]. Manganese dioxide can be used as catalysts and pigments, and potassium permanganate can be used as oxidants, preservatives, disinfectants, and chemical indicators. In the battery industry, foreign scholars have made a lithium-ion battery using manganese metal as anode material to replace traditional cobalt or nickel. This work could provide cheap and costly alternatives to these increasingly expensive and limited resources.

Recently, high-grade manganese ore resources are gradually being consumed, and mining and utilization of low-grade manganese ore resources have received increasing attentions [4]. At present, the mining of manganese oxide mainly includes three processes: crushing, grinding and beneficiation. In the beneficiation stage, manganese powder is sent to the magnetic separator for magnetic separation to remove impurities, followed by a conventional drying process to obtain the refined manganese powder. The purpose is to remove water from the manganese powder and effectively reduce the probability of the blast during smelting process of manganese. However, the energy utilization rate of the conventional drying process is low and an environmentally friendly and energy-saving heating method is required. Microwave heating, well known as a novel type of heating pretreatment method, is a promising alternative for the drying of manganese powder.

Compared with the traditional heating mode, microwave heating has the characteristics of selective heating, fast heating speed, uniform heating and fast thermal efficiency [5], [6], [7], [8]. Microwave heating technology has played an active role in industry, metallurgy [9], agriculture, communication, material preparation [10], [11], [12], [13], etc. and has a broad application prospect. In the field of metallurgical resource utilization, calcination [14], reduction [15], roasting [16], [17], oxidation [18] and leaching [19], [20], [21], microwave technology has shown unique advantages [22]. Zhang et al. pointed out that in the study of microwave calcination of Portland cement, microwave calcination was used compared with electric furnace calcination, the dimension temperature was decreased by 50 °C and the calcination time was reduced by about 70%.

However, there are few studies on the characteristics and kinetics of microwave drying of manganese ore. In terms of microwave drying kinetics [23], [24], scholars have applied the drying kinetic model to other mines [25]. In the study of microwave drying kinetics, Li et al. [26] reported that the drying process of titanium concentrate is more agree with the Henderson and Pabis index model than the Page semi-empirical model; Tahmasebi et al. [27] considered the Page model to be the most suitable model for describing the kinetics of lignite microwave drying.

In this work, microwave drying was used for the pretreatment of manganese ore, and the microwave drying characteristics and drying kinetics of manganese ore under different particle size distributions and powers were studied based on the drying kinetic models including Lewis, Page, Modified Page, Diffusion approach, Henderson and Pabis, Logarithmic, Simplified Fick diffusion. The purpose is to provide a reference for the industrial application of microwave drying technology for manganese ore.

Section snippets

Materials

The raw material for this experiment study was manganese ore provided by Dameng Manganese Industry Group Co., Ltd (Nanning, Guangxi Province, China). The method recommended by the National Standard of the People's Republic of China (GB/T 1506-2016) was adopted to analyze the element content of the as-received manganese ore. The results of chemical composition analysis are shown in Table 1, indicating that the contents of main components TMn, Mn4+, and SiO2 are 28.81%, 26.03%, and 25.85%,

Effect of particle size on microwave drying of manganese ore

Manganese ore with three particle size distributions [29] and an initial water content of 14% were dried by microwave heating at a microwave power of 450 W. Microwave drying characteristic curves of manganese ore with different particle size distributions were obtained, as shown in Fig. 2.

As can be seen from Fig. 2(a) and (b), the increasing particle size was promoting the reducing of moisture content [30], [31], indicating the particle size has a great influence on the microwave drying process

Conclusions

Different from the traditional drying method, under the microwave drying, because the microwave heating has the characteristics of selective heating, high water content part can absorb more microwave energy. With the increase of particle size, the specific surface area decreases, and the amount of absorbed microwave energy was positively correlated with the drying rate of the manganese ore particles, whilst negatively correlated with the time required for drying. The increase of microwave power

Acknowledgements

Financial supports from the National Natural Science Foundation of China (No: U1802255), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), and Innovative Research Team (in Science and Technology) in University of Yunnan Province were sincerely acknowledged.

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