Article pubs.acs.org/IECR
Numerical and Experimental Investigation of Arsenic Removal Process from Phosphoric Acid by Vertical Zone Melting Technique Yongsheng Ren,*,† Xiaoxiao Duan,† Baoming Wang,‡ Meiying Huang,§ and Jun Li‡ †
Department of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, People's Republic of China Department of Chemical Engineering, Sichuan University, Chengdu 610065, People's Republic of China § Department of Chemistry and Environmental Engineering, Chongqing Three Gorges University, Chongqing 404000, People's Republic of China ‡
ABSTRACT: A combined numerical and experimental approach was undertaken to investigate the arsenic removal process from phosphoric acid by vertical zone melting technique. The numerical model, which predicts the arsenic re-distribution at any point of the single zone pass, multiple molten zone passes, or ultimate distribution, is described for providing simulations. Experimental work on arsenic removal by vertical zone melting process was carried out with prepared phosphoric acid samples as the starting material. The experimentally obtained axial concentration profiles were compared with the theoretical predictions, which proved the efficiency of the proposed model in removing the arsenic concentration significantly. Following the model and as evidenced from the theoretical predictions, the arsenic concentration reduced to 1 except that a depleted rather than an enriched layer builds up. Impurities having keff values close to 1 are difficult to segregate. Thus, impurities become concentrated in one, the other, or both ends of the ingot, thereby purifying the remainder.6 The zone melting technique is much more efficient for impurities with keff further from one and lower initial content. Thus, this process is capable of producing a varity of organic and inorganic materials of extremely high purity.7−11 Highpurity phosphoric acid is used widely in many fields such as medical, food, environmental protection, and electronic industry.12 However, phosphoric acid by the traditional way (furnace process) contains many impurities such as As, Fe, Al, Mn, Cd, Ni, Pb, Cu, and Si, which can hardly meet the requirement for high purity grade application. Varieties of chemical and physical methods13−16 were developed for preparation of high-purity phosphoric acid. However, the process of chemical purification method requires multiple steps. Further, the chemical reagents are difficult to recycle and present a major waste disposal problem. Out of the various purification techniques for furnace process phosphoric acid, the zone melting technique is an attempt as the major impurity, arsenic, has a small keff.
1. INTRODUCTION Zone melting was first proposed by Pfann in the early 1950s1 and used to purify germanium for transistors.2 For some advantages such as high separation efficiency, low operation costs, and environmentally harmless, the process has found widespread application in ultrapurification material. However, research continues to improve the purification process.3,4 An efficient application of zone melting technique requires the understanding of the mechanism responsible for the impurity re-distribution along the whole process. Zone melting is used to achieve ultrapurification by a slow unidirectional movement of a molten zone, or a series of molten zones, through a solid material ingot, as shown in Figure 1. When the molten zone
Figure 1. Schematic representation of vertical zone melting process.
moves, the solid material ingot ahead melts at the melting interface, and the melt (liquid) solidifies at the freezing interface. Impurities move with, or opposite to, the molten zones. The impurity elements are divided into two categories depending on the values of keff and C0. The keff was defined by © 2012 American Chemical Society
Received: Revised: Accepted: Published: 4035
November 27, 2011 February 10, 2012 February 15, 2012 February 15, 2012 dx.doi.org/10.1021/ie2027553 | Ind. Eng. Chem. Res. 2012, 51, 4035−4040
Industrial & Engineering Chemistry Research
Article
This paper aims at studying the arsenic removal process of phosphoric acid by vertical zone melting technique and validating by existing numerical model. Using prepared phosphoric acid of arsenic content 45.5 μg·g−1 as the starting material, after multiple molten zone passes (defined as the number of single molten zone, which pass through the solid ingot), the representative impurity, arsenic concentration, was analyzed and compared with theoretical calculations to validate the efficiency and predictions. In addition, we investigate and optimize the crucial zone melting parameters, the number of passes, the molten zone length (namely, the length of one heater), and the molten zone velocity on the separation performance of phosphoric acid.
The solution is ⎛ m − i ⎞keff − 1 ⎟ C1i = C1m − b⎜ ⎝ b ⎠
(1)
l = b dx
(2)
k Cn0 = eff b
(3)
keff
1 i dCni = (Cni+ − 1 − Cn) dx
j=0
C 1 Cni = 0 − b
0≤i
