Energy Technology 2012: Carbon Dioxide Management and Other Technologies Edited by: Maria D. Salazar-Villalpando, Neale R Neelameggham, Donna Post Guillen, Soobhankar Pati, and Gregory K. Krumdick TMS (The Minerals, Metals & Materials Society), 2012
DRY GRANULATION OF MOLTEN BLAST FURNACE SLAG AND HEAT RECOVERY FROM OBTAINED PARTICLES Yuelin Qin, Xuewei Lv, Chenguang Bai, Guibao Qiu College of Materials Science & Engineering, Chongqing University, Chongqing 400030, China Emails:
[email protected].
[email protected]. Keywords: Blast furnace slag, Heat recovery, Pyrolysis, Printed circuited board
Abstract Blast furnace slag (BF slag), is the main by-product in the ironmaking process, which contains large amounts of sensible heat. To recover the sensible heat, this paper described the hot experiments which were carried out in a rotary multi-nozzles cup atomizer, where the molten BFS was transformed into granules without water impingement; subsequently, a new method—pyrolysis the printed circuited board (PCB) with the hot BFS particle, was proposed for recovering the sensible heat in this study. The gaseous products were analyzed by gas analyzer. The residual of the PCB was analyzed by FT-IR and XRD respectively, and the obtained slag particles were analyzed by XRD. The results showed that it is a feasible process of pyrolyzing PCB powder with the hot BFS particle. The sensible heat of hot BF slag could be converted to chemical heat and a large amount of combustible gas could also be generated during the process. Introduction Due to the rising of the product cost of blast furnace iron (BFI) and the increasing public awareness of environmental problems, the steel industries have to focus on the energy saving and emission reduction. Blast furnace slag (BF slag) is a byproduct from blast furnace exhaust temperature of 1450℃ or so, is a high-quality waste heat resources. In China, the steel industry produces approximately 220Mt of BFS per year, and the enthalpy of those slags is approximately equivalent to 13.3% of the whole energy consumption in the ironmaking process[1]. It can be seen, recycling waste heat is an important way of energy saving for iron and steel enterprises. The molten slag was treated by conventional water quenching method without any recovery of heat in spite of its huge potential. Also, the water quenching process consumes a plenty of water, pollutes the air and water[2]. Additionally, the extra energy is necessary for the
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drying process of slag to produce cement. As a kind of secondhand resource which can be reused, the BF slag contains a large amount of sensible heat. To recover the sensible heat of BF slag, three key problems should be considered, which refer to (1) the dry granulation of BFS without consuming water, (2) how to recover the heat of slag particles efficiently, (3) how to use the slag particles after heat recovery. In view of the current process of BF slag drawbacks, many dry quenching and granulating processes without water have entered the pilot phase at home and abroad, such as rotary cup atomizer (RCA), rotating disc or drum, rotary cylinder atomizing (RCLA), etc[3-7]. The RCA method is a typical dry quenching process for producing slag particles, which is first introduced by Pickering et al. to granulate molten slag; and the heat of slag particles could be used to produce steam or hot water. In Japan, Akiyama et al. had been studied the effects of the different cup shape, rotating speed, and the temperature and the composition of the slag on particle size and distribution. However, it is difficult to control the particle size and the outlook, and the particle distribution is extremely wide. Therefore, in the present study, the rotary multi-nozzles cup atomizer (RMCA) was proposed to control the particle size, distribution and the outlook in the granulating process; to make uniform particles by spouting the molten slag from the nozzle. Recently, the annual output of electronic waste is increasing with the rate of 18%[8], which is the fastest increase of waste in the world. Printed circuited board (PCB) is an indispensable part of electronic waste. At present, the technology of pyrolysis PCB is proposed as a new harmless treatment method which is characterized with quick processing speed, large treatment of capacity and low toxic exhaust[9, 10]. The PCB pyrolysis is an endothermic process, which consumes huge amount of energy to ensure the reaction continuously. However, the obtained slag particles contain a huge of high qualities heat in the experiment of dry granulation molten BF slag. So a new method—pyrolysis PCB with the hot BFS particle, was proposed for recovering the sensible heat in this study. The sensible heat of slag particles was converted to chemical heat through the pyrolysis reaction. Experimental Part 1: Dry granulation of molten BFS
Materials The BF slag used was from Chongqing Iron and Steel Co. China in this experiment. The main chemical compositions of the slag are shown in Table I. The slag was dried in a oven before the experiment began. Table I. Chemical compositions of BF slag SiO2 Al2O3 CaO MgO MnO Composition Content (wt%) 33.48 15.32 38.19
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9.58
0.59
S
TiO2
1.34 2.65
Apparatus and procedures Fig. 1 shows the schematic diagram of the apparatus of RMCA used in the experiment, and mainly consists of four parts. (1) A container with a hole at the bottom. (2) A multi-nozzles rotary cup with 56 holes, 7×8, 4mm in diameter. The multi-nozzles rotary cup is made of a graphite crucible with 40mm inside diameter and 100mm height. (3) A rotation driving equipment. The rotating speed will be adjusted through the frequency converter, and the maximum rotating speed is 3000rpm. (4) The slag particles collector. The slag was firstly melted in a graphite crucible by an induction furnace and kept it at a given temperature, about 1450~1500℃. The container and the cup of the RMCA were preheated for 15minutes using a burner with nature gas to avoid the breakage due to the thermal shock. Fix the rotation speed at a desired value, the molten slag was poured into the centre of the rotating cup, and then was spread out to form the granules of particles. The obtained slag particles were analyzed by XRD.
Fig. 1
Schematic of experimental apparatus
Fig. 2 The apparatus of pyrolysis PCB powder
Part 2: Heat recovery from obtained slag particle thought pyrolysis PCB
Materials The hot BF slag particles were from the obtained slag particles in the experiment part 1. TableⅡshows the major characteristics and properties of BFS particles. In this experiment, the obtained slag particles sizes vary from 1mm to 5mm were employed as the samples. Table Ⅱ. The major characteristics and properties of BF slag particles Basicity 1.14
Temperature 1500℃
Destiny 3
3
2.6×10 kg/m
Special heat
Enthalpy
Heat conductivity
1.2kJ/(kg∙℃)
1700MJ/ton
0.1~0.3W/(m·K)
The typical kind of PCB, FR4 PCB, was employed as experimental materials, which was from an abandoned telephone in this study. The resistor, capacitor and soldering on the PCB were firstly removed mechanically, and then were grinded into
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powder with the size of approximately ≤200 meshes. The major compositions of PCB powder are glass fiber, dicyandiamide and brominated epoxy resin, and the typical chemical structure of PCB is showed as follow.
Apparatus and procedure The experimental apparatus for pyrolysis PCB is schematically shown in Fig. 2. The PCB powder samples were loaded in the reactor, and the hot BF slag particles was added into from the top of reactor. The reactor was made of graphite with a inside diameter of 70 mm and a height of 100 mm. The gas was flowed in from the bottom of the reactor, and the flow rate was measured with a flowmeter. The component of the gaseous during the pyrolysis of PCB was analyzed using a gas analyzer. The residual of the PCB was analyzed by FT-IR and XRD respectively. The various conditions for the pyrolysis of PCB were as follows: (1) The initial temperature of the BFS, 600, 700, 800, 900℃; (2) The mass ratio of the PCB samples and the BFS, 1:5; (3) Reaction atmosphere, Ar gas. The experimental procedure includes: (1) Loading the PCB powder samples into the reactor; (2) Flowing the inert gas (Ar) at the desired rate; (3) Heating the BF slag samples to desired temperature holding for 30 min in a box resistance furnace, and then putting the BF slag samples into the reactor rapidly; (4) Analyzing the gaseous product using a portable infrared gas analyzer which was made by Wuhan Cubic Optoelectronics Co. Ltd, China. Results and Discussion Outlook and amorphous of the obtained slag particles Table Ⅲ shows the outlook of the obtained slag particles in different rotary speed. The outlook of slag particles are sphere and the smaller slag particles the better sphericity. The sphericity of slag particles depended mainly on the heat transfer between slag particle and cooling medium and the flying distance before hitting wall and collector. With the increase of rotary speed, the particle hits the wall or collector before solidification so that the final appearance of particles deformation. The smaller slag particles exhibited better sphericity because of the large specific surface helping to improve the heat transfer between particle and cooling medium. Table Ⅲ. The outlook of obtained slag particles in different rotary speed Particles size (mm) Rotary speed (rpm)
1
