Fundamental Research On The Preparation Of Cast Stone Directly From Molten Blast Furnace Slag

Utilization of high-temperature waste heat of molten slag and large-volume consumption of difficult-to-reuse slag are one of the important bottlenecks encountered in the current development of green metallurgical industry.Aiming at the problem of waste heat utilization of molten slag and high-value utilization of metallurgical slag,this paper adopted the technical route of integrated utilization of"slag" and "heat" for molten slag.Under the heat limitation of melting the cold modifier directly by the sensible heat of the molten slag,modification of the its composition and crystallization performance,and nucleation-crystallization one-step method during molten slag cooling process(Petrurgic method)were adopted to prepare low-cost and large quantities of artificial granite stone materials.By using high temperature X-ray diffraction(XRD),X-ray Fluorescence(XRF),field emission electron microscope(SEM),differential thermal analyzer(DTA),Factsage and so on,combined with performance testing of samples,the research investigated the melting ability of modifiers by using the sensible heat of molten slag,and systematically study on the crystallization behaviors and their influence on the properties of cast stone of blast furnace slag,modified blast furnace slag,and different titanium-containing blast furnace slags.Subsequently,the high-temperature crystallization mechanism of titanium-containing blast furnace slag in a supergravity field was discussed and verified,and the molten slag cast stone gradient materials were further prepared.Based on it,the ton-level experimental verification of cast stone prepared by molten blast furnace slag was carried out.The research has obtained the following important results:(1)The crystal growth method of blast furnace slag during the cooling process belonged to surface crystallization.The main crystal phase of blast furnace slag was melilite,which had poor mechanical properties.Therefore,the blast furnace slag needed to improve its structure and performance through composition adjustment.The heat balance calculation of the molten blast furnace slag tempering process showed that the maximum addition amount of quartz as the modifier to molten blast furnace slag with a slag discharge temperature of 1500? was 10%.(2)The study on the crystallization behaviors of different modified blast furnace slag with 10%modifier(quartz sand,boron mud and rutile sand)showed that titanium dioxide was a better modifier.Titanium dioxide played two roles in the molten slag cooling and crystallization process:on one hand,the highly electronegative Ti4+promoted the phase separation of the molten slag,forming silicon-rich and titanium-rich phases.Part of the Ca2+entered into the titanium-rich phase,which increased the silicon-to-calcium ratio of the remaining silicon-rich phase,and provided thermodynamic conditions for the subsequent precipitation of pyroxene and other minerals.On the other hand,the titanium-rich phase perovskite was precipitated at a high temperature firstly,which provided a nucleus for the subsequent precipitation of silicate minerals such as pyroxene and promoted the overall crystallization of the molten slag.The modified slag with 10%rutile sand not only precipitated melilite phase,but also precipitated pyroxene phase and perovskite phase,whose crystallization process changed from surface crystallization to overall crystallization and Avrami parameters changed from the original slag 2.11 and 2.26 to 4.17 and 6.13.(3)Research on the crystal phase and properties of cast stone prepared by using titanium-containing blast furnace slag with different titanium dioxide content showed that excessive titanium dioxide precipitated a large amount of perovskite,which reduced its mechanical properties.Therefore,controlling the titanium dioxide component in the molten slag to promote the precipitation of pyroxene without excessive precipitation of perovskite was the key to preparing cast stone with good properties.The cast stone prepared by medium titanium blast furnace slag with TiO2 content of 11.50 wt%had the best mechanical properties,its compressive strength was 286 MPa,and the precipitated crystal phases were pyroxene,melilite and a small amount of perovskite phases.(4)An experimental method for analyzing the process of melt crystallization at high temperature using melt high-temperature supergravity centrifugal separation was proposed.The experiment of solid-liquid supergravity separation of titanium-containing blast furnace slag in the cooling process showed that the titanium-containing slag can be separated under supergravity,and the titanium-rich phase dominated by perovskite in the filter layer(24.44 wt%TiO2,the recovery rate of TiO2 is 75.98%)and the remaining slag passing through the filter layer were obtained under the supergravity coefficient G=1000 and 1250?.In addition,the remaining slag was heat-treated to form cast stone,which had a dense structure,a pyroxene-based mineral phase composition,a lower titanium oxide content(9.46%TiO2)and better mechanical properties(flexural strength 40.54 MPa).It was further verified the mechanism of titanium oxide with phase separation to promote pyroxene crystallization and as a nucleating agent.(5)Considering the characteristics of high-temperature crystallization of molten slag,the high-titanium blast furnace slag cast stone gradient material was prepared by using the supergravity forming method.The studies showed that:when the molten slag was cooled to 1400?,the supergravity(supergravity coefficient G=1000)was applied and the cooling was continued.Simultaneously,the viscosity of the slag was small,and the precipitated perovskite gradually moved to the bottom layer under the action of supergravity due to its higher density,so that the bottom layer had the highest perovskite content and the largest crystal grains(about 80 ?m in length).At the same time,the bubbles in the slag gradually moved to the top layer under the action of supergravity due to its lighter density,forming a gradient distribution in which the number of voids gradually increased from the bottom to the top,so that the compactness and bulk density gradually decreased from the bottom to the top.The cast stone sample in the lower middle layer had suitable perovskite content and compactness,and its performance with a bending strength of 35.9 MPa was the best among different positions of gradient material.This method provided a new way to prepare such gradient materials.Among them,the formation of perovskite minerals enriched to one side of the material can be expected to maximize its functional effects.(6)The ton-level scale-up test of titanium-bearing blast furnace slag cast stone showed that the Petrurgic process,which directly cools and keeps from molten slag,can produce low-cost cast-stone artificial stone with good performance.Holding temperature and decreasing the temperature difference in the solidification and crystallization interval were the key links to the large-volume slag cast stone.The tonnage large-volume cast stone rough prepared by slowly cooled and heat-preserved with a heat-preserving mold forms a massive artificial stone after being cut.It has good performance and an average flexural strength of 39.31 MPa,which meets the standard requirements of natural granite building boards.The large-volume cast stone plate(500×500 mm)prepared by the traditional stone casting process can obtain excellent performance by the roller kiln,which was heat treated by crystallization at 1100? for 1 hour and annealing at 650? for 1 hour.The compressive strength reaches 268 MPa and the bending strength reaches at 56 MPa,it meets the performance requirements of artificial stone and can be mass-produced.