STEEL SLAG DEEP DISPOSAL AND MULTI-PURPOSE UTILIZATION TECHNOLOGY
BY
GUO XIUJIAN*
SYNOPSIS:
Based on the analysis of component, stability and activity affecting the utilization of slag, solutions are raised herein. Through comparison of slag cooling process and utilization method, this paper puts forwardself-disintegrating process by stream to realize slag cooling, during the process, Fe can be separated and iron-contained materials and slag for flux be recovered. Slag tailing is controlled in grain size; re-treatment processes are expected to produce slag powder and block for building materials or directly used for road construction and landfill. All these measures are directed to comprehensively utilize slagand to realize the steel slag’s “zero discharge/waste”.
Keywords:steel slag,stability, activity, modification, slag self-disintegrating process by stream, powder, multi-purpose utilization
*Engineer of Environmental Science Research Office, Master, CISDI Group Co., Ltd, Chongqing, China
I. General
Steel slag as the byproduct of steelmaking process will account for 13～16% of liquid steel output. For converter steelmaking, desulphurized slag, converter-melt slag and post-cast slag can be produced. For EAF steelmaking, EAF-melt slag and post-cast slag can be produced. Quantity of slag will be different from various steelmaking processes, normally, desulphurized slag to be 12~15kg/t hot metal, converter-melt slag 100~110kg/t liquid steel, EAF-melt slag 120~150kg/t liquid steel, and post-cast steel 25~30kg/t liquid steel.
In 2010, the global crude steel has reached 1.414 billion ton output, in which, Asian 0.811 billion ton, among the rest, China’s numbered the 1st with her 0.6267 billion ton, 44.3% of globally total. Converter steelmaking plays the main role in producing the crude steel to that scale by its 2/3 of the total. China’s 90% or so of melting shops are converter based, which can produce over 85,000,000 tpy slag. As of today, the steel slag is still under-evaluated for utilization. It has become an urgent and significant subject for study.
II. Factors to Affect Slag Utilization
The steel slag is a kind of mineral slag formed from the reaction between slagging agent (lime or others) and acid oxides for the purpose of desulphurization, dephosphorization and decarbonization. During such slag production and treatment process, the following factors will render influence on its utilization.
2.1 Composition
Since steelmaking is a discontinuous process, each heat of liquid steel is experienced with different working condition, and the slag tapped at different melting stage can be made of different compositions. The slag mixed from different heats or procedures will lead to unstable compositions of slag and even inconvenient for downstream users.
The smelting scheme is preset for a relatively fixed technological process to produce qualified liquid steel. In that sense, to achieve stable compositions of steel slag will be mainly dependent on reasonable slag tapping and treatment system.
2.2 Stability
In steel slag are contained free CaO (f-CaO), free MgO (f-MgO), C3S and C2S, which are unstable compositions under certain circumstances. In smelting, the free CaO and MgO can be enclosed by some minerals, thus increasing the free CaO and solid-solving FeO of certain concentration. It produces the dead-roasted lime block. In cooling of steel slag, C3S can be gradually decomposed to be C2S and free CaO at the temperature of 1,250~1,100℃. Furthermore, C2S can be phase-changed from β-C2S to γ-C2S at temperature of 675℃. In the meantime, the volume is expanded.
The free CaO and MgO will happen expansion when meeting with water. In particular, it will take 20 years long for free MgO to completely solve in building material at normal temperature. Therefore, the steel slag containing free CaO and MgO cannot show stable performance at normal temperature; only until free CaO and MgO dissolve completely or have a little content will the steel slag not be stably existent.
10% expansion of volume;
98% expansion of volume;
148% expansion of volume.
Among the above-listed materials causing slag expansion in contact of water, free CaO is predominant so as to swell the road, building materials productsor buildings. That requires stabilizing treatment on steel slag proposed to act as building materials products, road or landfill.
How to improve the stability of steel slag? It’s recommended to take following measures:
a) Add stability modifier in the molten state to remove the instability of free CaO;
b) Realize self-disintegrating process by stream under normal or high pressure to reduce the content of free CaO;
c) Store slag at slag yard (for at least 6 months) and spray water regularly for accelerated solving;
d) Mix with other materials for use to relieve the impact of slag expansion.
2.3 Basicity and activity
The steel slag is solid solution comprised of multiple minerals like CaO, SiO2, Al2O3, F2O3, MgO and a little amount of MnO, FeO, P2O5 and Fe. The C3S and C2S contained herein are active minerals, characteristic of hydraulic coagulation. Similar to the chemical compositions of Portland cement clinker, the steel slag has lower percentage of Si and Ca included [1].
The basicity [2] of steel slag is expressed as:
In which, it’s low-basicity slag when M<1.8, medium-basicity slag when 1.8≤M≤2.5, high-basicity slag when M>2.5.
When the basicity M is larger than (>) 1.8, the slag contains 60~80% C3S and C2S, displaying certain coagulation[3]; with higher M, the content of C3S is increased; when M reaches 2.5 and higher, slag is mainly laden with C3S. In case that the slag is utilized as gel, C3S herein will be required as much as possible.
In cooling of slag, it’s seen that water spaying can help solving of free CaO; simultaneously, bulge of volume makes the slag block be rapidly broken or powdered. It’s beneficial to both granulation and stabilizing. However, for the gel-purposed slag, water spraying can cause pre-hydration which unluckily lowers the hydraulic activity of slag, especially the initial activity [4].
How to improve the activity of steel slag? It’s recommended to take following measures:
a) Improve basicity by adding more properly CaO in melting, or adding CaO in slag tapping and mix them evenly;
b) Apply proper cooling technology of slag by comprehensively balancing the factors affecting its stability and activity;
c) Apply mechanical excitation to improve the fineness of slag through having the crystal lattice of minerals misaligned, defective and recrystallized, in so doing, forming water-soluble amorphous structure on the surface; increase the contact area of water and minerals in slag, creating more acting force between them and easier access of water molecule into minerals for quicker hydration reaction and higher activity.
d) Apply chemical excitation by introducing chemical constituent to generate a basic environment which can make vitreous in slag be fully depolymerized and hydration; usually taking use of alkali metal salt and gypsum and other excitant for the process.
e) Apply high-temperature excitation to break the vitreous network-formed keys so that the vitreous is accelerated polymerization and hydration reacted, and finally slag activity excited; with regard to different uses of slag, two methods of high-temperature excitation can be employed: i) pressure evaporation for higher temperature of hydration when preparing silicate product; ii) heat release from gelling hydration for higher temperature when using slag as additive into concrete.
III. Slag Cooling
Different cooling process will produce different states of slag, granulated, bulk and powder as shown in Table 1.
Table 1 Slag Cooling Methodology
S/No. / Cooling Mode / Principle / Advantage / Disadvantage1 / Self-disintegrated by stream / Spray water to generate physical and mechanical action and water solution of free CaO, to granulate slag; steamas generated from water, for deeply discompose free CaO. / Applicable for various hot slag, presenting pretty high activity and good stability; steam discharged orderly or collected in unified way. / Long cycle of treatment, large land use.
2 / Layer Pouring process / Pour hot slag and spray water to break it because of temperature stress; hydration of free CaO further cracking slag. / Quick rate of slag handling, short time of cooling, easier for mechanical operation, higher activity of slag. / Steam discharged disorderly, worse stability of slag.
3 / I.S.C process / Pour slag on pan: spray water onto solidified surface for quench, then pour into pool for further cooling. / Quenching, large volume of treatment / Applicable for liquid slag, worse activity of slag.
4 / Water-granulated process / Split slag by press water, granulate slag by impacting, and break slag by stress concentration of quenching contraction. / Quick rate of slag handling, small land use, smaller grain size of treated slag (about 5mm) / Applicable only for liquid slag, worse activity of slag.
5 / Rotary cylinder / Put liquid slag into high-speed rotating cylinder, quench by water to solidify and break slag. / Quick rate of slag handling, small land use, stable performance of granulated slag. / Applicable only for liquid slag, worse activity of slag.
6 / Water-granulated process / Cut slag by compressed air at high speed, quench, modify and granulate slag. / Quick rate of slag handling, small land use, even size of treated slag (<5mm) and smooth, excess heat recovered, high activity of slag. / Applicable only for liquid slag, worse stability of slag.
7 / Granulating by gear wheel / Fall slag onto high-speed rotating gear wheel, broken and granulated, meantime spray water for cooling. / Quick rate of slag handling, steam discharged or collected orderly. / Applicable only for liquid slag, serious corrosion of equipment, worse activity of slag.
The Layer Pouring processis the mostly-used slag cooling process in view of its low investment, easy operation and high productivity. However, truly is demanding such better targets as slag stability and activity; the self-disintegrating process by stream is paid more and more attention and under rapid development.
IV. Slag Integrated Utilization
4.1 Slag Utilization Path
Internal and external meltshop are generally considered for slag utilization, shown as Table 2.
Table 2 Slag Utilization Path
S/No. / Path / Requirement / Reason for Utilization / Reason for Limit Use1 / Internal Meltshop Utilization / Recover Fe / Converter-melting grain size: ≥10mm; Sintering grain size: <10mm / MFe in slag recovered by breakage, screening and magnetic separation. / High content of P and S, limited use for converter process.
2 / Flux for sintering / Composition variation: ≤±2%
Grain size: <10mm / CaO and Fe for higher availability of sintering machine and sinter quality, and lower fuel consumption. / Enriched P and S adverse to softening-melting and basicity of pellet.
3 / Flux for ironmaking / Grain size: 10~40mm
High basicity / Replace limestone partly. / Enriched P and S, against concept of concentrate charge in BF.
4 / Additive for steelmaking / Grain size: 5~50mm
High basicity, low P / Benefit primary slagging, lower consumption of auxiliary material, longer campaign life of converter / Enriched P and S, limited quantity for use.
5 / External Meltshop Utilization for Building Material / Production of Portland cement / Basicity ≥1.8, Fe <1%, free CaO≤5% / Active mineral C3S and C2S contained for sound hydraulicity; low heat of hydration, high secondary hardness, anti-corrosion and wearable. / Specific surface area below 400m2/kg, failed excitation full activity.
6 / Production of white cement / Reducing slag for EAF / Large quantity of C3S and C2S contained, high whiteness, due for 325# cement production (white cement) / Limited quantity in use.
7 / Production of micro powder
/pulverizing / ≤25mm (horizontal roller-grinding) / Grind to beyond 400m2/kg, for mechanical excitation of activity. / Pretty high energy consumption of grinding.
8 / Production of building block / Grain size ≤20mm / Mix with fly ash in due proportion to form and cure, for production of brick and tile. / Subject to influence of market demand, free CaO content in and basicity of slag.
9 / Road building and landfill / Powdering rate: no more than (≤) 5%,
Grain size: ≤300mm. / High strength, good activity, easy caking, prefer to mix with fly ash, slag or clay. / Stabilizing treatment, limited quantity in use.
10 / External Meltshop Utilization for Agriculture / Si fertilizer / SiO2>15%, grind fine below 60-mesh / Si can be used / Strict requirement on stock material, limited quantity in use.
11 / P fertilizer / P2O5>4% / P can be used / Strict requirement on stock material, limited quantity in use.
12 / Acid soil modifier / High basicity / CaO neutralizing with acid soil / Strict requirement on stock material, limited quantity in use.
13 / External Meltshop Utilization for Others / Absorbent for waste water treatment / Loose and porous / Absorb Ni, Cr, Pb and Cu and other particles in water. / Hard control of grain size, subject to damage original pore by grinding.
14 / Desulphurizer for flue gas / High basicity / CaO to remove SO2 / Low content of CaO, hardly for pulping; corrosive to piping.
4.2 Slag Treatment and Utilization Technology
The steelmaking process will tap large amount of slag; if not treated in time, it will occupy large space and pollute environment. Single technology will not work for taking use of slag resource. It shall consider the characteristics of slag, from generation, to take integrated measures on utilization in stepwise and profound way. Treat slag as a resource and achieve “zero waste”. To that end, the technological process flow is shown in Figure 1 below for slag retreatment and comprehensive utilization.
Fig. 1 Technological Process Flow of Slag Comprehensive Utilization
a) Slag modified treatment
The modifier for slag stability is added into molten slag. Such modifier contains mainly SiO2, and some adjustment agent such as Al2O3, CaO and MgO. Among the rest, SiO2, Fe2O3 and free CaO can react to produce calcium silicate or calcium ferrite. In this way, the stability is strengthened; the content of wearing cement mineral is increased, so is the slag use safely [5]. Oxygen may be blown during slag modification; but oxidation of Fe hereof shall be controlled; otherwise, Fe recovery rate may be less.
It’s noteworthy that the sensible heat of slag can be taken use in slag calcium treatment process, which can be expected to solve slag stability problem from the source of slagging. It’s still in test, prospecting research of significance.
b) Slag self-disintegrating process by stream
Considering mainly directed to building steels, especially, gelling materials, the steel slag shall be removed instable factors in pre-treatment, i.e., cooling process to improve its downstream stability. The self-disintegrating process by stream of slag can be described this way: the high-heat slag is sprayed water to be broken by its temperature stress; free CaO and MgO will besolved through absorbing water in the steam and expand to crack the slag in cooling process and even make it powder; less and less free CaO and MgO are remained to reduce slag size. This self-decomposition process takes place under either normal pressure or higher pressure. Presently, theself-disintegrating process by stream under normal pressure, typical smouldering, is ushering in bright future in slag cooling technology development.
It’s emphasized that the well-designed water spray schedule and well-controlled ratio of slag to water and treatment time will be the key elements to get ideal target of the above technology.
c) Slag primary grading[微软用户1] and recovery
The cooled slag is sized unevenly, still some blocks, and containing 15%~17% Fe. To recover the Fe and facilitate slag downstream use, it’d better crush the slag and do magnetic separation to recover Fe as much as possible.
Steel slag can be pre-treated with mechanical crushing and self grinding technologies. Most steel works prefers stage 1~3 mechanical crushing and magnetic separation. The finer the crushed slag, the more Fe is recovered. Specifically, 6.4% Fe can be recovered when slag is crushed to 100~300mm size; 7.6% Fe can be recovered when slag is crushed to 80~100mm size; 12~15% Fe can be recovered when slag is crushed to 25~75mm size[6].
Generally after magnetic separation, the scrap and slag in size of larger than (>) 10mm will be delivered to steelmaking, those in size of smaller than (<) 10mm to sintering shop; and some slag containing low percent of P and S but high percent of CaO will be used as flux for sintering, ironmaking or steelmaking. Besides, the slag can be used for road construction or landfill after simple grading.
The key to apply slag primary grading is to match the process to desired product and select reliable crushing and separation equipment so that slag size and Fe recovery ratio can be properly controlled and advanced.
d) Slag pulverizing
It’s to use pulverizer to grind slag to powder, affiliated to mechanical excitation of slag activity, capable of improving the physical action of fine slag in concrete work. The molten slag as produced by high-temperature melting can form the C2S and C3S lithofacies after cooled, which is similar to that of common Portland cement clinker. It’s suggested controlling the specific surface area of pulverized slag within 400~500m2/kg. Such a range is inferred from the fact that when the specific surface area is lower than 400m2/kg, the slag activity cannot be fully excited, and that when the specific surface area is higher than 500m2/kg, the slag strength cannot be remarkably enhanced; and it’s not easy and economical way [7] to call for bigger specific surface area by consuming more energies.
The slag powder can be directly mixed in commercial cement as equivalent substitute of cement, or mixed with fly ash and BF-granulated slag (two or three of them) to replace cement. It’s contributed to higher workable concrete, with better water retentiveness, durability, strength and corrosion resistance on the one hand, and saving concrete cost on the other. The steel powder can also be mixed with BF-granulated slag powder, gypsum powder and others to act as slag gelling material, which is strong in wear resistance and corrosion to alkaline, low in hydration heat and cost. It’s dedicated to cement for road and manufacture.