Environmental and technical assessment of ferrochrome slag as concrete aggregate material

Highlights

• Chromium release from the ferrochrome slag limits its utilization/disposal.

• The slag material has all the desirable properties for its use as concrete aggregate.

• Successful immobilization of chromium occurs in cement-concrete matrix.

• Concrete with slag as aggregate satisfies the desired technical properties.

• Concrete with slag is environmentally compatible with low chromium leaching.

Abstract

Ferrochrome slag is a major solid waste generated from submerged electric arc furnaces during manufacturing of ferrochrome alloy. The waste slag has excellent mechanical and engineering properties for utilization as concrete aggregate material. But it contains about 6–12% of residual chromium which has the potentiality of releasing hazardous chromium compounds to the environment. This research work carried out the mineralogical and chemical characterization study to find out the major chemical elements and mineral phases in the solid slag matrix. Physico-mechanical experimental studies indicated the suitability of the slag as aggregate material in concrete work. The concrete product with ferrochrome slag as coarse and fine aggregate showed excellent results with respect to compressive strength and were found to be suitable for general purpose concrete work. The standard leaching experimental results showed that the leachable chromium remains well immobilized in the cement and concrete matrix with very low to non-detectable level of chromium leaching. The results indicated the technical acceptability and the environmental compatibility of the slag as concrete aggregate material.

Introduction

High Carbon Ferrochrome (HCFeCr) is the most common alloying material for the production of different grades of stainless steel. Chromium and iron in chromite ore can form a continuous series of solid solution under certain conditions of heat treatment containing 45–80% of chromium. It is manufactured through direct smelting in Submerged Arc Furnace (SAF) at a temperature above 1500 °C. The furnace has the suitable system for tapping heavier metal and lighter slag and their handling. For the production of each Metric Ton (MT) of ferrochrome about 2.5–2.6 MT of chromite ore, 0.5–0.6 MT of coke and 0.3 MT of fluxing agents are required. There is a generation of 1–1.2 MT of solid waste slag for each MT of ferrochrome product. The waste slag material can be made available in different sizes under different cooling conditions and after material recovery. It contains about 6–12% deleterious substances like chromium as chromium oxide and has the potentiality of releasing hazardous chromium compounds to the environment restricting its use and disposal. Chromium is one of the most common toxic heavy metal found in the environment. It exists in the common oxidation states of hexavalent chromium Cr(VI) and trivalent chromium Cr(III). While chromium as Cr(III) is less mobile and less harmful, Cr(VI) is highly leachable and extremely toxic under all environmental conditions. As per Occupational Safety Health Administration (OSHA) [1], the major health effects associated with exposure to Cr(VI) include lung cancer, nasal septum ulcerations and perforations, skin ulcerations, and allergic and irritant contact dermatitis etc. Toxicity of chromium ranges from pulmonary to dermatological problems. As per US EPA [2], it is a suspected carcinogen. That is why there exists stringent Indian discharge standard such as 2.0 mg/l for total chromium (total Cr) and 0.1 mg/l for Cr (VI) [3].

In ferrochrome manufacturing process, the slag is a reactive medium, where most of the reduction reaction takes place. Ferrochrome (FeCr) slag is found to consist of mainly silica, alumina and magnesia with significant amounts of chromium and iron oxides in the form of Partially Altered Chromite (PAC) and entrained ferrochrome alloy [4]. At smelting condition, chromium is reported to exist as divalent CrO above 1600 °C. It is converted into metallic and trivalent chromium on cooling under ambient condition. The precipitated Cr₂O₃ is found to be in the needle shape and the metallic chromium inclusions are normally associated with these precipitates. The Cr₂O₃ and metal phases are normally concomitant and are dispersed in silicate phase [5]. Typically the granulated slag is reported to have three different phases, namely the amorphous glass phases including that of magnesium and calcium silicates, zonal (Fe, Mg, Al, Cr) oxide spinel phase and entrained metallic ferrochrome alloy [6]. A Scanning Electron Microscopy (SEM) study showed the slag having a partly crystalline and hypidiomorphic spinel (Mg, Fe) (Fe, Al, Cr)₂O₄ crystals totally enclosed in a condensed and homogenous glass matrix [7]. An Electron Probe Micro Analysis (EPMA) study result indicated that chromium spinel phase is found to exist in three modes as remnants of PAC having irregular space and lying within the framework of original particle as rims of chromite particles in an earlier stage of alteration or as separate euhedral crystals that recrystallized out of slag [8]. As per Kimbrough, Cr(III) stability occurs over a wide Eh and pH range under both reducing/oxidizing and acidic/alkaline conditions. Cr(VI) stability occurs in a much narrower zone like oxidizing and alkaline conditions. The hexavalent state is stable in an oxidizing alkaline environment, whereas the trivalent state is stable in a reducing acidic environment [9]. It is reported that, all hexavalent chromium salts except calcium, barium and lead chromate are found to be soluble in water under all pH conditions. Chromium being an amphoteric element is found to be insoluble as Cr(III) in the intermediate pH range showing minimum value in the pH range of 5.5–8.5. Above pH 5, chromium leaching is dominated by hexavalent chromium and in acid medium, that is below pH 5, total chromium in leachate is increased considerably because of dissolution of immobile Cr(III) from the slag. Cr(III) leachability is mostly controlled by the formation of insoluble oxides and hydroxide compounds and by the surface adsorption mechanisms. The mobility of chromium is determined by the competition among the mechanisms like dissolution/precipitation, redox transformation and adsorption/desorption [10], [12], [13]. Kilau reported the formation of picrochromite (MgO⋅Cr₂O₃) with the availability large amount of MgO preventing the release of chromium even under mild acid conditions [11]. Standard leaching tests indicated only very low leaching of chromium from ferrochrome slag because of its inherent chromium fixing ability in to spinel phase and of the structural encapsulation of the dispersed crystals inside an impermeable and chemically stable glass phase [7]. There is remote possibility of conversion of Cr(III) to Cr(VI), because of highly reducing conditions existing in the furnace, and the high redox potential of Cr(VI)/Cr(III) couple. The oxidation of Cr(III) to Cr(VI) by the naturally available oxidants like dissolved oxygen and MnO₂ under field condition, is reported to be too slow to cause any significant leaching of chromium [14], [15].

Zelic in his experimental study found that FeCr slag satisfied the criteria as aggregate for concretes and the concrete prepared from the slag showed better compressive strength results and was found suitable in pavement and general purpose concrete work. The slag was found to be particularly suitable for high concrete brands (M-50 and higher) where the natural aggregates could not provide the desired results [16]. The mechanical and the physical properties of the FeCr slag satisfied the requirement of the aggregate for granular layers of flexible pavement [17]. Lind justified the use of FeCr slag with minimum adverse environmental impact in road construction [18]. FeCr slag is found to be suitable as construction material due to its excellent technical and material properties but its use has been limited by the environmental concern of release of chromium from slag. Blended with fly ash there is a decrease in chromium leaching from the slag matrix [19]. Korkut et al [20] reported the usage of FeCr slag in epoxy resin up to 50% improving the nuclear radiation shielding performance of the blended epoxy resin.

Immobilization of chromium in cement-concrete matrix depends upon its oxidation state. It has been extensively reported in literature that Cr(III) is effectively immobilized in all types of cement-concrete matrix reducing chromium leachability to minimum, well below the US EPA TCLP limit and Indian discharge standards [21], [22], [23], [24], [25]. Cr(III) is immobilized in C₃S in OPC matrix due to formation of Cr(OH)₃. Chromium as Cr³⁺ is found to be substituted by Al³⁺, Si⁴⁺and Ca²⁺ in Calcium Aluminate Hydrate (CAH) phase and in Calcium Silicate Hydrate (CSH) phase. The necessary containment frame work is provided by the stable CSH network preventing chromium leaching from slag [26], [27]. But it was reported by Glasser that Cr(OH)₃ is not formed in the hardened product. Solubility in pore water is found to be much less than the solubility of Cr(OH)₃ at the same pH. The cement hydration products having octahedrally coordinated Al³⁺, is readily replaced by Cr³⁺. Chromium like aluminium is amphoteric, both remain insoluble in the cement environment. As the solubility of aluminium in the normal cement of pH range of 11–13 is controlled by the solubility of calcium aluminate hydrate and not by Al(OH)₃, so also the solubility is limited by the solubility of calcium chromium aluminate hydrate and not by Cr(OH)_3. Therefore it can be concluded that chromium is immobilized and stabilized in the cement and concrete matrix and thus reducing its leachability [23], [28], [29], [30], [31]. On the other hand Cr(VI) is not effectively immobilized in cement-concrete matrix with Ordinary Portland Cement (OPC), and is found to be soluble in entire pH range [21], [23], [32], [33]. Cr(VI) substitution in ettringite is not expected in OPC matrix with its very formation is inhibited by the presence of chromium. It is reported that the slag type cement with Granulated Blast Furnace Slag (GBFS) is found to effectively immobilize Cr(VI) because of the low redox potential and reductive capacity of GBFS with sulphide content [23], [24], [34], [35].

The objective of this research work is to evaluate the technical performance of concrete material with the use of the ferrochrome slag as aggregate material and is to assess the environmental compatibility of the waste slag with chromium immobilization in cement concrete matrix and its release behaviour from the concrete monolith having slag as aggregate material.