Smelting Slag Research Articles

The advances in the recovery process for precious metals from nickel slag, a review

The creation of nickel-smelting products including important metals like nickel, cobalt, and copper is a persistent issue in the nickel mining business. Another secondary source of precious metals is nickel slag. In addition, the massive amounts of nickel slag generated by the nickel smelter sector will pollute the environment, particularly the soil, since the smelting slag contains hazardous materials. This study examines techniques for recovering precious metals from nickel slag by reviewing publications from Springer Link, Google Scholar, MDPI, ScienceDirect, Membrane Journal, and other authors. The two types of metal recovery techniques from nickel slag that were examined were hydrometallurgical and pyrometallurgical techniques. This review article describes a pyrometallurgical method that involves roasting and selective reduction. In the meantime, the hydrometallurgical techniques were examined in the high-pressure oxidative acid leaching process, the atmospheric acid leaching method, and the bioleaching process. A roadmap for research designs that can be used to recover valuable metals from nickel slag sustainably has been created due to the completed literature evaluation.

Smelting characteristics of nickel‑chromium‑manganese bearing prereduced pellets for the preparation of nickel saving austenite stainless steel master alloys

Nickel saving austenite crude stainless steel master alloys were innovatively produced by smelting of nickel‑chromium‑manganese bearing prereduced pellets in an effective and economical way. The thermodynamics analysis and systematic process mineralogy were also conducted to further reveal the smelting characteristics of prereduced pellets and the related enhanced mechanism. The results showed that the preferable stainless steel master alloys with 66.29% Fe, 4.21% Ni, 17.28% Cr, 6.53% Mn and 4.76% C can be prepared from the nickel‑chromium‑manganese bearing prereduced pellets with a corresponding recovery of valuable metals more than 95%. Furthermore, during the enhanced smelting process, the optimization of slag types can further realize the phases transformation from high-melting-point enstatite and forsterite to low-melting-point diopside and monticellite to obtain a decreased smelting temperature and viscosity of slag. The smelting duration and the alloy grain included in the smelting slag were both obviously decreased, resulting in an improved smelting performance.

Laboratory Scale Evaluation of the Slag Foaming Behavior

Due to the ambitious climate targets of the European Union, one can expect that the electric arc furnace (EAF) will gain greater importance in the future of steelmaking. Since slag foaming is a decisive factor in an efficient process, understanding this phenomenon is essential when applying hydrogen-based direct reduced iron (DRI). Therefore, a method was developed to check different slag compositions concerning their foaming behavior. Slag samples are melted, and a carbon carrier is added. After a selected reaction time, the crucible is quenched in liquid nitrogen, superficially freezing the state while foaming. Afterward, it is halved, providing metallographic examination and height measurement possibilities. Three slags were tested, MgO-saturated EAF slag, MgO-unsaturated EAF slag, and electrical Smelter-like slag. Digital and scanning electron microscopy with energy dispersive x-ray spectroscopy are used to compare the slags and evaluate the method. The Smelter slag shows no foamability, unaffected by the FeO content. Contrary, good foamability can be observed for EAF slags.

Mechanistic insight into phosphorus migration pathways from oolitic hematite using in-situ observations during roasting and reduction process

Oolitic hematite is a promising alternative resource, in facing the iron ore downgrading. As oolitic hematite is often rich in phosphorus, an efficient separation of iron and phosphorus is necessary, in terms of utilizing both elements. Therefore, a close insight into the transformation of phosphorus-bearing minerals in the unique oolitic structure during the reduction process is important. To capture the real-time transformation of the apatite ring in the oolitic structure, in-situ observations under roasting, and CO/carbon reducing conditions were adopted by high-temperature confocal laser scanning microscopy. The results indicated that the apatite ring was transformed by four stages during the roasting process from 25 °C to 1400 °C: stabilization, defluorination, partial melting, and complete melting, which was: Ca5(PO4)3F → Ca3(PO4)2 → Ca3(PO4)2 + P2O5 liquid → P2O5 liquid. Under the CO reduction, the partial reduction of the phosphate at 1400 °C generated a Fe-Plow alloy. While the carbon reduction promoted iron phase absorption of phosphorus, forming Fe-Phigh alloy at 1400 °C. In addition, interfacial phenomena between apatite and slag melt were observed, and the dissolution of apatite (Ca/P molar ratio) into the slag melt was affected by the Fe/(Al + Si) molar ratio of the slag. Finally, using inverse error function methods, the diffusion coefficients of P and Ca increased significantly from the roasting to CO reduction and then to carbon reduction. These findings can benefit to understand phosphorus migration mechanisms, improving effective dephosphorization strategies for high-phosphorus oolitic hematite.

ОЦІНКА СТРУКТУРИ ШЛАКОВИХ РОЗПЛАВІВ НА ПІДСТАВІ АНАЛІЗУ ТЕМПЕРАТУРНИХ ЗАЛЕЖНОСТЕЙ В’ЯЗКОСТІ ТА ЕЛЕКТРОПРОВІДНОСТІ З ВИКОРИСТАННЯМ АДАПТИВНОЇ СЕГМЕНТНОЇ РЕГРЕСІЙНОЇ МОДЕЛІ

The expediency of using an adaptive segmented regression model for the analysis of logarithmic dependences of viscosity and electrical conductivity on temperature to assess the structure of slag melts is shown. Calculated values of the activation energies of viscosity ( ) and electrical conductivity ( ) in different temperature ranges. The obtained calculation results confirm that the viscosity and electrical conductivity of slag melts characterize the interaction of various structural particles (ions and/or their groups) and can be used in the study of the structure of slag melts. It was established that the activation energy of viscosity and electrical conductivity decreases as the temperature of slag melts increases. The performed calculations of the indicator n=En/Ex indicate that it can be a criterion for evaluating the structure of the slag melt, when n >1, the slag melt is a heterogeneous system, and when n < 1, it is a homogeneous system. Viscosity and electrical conductivity are interdependent properties and can be predicted if there is data on one of the properties, provided the value of n is known.

Effect of TiO2 on the Phase Transformation and Microstructure Evolution of Ti-Containing Melting Slag in the Alkali Fusion Process

Effect of TiO2 on the Phase Transformation and Microstructure Evolution of Ti-Containing Melting Slag in the Alkali Fusion Process

Ball milling coupled cascade magnetic separation to recover valuable metals from alkali disaggregation copper smelting slags

Due to the lack of effective reduction and recycling methods, copper smelting slags (CSS) containing a large number of valuable metals are stockpiled. This work proposed a novel strategy of ball milling coupled cascade magnetic separation to achieve recovery of magnetite and ferrite and enrichment of Cu and Zn in alkali disaggregation CSS. The Fe grade of recovered magnetite reached as high as 72.43%, and 15.51% of Cu and 16.63% of Zn were recovered efficiently as spinel ferrites. Simultaneously, 70.54% of Cu and 58.18% of Zn were enriched in non-magnetic fraction. Compared with primary magnetic fraction, the grade of Fe in the secondary magnetic fraction increased by 19.19%. Additionally, compared with the control, the recovery efficiency and the grade of Fe in the magnetic products after ball milling increased by 52.11% and 49.20%. The main mechanisms were as follows: (1) The significant dissociation of Fe3O4 from non-magnetic impurities and the transformation of Cu and Zn to ferrite were promoted by ball milling. (2) The separation efficiency of weak magnetic ferrite and Fe3O4 was improved by cascade magnetic separation. Therefore, the proposed strategy could provide a green and sustainable solution for CSS processing.

EXPERT ASSESSMENT OF THE THERMODYNAMIC STATE OF THE "METAL-SLAG" SYSTEM IN REDUCING AND OXIDIZING CONDITIONS

A new approach has been developed to assess the thermodynamic state of the "metal-slag" system in the production of cast iron and steel using the parameters of interatomic interaction. As a result of computational and analytical studies of the indicators of the melting products of the blast-furnace and steelmaking processes, the regularities of the coordinated change in the chemical compositions of the metal and slag melts were revealed. A close relationship has been established between the integral indicators of metal (ZY) and slag (, e), characterizing the chemical and structural state of their melts. As a criterion for evaluating the thermodynamic matching of melts of the "metal-slag" system when it tends to equilibrium, an analytical expression is proposed in the form , which can be implemented in automated process control systems to solve the problems of controlling the thermal and slag modes of iron production and steel of the required composition.

Incorporation of reduced graphene oxide in waste-based concrete including lead smelter slag and recycled coarse aggregate

Waste-based concrete stands as an environmentally friendly and sustainable optifon compared to conventional concrete. The waste-based concrete holds the potential to reduce the unfavorable ecological effects associated with the concrete production. In this research, along with ground granulated blast furnace slag (GGBS) and fly ash (FA) as alternative binders to cement, lead smelter slag (LSS) as sand and recycled coarse aggregate (RCA) have been utilized in the production of the waste-based concrete. The influence of reduced graphene oxide (rGO) on mechanical and durability characteristics of the waste-based concrete has been assessed. Four rGO dosages of 0%, 0.05%, 0.1% and 0.3% by weight of binder were incorporated. Scanning electron microscopy and thermogravimetric analysis were conducted to assess the rGO-reinforced concrete's microstructure. The findings indicate that rGO improves the characteristics of the waste-based concrete. 0.1% is the optimum dosage of rGO to increase the compressive strength (47%), splitting tensile strength (44%), and flexural strength (42%) and decrease the water absorption (45%) and drying shrinkage (35%) of the concrete. Further dosage leads to reduced properties of the concrete. In addition, the waste-based concrete incorporating 20% FA, 50% GGBS, 100% LSS, and 100% RCA reinforced with 0.1% rGO exhibits similar mechanical and durability properties to the conventional concrete. The enhanced performance of the waste-based concrete can be related to the reduced porosity, refined pore structure, densified hydration products and increased hydration degree of the concrete by incorporating rGO.

Lead removal from tin chloride solution by barium sulfate and coprecipitation of (BaxPby)(SO4)2 for the processing of Pb[sbnd]Sn smelter slag

Owing to the characteristics of mineral paragenesis (equilibrium sequence of mineral phases), the hydrochloric acid leach liquors of Sn ores and Sn-containing waste materials typically contain large amounts of Pb. In this study, barium sulfate coprecipitation was employed to efficiently remove Pb from a tin chloride solution. The lead removal efficiency, reaction mechanism, and limitations were investigated by varying the Cl− and SO42− concentrations. The results showed that the structural formula of the precipitates was (BaxPby)(SO4)2 (x + y = 2), and its solubility product was in the range of 1 × 10−12–1 × 10−13, indicating that Pb2+ and Ba2+ were more easily precipitated in the form of complex salts. In addition, thermodynamic analysis revealed that when the Cl− concentration was <1 mol/L in the Pb2+–Sn2+–Cl−–H2O system, [Pb2+] and [PbCl+] were the primary species. These positively charged species were readily adsorbed onto the surface of BaSO4 via electrostatic forces, resulting in a significant increase in the Pb content of (BaxPby)(SO4)2. Therefore, the Pb removal efficiency significantly improved as Cl− concentration decreased. The lead removal mechanism involves a combination of adsorption and replacement processes. Based on the aforementioned fundamental study, a method involving dechlorination and coprecipitation was proposed to remove lead from an acid leaching solution of Kaldo slag. With the dechlorination via distillation, the Pb removal efficiency could reach 99.9% at 1 mol/L Cl− and 0.5 mol/L SO42−.

Vertical migration behavior simulation and prediction of Pb and Cd in co-contaminated soil around Pb-Zn smelting slag site

Heavy metal migration in soil poses a serious threat to the soil and groundwater. Understanding the migration pattern of heavy metals (HMs) under different factors could provide a more reasonable position for pollution evaluation and targetoriented treatment of soil heavy metal. In this study, the migration behavior of Pb and Cd in co-contaminated soil under different pH and ionic strength (NaCl concentration) was simulated using convective dispersion equation (CDE). We predicted the migration trends of Pb and Cd in soils after 5, 10, and 20 years via PHREEQC. The results showed that the migration time of Cd in the soil column experiment was about 60 days faster than that of Pb, and the migration trend was much steeper. The CDE was proved to describe the migration behavior of Pb and Cd (R2 > 0.75) in soil. The predicted results showed that Cd migrated to 15–20 cm of soil within 7 years and Pb stayed mainly in the top 0–6 cm of soil within 5 years as the duration of irrigation increased. Overall, our study is expected to provide new insight into the migration of heavy metal in soil ecosystems and guidance for reducing risk of heavy metal in the environment.

Selective recovery of rare earth elements by smelting of magnets

Rare earth elements (REEs) play a crucial role in many technologies from daily appliances in cell phones to more advanced wind turbines and electric cars. Permanent magnets account for a quarter of total global REEs production and have high recycling value. In this study, smelting process was used to selectively oxidize REEs in the permanent magnets by adding Fe2O3. This separates REEs into a slag phase from an iron-rich metallic phase. B2O3 was also added to the system as a flux to lower the slag melting temperature. This minimizes REEs loss to the metallic phase and allows a more efficient phase separation. The effect of flux and oxidizing agent addition was investigated on both regular and cerium-rich NdFeB (NdCeFeB) magnets. At 1350 °C and for 1 h, the slag phase was successfully separated from the metallic phase with the addition of 0.8 stoichiometric amount of Fe2O3 and 40 wt% of B2O3. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) analysis reveals that REEs in the magnet do not migrate to the metal phase while the REE-rich slag phase contains almost no iron. After the selective removal of iron into the metallic phase, REEs are recovered from the slag phase through an acid leaching process allowing >99% of REEs recovery. Boron in the magnet can also be recovered as useful boric acid by evaporation and crystallisation technique. The proposed process in this study is reagent and energy-efficient with almost complete valorisation of both NdCeFeB and NdFeB magnets.

Recovery of Metals from Copper Smelting Slag Using Coke and Biochar

With the purpose of recovering the metal values, in this study the copper slag was reduced by coke and biochar at 1250 °C in an argon gas atmosphere using the isothermal reduction/drop quenching technique. The phase compositions of metal, matte, and slag were determined using electron probe microanalysis (EPMA). The effects of reduction time and amount of reductant were investigated. The distribution of elements between metal/matte and slag was ascertained based on the elemental concentrations determined by EPMA. It was found that copper concentration in slag can be effectively decreased to approximately 0.4–0.6 wt% within 5 min by coke and biochar. Copper and nickel can also be successfully recovered into the copper alloy phase once settling has been accomplished.Graphical

New approach to evaluating the thermodynamic consistency of melts in the ‘metal-slag’ system based on interatomic interaction parameters

The article presents a new approach to evaluating the thermodynamic state of the ‘metal–slag’ system during metal smelting in oxidizing-reducing conditions. The interaction between the metal and slag is analyzed using the model of the structure of metallurgical melts, which considers cooperative ion exchange processes and interatomic interaction parameters. As a result of analyzing experimental data on the compositions of reacting melts during pig iron and steel smelting, criteria were developed for assessing the degree of achieving the equilibrium in the system regarding sulfur. The charge state parameters of the metallic system ZY and the slag system Δe, the slag stoichiometry index ρ, and the charge state parameter of the melt components Zi were used. The regularity of a consistent formation of metallic and slag melts has been established, which is evidenced by a significant correlation between the chemical equivalent of the metal composition ZY and the slag Δe. Analytical dependences were obtained in the form of ZY = f(Δe,ρ). The identified patterns and criteria can be integrated into automated process control systems for regulating the slag regime and producing high-quality pig iron and steel.

A metallurgical approach for separation and recovery of Cu, Cr, and Ni from electroplating sludge

Electroplating sludge is extensively produced in chemical precipitation–based treatment of electroplating wastewater. It poses a huge threat to environmental safety if not properly disposed, ascribed to its high contents of heavy metals. An innovative metallurgical approach was proposed a to recycle Cu, Cr, and Ni from it. Ammonia leaching was firstly performed to selectively leach Cu from Cr, in which the Cu oxide and sulfide were leached into the leachate while the Cr oxide and Ni carbide (NiCx) retained in the residue. (NH4)2SO4 increased the Cu leaching rate via increasing the dissolved oxygen amount in the ammonia leachate and converting CuS to Cu2+. Under the optimal conditions, the leaching efficiency of Cu achieved 96.5 % while that of Cr was only 0.1 %. In the followed aluminothermic reduction, C in the leaching residue could be effectively removed via a thermal oxidation, which in turn decreased the formation of a C-containing compound of high melting point and benefited the Cr and Ni recovery. Cr and Ni from the residue were reduced and recovered in a Cr-Ni alloy, and the reductant of Al first changed to a refractory Al2O3 and then transformed to a low melting point 12CaO·7Al2O3 with the additive of CaO. This transformation increased the molten slag fluidity and promoted the separation of Cr-Ni alloy from slag. Moreover, the excessive Al increased the Cr and Ni yields and concentrated all of them to be together. Partial Al was used as reductant, and the other Al transferred into Cr-Ni alloy to decrease its melting point. Cr and Ni contents in the smelting slag could be decreased to 0.11 wt% and 0.12 wt% respectively, showing an efficient recovery. This work provided a high efficiency method to recover Cu, Cr, and Ni from waste electroplating sludge.

Relationship between viscosity, foaming, and structure of CaO–SiO2–Al2O3–MgO–FeO slag with the addition of SiO2 and Al2O3

Optimizing the foaming characteristics of slag in electric arc furnaces (EAF) presents a challenging task, affecting electrode heating efficiency and increasing smelting cost. This study explores the impact of adding silicon dioxide (15–35 wt%) and a small quantity of alumina (5–10 wt%) on the viscosity and foaming properties of CaO–SiO2–Al2O3–MgO–FeO slag. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to investigate structural changes in the slag melt. The findings reveal that higher levels of silica or a slight increase in alumina content enhance slag polymerization and foaming efficiency. Specifically, a 10 wt% alumina content, in contrast to 5 wt%, leads to high silica content, leading to faster polymerization but relatively lower viscosity. Structural analysis confirms that increasing alumina content, particularly when silica content is high, results in a less stable Al–O–Si structure, causing a shift from Q0 and Q1 units to Q2 and Q3 units, which ultimately reduces viscosity. When the alumina content remains at 5 wt% and silica content increases from 15 wt% to 35 wt%, foaming efficiency improves from 13.06 cm min to 94.97 cm min, a remarkable 627% rise. Moreover, when silica content is 30 wt% and alumina is slightly increased by 5 wt%, there is a notable enhancement in foaming efficiency, rising from 47.11 cm min to 89.16 cm min, while the effect on viscosity is not prominent.

Preparation of Cementitious Materials from Mechanochemically Modified Copper Smelting Slag Compounded with High-Aluminum Fly Ash.

Copper smelting slag discharged from mining and high-aluminum fly ash generated during the combustion of coal for energy production are two typical bulk solid wastes, which are necessary to carry out harmless and resourceful treatment. This research proposed an eco-friendly and economical method for the co-consumption of copper smelting slag and high-aluminum fly ash. Cementitious materials were compounded with copper smelting slag and high-aluminum fly ash as the main materials were successfully prepared, with a 28-d compressive strength up to 31.22 MPa, and the heavy metal leaching toxicity was below the limits of the relevant standards. The optimum mechanical properties of the cementitious materials were obtained by altering the material proportion, ball mill rotation speed, and CaO dosage. Under the combined effect of mechanical ball milling at a suitable speed and chemical activation with a certain alkali concentration, the prepared cementitious materials had an initial activation. The pastes of the cementitious materials generated a gel system during the subsequent hydration process. The two steps together improved the mechanical strength of the cured products. The preparation was simple to operate and offered a high stability of heavy metals. The heavy metal contaminants were kept at a low content throughout the process from raw materials to the prepared cured specimens, which was suitable for application in practical environmental remediation projects and could provide effective solutions for ecological environment construction.

The Direct Alloying of Steel through Silicothermic Self-Reduction of Chromite Ore Utilizing Si-Containing Solid Waste

Organosilicon materials generate copious amounts of Si-containing solid waste during production, leading to severe environmental pollution and substantial resource squandering. In pursuit of the resource utilization of Si-containing solid waste, this study conducted experimental research on the direct alloying of molten steel through the silicothermic self-reduction of chromite ore using Si-containing solid waste as a reducing agent. Additionally, thermodynamic analysis was performed, employing the thermodynamic calculation software FactSage 8.2 (Thermfact Ltd., Montreal, QC, Canada and GTT-Technologies, Aachen, Germany), to examine the equilibrium reactions of the silicothermic reduction of chromite ore and the variations in the thermodynamic equilibrium compositions of slag and metal phases. The results indicate a reduction sequence for the reducible components in chromite ore as Fe2O3 → Cr2O3. The introduction of CaO and Al2O3 into the silicothermic self-reduction compacts altered the forms of Fe and Cr oxides in equilibrium, significantly reducing the standard Gibbs free energy (ΔG0) of the silicothermic reduction reaction. The initial slag melting point decreased from 1700 °C without the addition of CaO and Al2O3 to 1500 °C with the addition of CaO and Al2O3. Correspondingly, the slag viscosity at 1600 °C decreased from 134.1 Pa·s without CaO and Al2O3 addition to 1.81 Pa·s with CaO and Al2O3 addition. The addition of CaO and Al2O3 accelerated the reduction of Cr oxide in chromite ore and enhanced the recovery of Cr, consistent with the thermodynamic calculation results. In the process of steelmaking through the direct alloying of chromite ore silicothermic self-reduction compacts, the final recovery rate of Cr increased from 86.4% without CaO and Al2O3 addition to 95.4% with CaO and Al2O3 addition.

Vertical distribution and contamination assessment of heavy metals in the vadose zone of a 50-year-old smelter dumpsite, San Antonio Bay, Argentina

This research aimed to assess the contamination levels and mineralogical association of heavy metals in the vadose zone beneath smelter residues that were freely deposited over natural sediments from the 1960 s to the 1980 s. The results revealed elevated concentrations of heavy metals, particularly Fe, Pb, Zn, Cu, and Cd. The highest concentrations were found within the superficial layer, corresponding to the smelter slag (203,703 – 219,998 mg kg−1 Fe; 73,708 – 136,390 mg kg−1 Pb; 18,922 – 29,484 mg kg−1 Zn; 6473 – 7887 mg kg−1 Cu; 20 – 48 mg kg−1 Cd). This residue is an acidic material mainly constituted by secondary sulfates and oxyhydroxides. Although concentrations decreased with depth, the levels of these metals remained higher than the background even at depths of 300 cm. Precipitation of secondary minerals (plumbojarosite, anglesite, Zn-sulfates, and Fe oxydroxides) from the percolating leachates was detected in the vadose zone based on SEM and EDS analysis. According to their vertical distribution, Cd and Zn showed a higher mobility than Cu, Fe and Pb, which were primarily concentrated at the surface layer. The geo-accumulation index (Igeo) was Pb > Cu > Zn > Cd > Ni. The vadose zone is severely polluted and represents a very high risk to the environment based on the ecological risk factor (Eri) and risk index (RI). The metals that contribute most to this condition are Pb > Cd ≈ Cu > Zn. Given the severity of the situation, remediation of the study area is recommended.

Degradation of typical flotation reagents using lead-zinc smelting slag as mediator for persulfate activation: Effect of gallic acid and Cr(VI) on the removal performance and fate of reactive oxygen species

To remediate the Cr(VI)-organic co-contaminants in a non-ferrous mining area, a gallic acid (GA) accelerated lead-zinc smelting slag (LZSS, a mine-sourced waste) mediated peroxodisulfate (PDS) Fenton-like system was constructed for degradation of two typical flotation reagents (benzotriazole and N-hydroxyphthalimide). LZSS acting as an in-situ Fe source in the Fenton-like process, could continuously release Fe species, while GA as a chelate with reducing properties was able to accelerate the rate-limiting step of Fe(III)/Fe(II) cycle to enhance the production of reactive oxygen species (ROS). In the LZSS/PDS/GA system, produced SO4•-, •OH and Fe(IV) jointly contributed to the contaminant removal through radical/nonradical pathways. However, when Cr(VI) coexisted with organic pollutants in the LZSS/PDS/GA system, the reduction of Cr(VI) consumed the electrons that otherwise would have been available for activation of PDS, resulting in fewer different ROS being produced. The increased concentration of GA, as an electron donor, promoted the production of SO4•-, but this promoting effect gradually diminished with increasing Cr(VI). Overall, the dominant ROS gradually transformed from Fe(IV) to SO4•-/•OH as the GA level increased or the Cr(VI) level decreased. Therefore, regulation of the relative roles of ROS by adjusting either the GA dosage or the Cr(VI) levels in the wastewater can improve availability of ROS for further specific removal of pollutants. This study offers an all-in-one solution for utilization of LZSS industrial waste and degradation of flotation reagents, and it also provides a new insight into the advanced environmental application of GA in remediation of Cr(VI)-organic co-contamination.