CEMENT-WASTE AND CLAY-WASTE DERIVED

0957±5820/01/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 79, Part B, January 2001

CEMENT-WASTE AND CLAY-WASTE DERIVED PRODUCTS FROM METAL HYDROXIDES WASTES Environmental Characterization  S1 , C. RUIZ1 , A. IRABIEN1 and F. CASTRO2 J. VIGURI1 , A. ANDRE 1 Departamento de QuõÂmica, Universidad de Cantabria, Santander, Spain Departamento de Engenharia Mecanica, Universidade do Minho, Azurem, Portugal

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etal hydroxide sludges from the wastewater treatment of metal ®nishing ef¯uents are complex wastes, mainly inorganic, with a high content of heavy metals and anions. This makes it necessary to employ careful environmental management. In this work, two selected metal hydroxide cakes from the `on site’ wastewater treatment of nickel and chromium electroplating activities and the anodizing of aluminium materials were treated with Portland cement and mixed with clay, offering two possibilities of solidi®cation/ stabilization prior to land®lling, and reutilization in ceramic products, respectively. The obtained products were characterized by the determination of ecotoxicity values, EC50, of the TCLP leachates and the chemical characterization of DIN-38414-S4 leachates. The uncon®ned compressive strength (UCS) was determined in all derived products. Cement formulations show values of EC50 higher than 3000 mg l ±1 and high concentrations of chromium in the DIN 38414-S4 leachates, due to the alkaline medium resulting from the cement addition. The clay/waste products are non-hazardous based on the EC50 and the low concentrations of heavy metals in the leachates. The paper establishes environmental characterization of both kind of products, showing that there are environmental bene®ts of waste dosi®cation in ceramic materials. Keywords: metal hydroxide cakes; metal ®nishing wastes; reuse; cement-waste products; claywaste products.

INTRODUCTION

considered to be a special waste by the majority of regulations1, and must be managed accordingly by disposal or reuse. Taking into account that the characterization of these wastes is the ®rst step in assessing their environmental impact and in establishing the best way to manage them, the characterization of wastes from different European metal ®nishing facilities, based on the Spanish Regulations and European Union (EU) Directives has been previously reported2. Ecotoxicity values and chemical characteristics of leachates have been studied, showing a complex composition and behaviour. Among the different ways of management of metal hydroxides wastes, solidi®cation/stabilization (S/S) has been demonstrated as an effective ®nal treatment step prior to disposal. Stabilization refers to the alteration of waste contaminants to a more chemically stable form and solidi®cation refers to the physical modi®cation of the waste, thereby resulting in a more environmentally acceptable waste form. Typically, stabilization processes also involve some form of physical solidi®cation3. Containment of hazardous components within the microstructure of the solidi®ed material is the most important feature of solidi®ed/stabilized wastes and it has been extensively investigated4±7. Environmental priorities establish the reutilization of

Metal ®nishing is a necessary step in the manufacture of commercial metallic products. The variety of different speci®cations has led to the introduction of a high number of Small to Medium Sized Enterprises (SME) with metal ®nishing facilities, whose primary business is the ®nishing of metal products coming from other ®rms (job shops) or where metal ®nishing is performed as part of the production process (captive shops). Both facilities spread over a range of industrial activities classi®ed in section D, subsections I, J, K, M and N of the NACE codes (statistical classi®cation of economic activities). Metal ®nishing facilities involve several operations to modify the surface properties of metallic products, based on three main steps: surface preparation (cleaning and surface activation), plating (electroplating, coatings, anodizing, etc.), and post-treatment processes (chromating, passivating, phosphating, etc.). Each step is followed by rinsing with water, leading to high volumes of liquid ef¯uent containing a variety of pollutants, mainly heavy metals and inorganic anions. Waste management in metal ®nishing facilities includes process optimization, maintenance of the industrial facilities, and on-site treatment of ef¯uents. Wastewater treatment usually includes a physico-chemical facility which generates a sludge, which is generally 38

CEMENT-WASTE AND CLAY-WASTE DERIVED PRODUCTS FROM METAL HYDROXIDES WASTES wastes as a proper way to reduce the amount of wastes for disposal. As an alternative to disposal, the reutilization of metal ®nishing wastes in the formulation of bricks, tiles and other marketable building materials is considered8; in this case metal hydroxides are encapsulated in the ceramic matrix of the ®nal product. Ceramic products based on clays show a mineralogical composition of the silicate type decreasing the release of pollutants from the ®nal products9,10. Sintering of clay and metal waste mixtures to obtain ceramic products also contributes to the immobilization of heavy metals. The main environmental impact, which can be associated with solidi®ed/stabilized products and ceramic products, is the leaching of contaminants to surface and ground water. Therefore, leaching tests play an important role in assessing the compatibility of use and/or treatment. Leaching tests have been developed with different objectives and they have different applications) the objectives of leaching tests include regulatory purposes11±14, basic evaluation of leaching15,16, their use as indicators in the environmental impact assessment17, speci®c quality criteria of products18, and their use as management tools for environmental purposes19,20. In the present study, two metal ®nishing wastes based on metal hydroxides2 were selected and processed with Portland cement and clay. The ®nal products were characterized using the TCLP and DIN-38414-S4 leaching tests suggested by the Spanish and EU frameworks, respectively. These tests were applied to the cement and ceramic products in order to assess the hazard associated with the ®nal products, as well as the feasibility of treatment to meet regulatory limits. The experimental results allow a comparison of the environmental behaviour of both kind of products containing metal hydroxide wastes. The uncon®ned compressive strength of the ®nal products was also evaluated in order to examine the range of expected mechanical properties. MATERIALS AND METHODS Metal Finishing Wastes Two metal ®nishing wastes, W1 and W2, were selected because they are typical mixed metal hydroxides cakes, which are representative of two main categories in the metal ®nishing activities from the Cantabria region (Northern Spain) and the Minho region (North-Western Portugal)2. W1 comes from the nickel and chromium electroplating of bathroom ®ttings; W2 comes from a job shop, whose primary business is the anodizing of aluminium window frames fabricated by other ®rms. Both wastes were generated from on-site physicochemical wastewater treatment plants. The wastewater treatment is based on the neutralization of the ef¯uent with calcium hydroxide, in the case of W1, and sodium hydroxide, in the case of W2, followed by clari®cation and dewatering of the resulting sludge using a ®lter press. The hexavalent chromium from chromium electroplating is reduced to the trivalent form using chemical reagents (Fe 2+) before the precipitation step. The ®lter cakes used in the laboratory experiments were dried at 1058 C, crushed and sieved. The fraction passing a 4 mm-screen was considered as the waste and processed with cement and clay. The chemical composition of the Trans IChemE, Vol 79, Part B, January 2001

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Table 1. Main components of waste ®lter cakes by x-ray ¯uorescence of elements expressed as metal oxides (% dry weight). Oxides

W1

W2

Na2O MgO Al 2O3 SiO2 P2O5 SO3 Cl CaO Cr2O3 Fe2O3 NiO CuO ZnO PbO2

0.3 0.8 0.2 1.9 19.3 6.1 0.3 34.2 17.1 2.4 10.7 3.2 2.9 0.6

11.6 0.1 45 1.9 0.3 36 1.7 0.2 1.8 0.5 0.9 0.01 0.03 0.03

wastes from a x-ray ¯uorescence analysis is shown in Table 1. Cement and Clay Waste Derived Products Cement-waste products were obtained by mixing ASTM Type I Portland cement (PC), water and dried waste at room temperature (208 C) in a CEMEX W-20, X-02-G laboratory scale solid mixer prototype. The total amount of solids was constant (1025 g) in all the experiments. The amount of added water was kept constant and equal to 40 wt.% of the total solids. The waste/cement ratio was changed over a broad range representative of that typical for S/S processes, from 9/1 to 1/2 wt. ratio 21,22. Different subsamples of each mixture were placed into 7.1 cm diameter by 14.2 cm long PVC moulds. The samples were unmoulded after 24 h in a moisture chamber (75 % humidity) and cured for 28 days at room temperature (208 C+/-3) to obtain monolithic cylindrical samples. Clay-waste products, were obtained by mixing the dried waste with clay (C) and water at a solid/water weight ratio of 18, to obtain a workable paste. Clay for the experiments was taken from the extruder step of the forming processes in the manufacture of structural-clay products. Fifty percent of the clay passed through a 75- mm screen and 100% through a 6.35-mm screen. The wastes were initially mixed with the prepared clay and water in a mechanically stirred tank, the mixing took place at room temperature (208 C+/-3) and the total amount of solid was constant (1025 g) in all the experiments. The amount of added waste was 1 wt.% and 3 wt.% of the total solids. These formulations were selected based on previous work23,24 where these levels of waste addition did not produce signi®cant modi®cations of the mechanical properties of the ceramic products. The mixtures were placed into 7.1 cm diameter by 14.2 cm long moulds and were thermally treated by continuous heating for 9 hours from room temperature (208 C+/-3) to 10008 C, isothermally followed by 5 hours at 10008 C and cooling for 9 hours from 10008 C to room temperature. Table 2 shows the oxide compositions of the Portland cement and clay used as binders in the treatment processes and Table 3 gives the formulations and treatment conditions for obtaining the cement and clay products.

VIGURI et al.

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Table 2. Composition of Portland cement and clay (% weight). Oxide Binder

SiO2

Al2O3

Fe2O3-T²

MgO

CaO

Portland Cement (PC) Clay (C)

20.3 60.2

5.61 22.8

3.43 6.19

1.12 0.62

66.4 0.05

K2O

Na2O

0.62 1.97

0.08 1.24

SO3 1.21 0.02

LOI³ 1.31 6.97

² Fe2O3-T, total Fe calculated as Fe2O3 ³ LOI, Loss on ignition

Environmental and Physical Characterization of Cement/Clay Waste Products After the mixing and treatment the 28th day specimens were tested for uncon®ned compressive strength. The ®nal products were evaluated according to two different methods of characterization previously described in the literature 1,2: determination of the ecotoxicity, EC50 parameter, given by Spanish regulations11, and determination of selected chemical parameters according to the 1991 EEC land®ll of wastes Draft25. Replicate cylinder specimens were manually crushed and sieved to obtain particles in the 4.0-mm to 1.0-mm size range for use in the leaching tests, TCLP and DIN 38414-S4 procedures. The evaluation of the ecotoxicity (EC50) of the leachates was performed, after application of the Toxicity Characteristics Leaching Procedure (TCLP)13. A high liquid/solid ratio (2 l/100 g) is agitated by tumbling at 30 rpm for 18 hours. The extraction solution is selected as a function of the alkalinity of the sample, acetic acid or acetic acid/sodium hydroxide, depending on the subsample pH, according to the method used15. After extraction and ®ltration, the ecotoxicity (EC50) of the leachates was determined. The parameter EC50 is evaluated using a luminescence bioassay with the marine bacterium Photobacterium phosphoreum in a Microtox Toxicity Analyser, M-500. The standard method is based on the light diminution of bioluminescent bacterial cells, when mixed with toxic substances. The light output of Microtox reagent (a suspension of luminescent bacteria) is measured before and after the reagent is exposed (15 min) to the sample being tested. When toxic materials interfere with the metabolism

of the growing organisms, the light emission output drops in proportion to the toxicity of the medium. The TCLP leachates were adjusted to contain 2% sodium chloride and all subsequent sample dilutions were made with 2% sodium chloride solution, which is the control. Bioluminescence was measured at 158 C on the saline sample. For every sample, the simultaneous testing of a control with ®ve dilutions of the leachate was conducted to quantify toxicity and to determine the effective concentration of leachate that decreases the normal light production by 50% (EC50). Estimates of this value were obtained using linear regression analysis of the light decrease of the ®ve dilutions. Results were compared to the value EC50 # 3000 mg l ±1 given by the Spanish regulations for the ecotoxicological characterization of wastes (toxicity characteristic H14). The leaching behaviour in distilled water25 was evaluated according to the composition of the leachates obtained by application of the leaching test procedure DIN 38414-S426. This leaching test has a liquid to solid ratio of 1 l/100 g, which is slowly agitated by tumbling at 0.5 rpm for 24 hours. The leachate is ®ltered and pollutants analysed. Analytical evaluations of metals: Cd, Cr, Cu, Fe, Ni, Pb and Zn, were performed using Atomic Absorption Spectrometry, Perkin Elmer, 1100 B. Anion concentrations: Cl and SO24 were evaluated using a Waters 746 Ion Chromatograph with a conductivity detector Model 430, and IC-Pack A anionic column. The experimental results obtained in this work were compared with the concentration levels proposed by the 1991 EEC land®ll draft in order to classify the wastes as inert, hazardous or non-hazardous, for the purpose of land®lling. Uncon®ned compressive strength (UCS) was used to

Table 3. Mix details and treatment conditions to obtain cement and clay products. Process conditions Treated wastes

Waste addition

Cement

Clay

Water

ÐÐÐÐÐÐÐÐÐ % total weight ÐÐÐÐÐÐÐÐÐ Cement/Waste Products W1/PC1 W1/PC2 W2/PC1 W2/PC2 W2/PC3 Clay/Waste Products W1/C1 W1/C2 W2/C1 W2/C2

45 54 20 30 45

15 6 40 30 15

____

0.95 2.80 0.95 2.80

____

94 92 94 92

____ ____ ____

____ ____ ____ ____

Waste/Binder²

Process temperature

wt/wt

40 40 40 40 40

3 9 0.5 1 3

5 5 5 5

0.01 0.03 0.01 0.03

Mixing at 208 C ³ 208 C 1.8 8 C/min 10008 C (5h)

² Total solids: 1025 g ³ Curing time: 28 days

Trans IChemE, Vol 79, Part B, January 2001

CEMENT-WASTE AND CLAY-WASTE DERIVED PRODUCTS FROM METAL HYDROXIDES WASTES Table 4. Results of TCLP leaching test on the metal ®nishing treated wastes. Waste sample

Mixture

TCLP leachate

Waste/Binder (wt/wt)

Final pH

EC50 (mg/l)

waste 1

4.7 5.8

4729 48,770

3/1 9/1 1/2 1/1 3/1

7.1 5.5 10.1 9.4 7.5

74,830 1455 1,000,000 1,000,000 1,000,000

1/99 3/97 1/99 3/97

4.9 4.9 4.9 4.9

7883 7186 6072 6772

W1 W2 Cement/Waste products W1/PC1 W1/PC2 W2/PC1 W2/PC2 W2/PC3 Clay/Waste products W1/C1 W1/C2 W2/C1 W2/C2

waste 2

assess the physical durability of the products20,27. UCS was determined in accordance with the ASTM D 1633 Standard Test Method for Compressive Strength of Molded SoilCement Cylinders28. The UCS was measured on the 28th day after the waste was mixed, using a Schenk Trebel Universal Testing Machine, a standard error of +/ 20% has been established from replications. RESULTS AND DISCUSSION Leaching Behaviour of Cement/Clay Waste Products The values of pH and ecotoxicity (EC50) obtained in the TCLP leachates of the ®nal products are shown in Table 4. The ecotoxicity values of the cement-waste products show a strong in¯uence of the kind of waste, very low ecotoxicity for W2, cement products and higher ecotoxicity for W1, cement products. High Waste1/cement ratios (9/1) seem to increase the leachate ecotoxicity, which can be controlled by lower Waste1/cement ratios (3/1). Ecotoxicity values of the ceramic products containing low waste dosages are in the range of moderate ecotoxicity (6000 to 8000 mg l ±1) for both W1 and W2. Although solidi®cation with alkaline binders is widely used in the treatment of metal ®nishing sludges3,29±35, the

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results obtained in this work, show that there is not a clear relationship between formulations and ecotoxicity values of the TCLP leachates. While the literature contains detailed studies of the performance of treatments of de®ned metal ®nishing wastes with speci®c binders, it does not seem to be easy to assess the environmental impact of the ®nal products. Leaching tests based on distilled water have been widely considered for the environmental evaluation of solid materials. The experimental values of pH and selected metals and anions obtained in the DIN 38414-S4 leachate from the cement and clay/waste products are shown in Table 5. Since a non-acidic leaching medium (deionized water) was used, the leachate pH is indicative of the equilibrium pH of the product in pure water. The major role of the cement binder is to increase the pH of the contacting ¯uid. The mobility of metals is strongly dependent on the leachate pH2 and therefore a strong in¯uence of this variable can be expected. Cadmium, iron, lead and zinc do not show any detectable concentrations in the leachates (below 0.05 mg l ±1). A dramatic reduction of nickel and sulphate concentrations in the leachates and a moderate reduction of chloride concentrations were observed in all cases. The nickel reduction can be associated with water soluble salts in the waste which can be precipitated at higher pH in the cement and/or ceramic matrix. The sulphate reduction can be explained by the stabilization of calcium salts in the cement/ ceramic matrix. The concentrations of chromium seem to be strongly related to the pH of the leachates, being higher than EEC standard values proposed in the Land®ll Wastes Draft. It was observed that the concentration of chromium in the leachate may be due to the amphoteric behaviour of this element in the case of cement/waste products, and the oxidation of Cr(III) to Cr(VI) in thermal treatment process of the ceramic products36,39. A similar increase in the leaching behaviour of copper is obtained in the products. This behaviour can be associated with the alkaline medium of cement/waste products, which may be responsible for the copper remobilization. Taking as references the limit of ecotoxicity given by the Spanish regulation (3000 mg l ±1) and the suggested

Table 5. Results of DIN-S4 leaching of the metal ®nishing treated wastes. Chemical Leachate Parameters Waste sample

Waste Binder

W1 W1/PC1 W1/PC2 W1/C1 W1/C2 W2 W2/PC1 W2/PC2 W2/PC3 W2/C1 W2/C2

_____

Standard ²

_____

3/1 9/1 1/99 3/97

_____

1/2 1/1 3/1 1/99 3/97

pH

Cr

4±13

Trans IChemE, Vol 79, Part B, January 2001

Ni

Cl ±

SO24

ÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐ mg l ±1 ÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐ