Foundry processes: the recovery of green moulding sands for core operations

Transcription

1 Resources, Conservation and Recycling 38 (2002) 243/254 Foundry processes: the recovery of green moulding sands for core operations Maria Chiara Zanetti, Silvia Fiore * Department of Georesources and Territory, Politecnico di Torino, Torino, Italy Received 20 July 2001; accepted 18 October 2002 Abstract Three different treatment processes for the reclamation of bentonite bonded moulding sands, made up of silica sand, coal dust, and clay, are considered in this work. The studied processes are the following: two kinds of wet mechanical regeneration, performed by the Safond and Sasil plants situated in Northern Italy, and a dry mechanical regeneration, performed by Gemco Engineering in the Netherlands. The performances of each treatment process were evaluated, considering the inflows and outflows. Each sample was physically and chemically characterized by means of particle-size analysis and the determination of thinness index (AFS), acid request, coal dust, oolitic, and some metals contents. The results of the inflow and outflow characterization were compared for each process to evaluate the efficiency of the considered regeneration plant. From an economic point of view, dry mechanical regeneration has proved to be the best solution but wet regeneration allows a better quality product to be obtained. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Moulding; Bentonite bonded moulding sands; Green sands; Reclamation; Core operations; Cold box; Foundry wastes 1. Introduction Cast iron foundries create a large quantity of wastes: one-quarter to one ton of solid waste per one ton of casting (EPA, 1981), 30/60% of which is made up of core * Corresponding author. Address: Dipartimento di Georisorse e Territorio, Politecnico di Torino, C.so Duca degli Abruzzi, 24, Torino, Italy. Tel.: / ; fax: / address: sfiore@polito.it (S. Fiore) /02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S ( 0 2 )

2 244 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/254 and moulding sands. The recovery and reuse of wastes is the alternative with the lowest impact and leads to a saving of raw materials and a reduction of manufacturing residues. Foundry wastes can be recycled or reused in several ways, in foundry manufacturing itself or in different industrial processes (Tonda et al., 1997; Zanetti et al., 1999; Fiore et al., 1999; Zanetti et al., 2000). A cast iron foundry is composed of six main operating areas (EPA, 1997): core and mould preparation, furnace charge preparation, pattern making, melting and casting, cleaning and finishing and finally sand handling. The moulding operation is performed by shaping a suitable refractory material to form the cavity in which the molten metal will be introduced. The cavity has to retain its shape until the metal has solidified and the casting is separated from the mould by means of the shakeout operation. The material employed for moulding operations depends on the type of metal being cast and the desired shape of the final product. The most common moulding material is sand. Green sand moulding is by far the most diversified and well known method used in metal casting operations and is performed by 85% of foundries (EPA, 1981). This process uses a mixture of silica sand, which provides the structure for the mould, 2/ 10% b.w. of coal dust (commercially known as black mineral) to increase the refractory of the sand, and 3/10% b.w. of clay, used as a binding agent. Water is added, in a quantity equal to 3/4% b.w., to activate the clay transformation to gel, which gives an appreciable cohesion to the mould. The mixture is then tightly compacted in a pattern, made of metal or wood. The two half moulds are positioned together to obtain the complete external shape of the casting. The word green denotes the presence of moisture in the moulding mixture, and indicates that the mould is neither baked nor dried (Luther, 1997 /1998). This method is suitable for most metals, is adaptable to large or small quantities, and the pattern and material costs are relatively low. However, there are general limits concerning the complexity of design and the dimensional accuracy of the finished product. Sand mixtures are also used to create cores, which are the pieces that fit into the mould to create the internal passages in the metal piece. Cores must be strong enough to withstand molten metal, and sufficiently collapsible to allow removal from the casting after it has cooled. Resins or chemical binders are usually added to sand mixtures to obtain these properties. Depending on the binder used, cores can be either air, catalytically or thermally set. The most common core making processes are: (1) Shell moulding: in this process metal pattern halves are preheated, coated with a silicone emulsion release agent and then covered with the resin-coated sand mixture. The heat from the patterns cures the sand mix and the silicone emulsion acts as a core release agent, allowing the shell to be removed from the pattern after curing. (2) Hot box: this process employs about 1% b.w. of a phenolic resin (containing 6% b.w. of phenol and 6% b.w. of formaldehyde) and about 0.8% b.w. of a weak acid catalyst (ammonium nitrate and urea). These are mixed with sand to coat the surface of the silica grains. The core is then heated to about 230/260 8C until solidification takes place. The shell moulding process has the advantage of very fast curing times

3 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ (40 s for a core of 20 mm thickness) and a sand mixture consistency that allows the core boxes to be filled and packed quickly. It is therefore ideal for automation and the mass production of cores. The disadvantage is that more sand and binders are used in this system than in the previously cited process. (3) Cold box: this process employs a catalytic gas to cure the binders at room temperature. One hundred parts of sand are mixed with about 1.3% b.w. of liquid phenolic resin, about 0.9% b.w. of isocyanate and about 0.1% b.w. of trimethylamine gas as the catalyst. The isocyanate reacts with the phenolic resin, forming polyurethane which binds the sand grains. This process has a short curing time (lower than 10 s), is well suited to mass production techniques and the absence of heating results in substantial energy savings. A green sand mixture in a cast iron foundry is characterized by the following properties:. constant particle-size distribution (from 0.1 to 0.4 mm): to obtain a smooth and uniform surface of the metal piece. This property is measured using the thinness index (AFS), which becomes higher as the fine particles increase;. refractory: the moulding sand must be chemically and physically stable on contact with the melting metal. Black mineral is added to the mixture because its particles distil in response to the heat and create a protective gas layer of carbon dioxide between the mould and the casting. The proportion of black mineral must be limited to prevent excessive gas production, which leads to surface faults in the pieces;. permeability: this is the property that allows the passage of the gaseous substances, produced during the metal casting, through the mould and it is related to the existence of tiny channels among the sand particles, which are due to the constant particle size and the presence of a binding agent;. cohesion: this is the property that allows the mould to preserve the shape obtained through an opportune compression and is due to the binding agent. Many methods have been developed to recover green foundry sands for the production of moulds and cores (Heine, 1983) (EPA, 1997; Schleg, 2000). Green sands can be reused again and again for moulding operations without any significant refinement. The sand is sieved to remove large particles and new additional sand is added to account for the lost sand, then the material is remoulded for a different metal piece (mulling operation). There are generally three methods for the recovery of green foundry sands for core operations: dry mechanical reclamation, wet mechanical reclamation and thermal reclamation. Thermal sand reclamation employs heat in a rotary kiln, multiple-hearth furnaces, or a fluidised bed to combust binders and contaminants. The process can cause the existence of combustion products due to the fuel used to produce heat and the thermal cracking of the sand crystals. The thermal reclamation of green moulding sands, however, requires a further mechanical (pneumatic or attrition) pre-treatment

4 246 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/254 to eliminate any metallic or non metallic impurities and to reduce the sand to a preestablished grain-size, plus a final treatment to reduce the fines to less than 0.3% b.w. Dry mechanical sand reclamation is based on attrition and it removes lumps and binders from the sand. Mechanical scrubbing moves each sand grain through a sandto-metal or sand-to-sand interface to remove any impurities. This treatment can be effective for the recovery of moulding sands but produces large quantities of dust. Clay bonded sands are efficiently regenerated with the wet mechanical process as there is a wet mechanical attrition phase that is performed using water and hydrochloric or sulphuric acid. Three different treatment processes are compared in this work for the reclamation of green moulding sands for core production. The studied processes are: two kinds of wet mechanical regeneration processes, performed by the Safond plant of Montecchio Precalcino (VI) and the Sasil plant of Brusnengo (VC), both located in Northern Italy, and a dry mechanical regeneration process, performed by the Gemco plant of Globe Foundries (Weert, The Netherlands). Two samples of green sand, one representing the inflow and the other the outflow, have been considered for each treatment process. Each sample was characterized through a particle-size analysis and determination of thinness index (AFS), acid request, black mineral, oolitic and some metals contents. The inflow and outflow samples have been compared to evaluate the efficiency of each regeneration process and finally all the outflow samples were examined to assess the best treatment for the reuse of reclaimed green sand in core production using the cold-box process. 2. Materials and methods All the employed reagents are A.C.S. grade (i.e. they fulfil the American Chemical Society requirements concerning product quality) and the flasks and the glassware are class A. All the analyses were performed on dried samples that were obtained by heating the sand at the temperature of 110 8C for 2 h. The particle-size analysis was carried out on samples of about 5 kg reduced to a mass of about 400 g using a Jones splitter and a Ro-Tap Tyler mechanical siever (rotation speed: 243 rpm, stroke speed: 162 strokes/min), equipped with six Tyler mesh sieves (2 1/4 ratio) for 10 min. All the weighing operations were performed using a Mettler PC2000 balance (0.01 g sensitivity). The inflow samples were at first wet sieved at a mm dimension before the particle size analysis. The thinness index (AFS) shows the number of silica grains that remain on 1 mm 2 of the sieve surface: a high value of this parameter means the existence of a high number of fine particles. This index is calculated after the particle-size distribution analysis, each elementary percentage being multiplied for a fixed number, as reported in Table 1. The thinness index is determined by summing these values and dividing the total by the total weight of the sample. The acid request is the total acid quantity that the basic compounds contained in the recovered sand succeed in neutralizing. This parameter is very important for core

5 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ Table 1 Particle-size classes and coefficients for AFS determination d (mm) Coefficient d / B/d B/ B/d B/ B/d B/ B/d B/ B/d B/ B/d B/ B/d B/ B/d B/ B/d B/ d B/ production because its value shows whether the sand and the resin can work together. The experimental procedure employed in this study foresees the addition of 50 ml of distilled water and 50 ml of 0.1 N hydrochloric acid to a sample of a weight equal to 50 g. After 15 min of vigorous stirring, the sample is washed twice with 100 ml of distilled water, and the liquid phases are titrated, using a Orion 420 ph-meter, with a 0.1 N sodium hydroxide solution till a ph value equal to 5 is reached. The black mineral content was evaluated by roasting 10 g of dried sample at the temperature of 900 8C for 30 min. The weight difference (% b.w.) between the starting sample and the roasted one gives the required value. The oolitic content indicates the ageing degree of the sand, because the clay used as the binding agent after the melting creates a shell around the silica grains. This clay layer should not be very thick, or the metal pieces may present some surface defects. The oolitic content determination requires the addition of 200 ml of 6 N hydrochloric acid to a sample of a weight equal to 50 g that has previously been heated to a temperature of 900 8C for 4 h. The mixture is then boiled for 25 min. The sand is washed with distilled water until no more acidity is detected and then 250 ml of distilled water and 60 g of potassium hydroxide are added. The mixture is boiled for a further 25 min, the sand is washed again with distilled water until no more alkalinity is detected, and the residue is filtrated and dried. The difference (% b.w.) between the input sample and the treated one gives the oolitic content. Some metal contents (Na, K, Ca, Mg, Fe, Cr, Zn) were obtained through an acid digestion of 0.5 g sand with 6 ml of 37% hydrochloric acid and 2 ml of 65% nitric acid in a Milestone 1200 Mega microwave oven. The digested samples were filtered on Whatman grade 44 filter (16 mm particle retention) and the determination of the metal contents was performed on the obtained solutions using a Perkin /Elmer 1100B flame atomic absorption spectrometer.

6 248 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ The Safond wet mechanical regeneration treatment Safond plant of Montecchio Precalcino (VI), located in Northern Italy, performs a wet mechanical treatment for the recovery of moulding sands from several cast iron and steel foundries. This treatment is articulated according to the following phases:. starting magnetic separation;. breaking up and homogenisation of the non magnetic fraction in a mixing barrel;. screening to eliminate the d /1.6 mm particle-size fraction;. mixing the resultant fraction with water, in a solid-liquid ratio of 1:9;. cyclone separating of the thin and coarse fractions;. conditioning of the cyclone thin fraction using ferric chloride, lime and anionic polyelectrolyte to obtain the coagulation of the solid phase, which is then transferred to a settler from which the clarified water is recycled. There is also a final filter press that reduces the water content of the mud below 25% b.w.;. transferring of the cyclone coarse fraction, made up of 80% b.w. of solid and 20% b.w. of water, to ten attrition cells. The outflow product is then sieved and the d /0.1 mm particle size fraction is dried in a rotating dryer and magnetically separated to obtain the final treatment product. A design of the Safond plant is shown in Fig. 1. Fig */inflow; 2*/magnetic barrel separator; M 1 */magnetic fraction; 3*/mixing barrel; 4*/ vibrating screen 1.6 mm; S 1 */fraction d /1.6 mm; 5 */mixing pond (10% b.w. solid, 90% b.w. water); 6*/hydraulic cyclone; 7 */conditioner; a 1 */ferric chloride; a 2 */lime; a 3 */anionic polyelectrolyte; 8 */ settler; 9*/filter press; 10*/10 parallel attrition cells; 11*/vibrating screen 0.1 mm; S 2 */d B/0.1 mm fraction; 12*/rotating dryer; 13*/rotating barrel magnetic separator; M 2 */magnetic fraction; P */final product; 14*/pneumatic cyclone; S 3 */cyclone coarse fraction; 15*/bag filter; S 4 */bag filter dusts.

7 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ The Sasil wet mechanical regeneration treatment The Sasil plant of Brusnengo (VC), situated in Northern Italy, usually performs the treatment of materials from granite mining for glass and ceramic production. The treatment cycle for green moulding sand recovery is as follows:. starting screening to eliminate the d /100 mm particle-size fraction;. attrition treating with water;. separating the thin particles (d B/0.1 mm) using a screw conveyor;. drying phase;. sieving to eliminate the d /0.7 mm particle-size fraction;. magnetic separation;. sulphuric acid lixiviation;. drying phase. A design of the Sasil plant is shown in Fig. 2. Fig. 2. 1*/inflow; 2*/vibrating screen 100 mm; S 1 */particles d /100 mm; 3*/attrition cell; 4 */screw conveyor; S 2 */scrubber particles (d B/0.1 mm) 5, 11*/drier; 6, 12*/pneumatic cyclone; S 3, S 6 */cyclone coarse fraction; 7, 13*/bag filter; S 4, S 7 */bag filter particles; 8*/vibrating screen 0.7 mm; S 5 */particles d /0.7 mm; 9 */rotating barrel magnetic separator; M */magnetic fraction; 10*/lixiviator; 14*/final product.

8 250 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ The Gemco dry mechanical regeneration treatment The Gemco process is applied in Globe Foundries, located in Weert (The Netherlands), and it consists of a dry mechanical treatment for the reclamation of foundry sands for core production using the cold box process. The reclamation treatment is based on the grinding of individual sand grains, to free the particles of binding agents and other compounds and on the recovery of fine particles by means of a dedusting and filtering system. The environmental advantages of this process are as follows: / there is no need for an exhaust gas treatment; / the production of waste materials is relatively low in comparison to other processes; / the treatment is advantageous from an economic point of view and the recovered silica grains have a rounded shape that determines a lower percentage of cast defects. Unfortunately, the process is not continuous. The heart of the process is the sand cleaner: the dried sand grains are fed between metal blade drums that rotate at 1 rpm and a stone grinding wheel, which rotates at 40 rpm, thus obtaining the breakage of the oolitic shell and the rounding off of the silica particles. A design of the Gemco plant is shown in Fig. 3. Fig. 3. 1/inflow; 2 /drier; 3 /cooler; 4 /sand cleaner; 5/pneumatic cyclone; S 1 /cyclone coarse particles; 6/bag filter; S 2 /bag filter particles; 7/final product.

9 6. Results M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ The results of the physical and chemical analysis that were performed on the inflows (called in ) and outflows (called out ) of the three considered treatment processes are reported in Tables 2 and 3, and Table 4 and in Figs. 4 and 5, and Fig. 6. A comparison between the particle-size distribution of the inflows and outflows of each treatment plant led to an evaluation of the efficiency of each treatment process to separate the silica sand fraction that is useful for core operations (0.1 /0.4 mm). The weight limit for the fraction below 0.1 mm for the outflows is 0.3% b.w. and this requirement is satisfied by the Safond and Sasil samples and almost satisfied by the Gemco samples (Figs. 4/6). The difference (% b.w.) of the fraction above 0.4 mm between the inflows and outflows is mostly due to the moulding process concerning each sample. This difference is equal to 15% b.w. for the Safond process, to 20% b.w. for the Sasil process and to 0% b.w. for the Gemco process. The thinness index (AFS) values of the three samples are also in agreement with the prescribed interval of 44/52. The values obtained through the determination of the acid request, the black mineral content and the oolitic content are reported in Table 3 together with the prescribed foundry limits concerning these parameters for the reuse in the cold-box core making process. The Sasil samples comply with all the limits, whereas the Safond and Gemco samples are above the limits for the oolitic content, for the acid request value and oolitic content, respectively. The acid request, black mineral and oolitic values should satisfy the foundry requirements not to lower the compression strength of the produced cores. Globe Foundries produce cores using the cold-box process utilizing 100% of reclaimed sand and increasing the contents of resins and amine to achieve the required compression strength of the products. The Teksid cast iron foundry of Crescentino (Northern Italy, with a production of 800 t/d) employs 30% b.w. of Safond reclaimed sand and 70% b.w. of new sand in core production, thus satisfying all quality requirements. Sasil reclaimed sand could also be used in Teksid core production. The gathered data concerning the determination of several metals are reported in Table 4. The appreciable decrease of sodium and iron in the outflow samples are due to the loss of the bentonite and the magnetic particles during the treatment processes. Table 2 Particle-size analysis parameters Sample d B/0.1 mm (% b.w.) d /0.4 mm (% b.w.) AFS SAFONDin / SAFONDout SASILin 2 30 / SASILout GEMCOin / GEMCOout Foundry requirements 0.30 / 44/52

10 252 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/254 Table 3 Acid request, black mineral and oolitic contents Sample Acid request ph 5 (ml) Black mineral (% b.w.) Oolitic (% b.w.) SAFONDout SASILout GEMCOout Foundry requirements 0 / Table 4 Metal contents (% b.w.) Sample % Na % K % Ca % Mg % Fe % Cr % Zn SAFONDin B/ SAFONDout B/0.005 B/0.002 SASILin 0.06 / / / 0.77 / / SASILout 0.02 / B/0.01 / 0.04 B/0.005 / GEMCOin GEMCOout B/ Fig. 4. Particle-size analysis of the Safond samples. The iron loss in the Gemco samples is less noticeable as the process does not involve a magnetic separation step. The prescribed limit value of the foundry for the total oxide contents for cold-box core operations is 1% b.w. therefore, some problems could be expected with this limit compliance for the Safond and Gemco outflow samples because of the high iron content.

11 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/ Fig. 5. Particle-size analysis of the Sasil samples. 7. Conclusions Fig. 6. Particle-size analysis of the Gemco samples. The results of the analyses show that the wet mechanical treatment, particularly the Sasil process, is the most effective for the recovery of green moulding sands for cold-box core production. The values of the fine particles, oolitic content and acid request are better than those of the dry mechanical treatment. However, the wet mechanical treatment involves a noticeable sludge production and a relatively high treatment cost of about 0.02 /0.03 t/kg. The dry mechanical process is economical and effective, but, due to the high gathered acid request value (/10 ml), requires a change in the cold-box core making foundry operations if this recovered foundry sand is to be used, and an individual study of the organic additives mixture is necessary for each plant in order to comply with the core making operations. Another disadvantage is the low mill productivity of around 3 t/h that is essentially due to the discontinuity of the process. The treatment costs are about 0.01 t/kg. Foundry wastes is a problem that concerns foundries working in the field of secondary fusion of iron metals throughout the world. In Europe, because of the different rules and economic factors that exist in different countries, a solution that can be valid for the reuse of foundry wastes in one country might not be acceptable

12 254 M.C. Zanetti, S. Fiore / Resources, Conservation and Recycling 38 (2002) 243/254 in another country. However, a knowledge of the possible solutions could lead to choosing modifications that might be acceptable. Silica sands for foundry products in the different European countries are commercialised at different prices: in Italy the price is about 0.04 t/kg while in Belgium and Netherlands the same product is sold at about 0.01 t/kg. These differences and the landfilling costs justify the different solutions adopted in European countries for green moulding sands (recycling or reuse): recycling in Italy, re-use as capping for landfills and concrete production in Sweden, re-use for road construction in Belgium and so on. On the grounds of the mentioned observations, the wet mechanical process can be applied to green moulding sands obtained from several foundries at the same time and is able to satisfy the prescribed requirements for cold-box core operations (the total flow rate has to be at least / t/y); the Gemco process, instead, has to be applied to a single foundry because of the low flow rate and the necessity of having to change the core making foundry operations, according to the kind of cast iron production (large or small cores, difficult or simple cores). Because of the costs, the wet mechanical treatment process could be adopted in European Countries where silica sands have a high commercial price or where there are few other re-use opportunities such as in France, Spain etc. The dry mechanical treatment process, instead, could be applied in countries with a low commercial price for silica sands (Belgium and Netherlands). However raw material saving, which is a direct consequence of recycling and re-use, is also an important environmental aspect that is now considered all over the world. References EPA. Summary of factors affecting compliance by ferrous foundries, vol. 1, EPA-340/ ; pp. 34/49. EPA. Profile of the metal casting industry, EPA/310/R-97/004; pp. 15/22. Fiore S, Zanetti MC, Onofrio M. Treatment process definition for the reuse of manufacturing dusts from metallurgical slugs. Proceedings of the Second National Congress Valorization and Recycling of Industrial Wastes, L Aquila, Italy; pp. 117/125. Heine HJ. Saving dollars through sand reclamation-part 1. Foundry Management and Technology 1983;111(5):22/5. Luther NB. Metalcasting and molding processes vol. 7, No. 1 Casting Source Directory1997/1998. pp. 29/35. Schleg F. Guide to casting and molding processes. Engineered Casting Solutions 2000;2(3):18/27. Tonda E, Zanetti MC, Clerici C, Operto M. Iron recovery from foundry wastes, Proceedings of the First National Congress Valorization and Recycling of Industrial Wastes, L Aquila, Italy; pp. 89/95. Zanetti MC, Giordanetto L, Clerici C. Recovery of an old landfill: wastes treatment and recycle, Proceedings of Sardinia 1999, Seventh International Waste Management and Landfill Symposium, S.Margherita di Pula, Italy; pp. 595/600. Zanetti MC, Clerici C, Sandrin D, Operto M. Employment of foundry wastes, Proceedings of The XXI International Mineral Processing Congress, Rome, Italy. 2000;C12a. pp. 9 /14.