PROCESS FOR SEPARATING THE COMPONENTS OF HARDENED CONCRETE WASTE FOR PRODUCING RECYCLED CEMENT

Abstract

The present invention lies within the field of construction materials and concerns a process for separating the constituents of hardened concrete, with the aim of extracting the cementitious fraction to be used in the production of thermoactivated recycled cement, involving the essential steps of: (a) crushing the concrete waste; (b) screening the crushed material to separate material smaller than about 1 mm; (c) fragmenting material larger than 1 mm; (d) screening material smaller than 1 mm into various granulometric fractions; (e) high intensity magnetic separation of the material; (f) grinding of the cementitious fraction resulting from the magnetic separation in the previous step to a size that allows its efficient thermoactivation; and (g) obtaining a thermoactivated recycled cement.

Claims

1. Process of separating the components of hardened concrete waste to obtain a recycled cement with the said concrete resulting from construction and demolition waste characterised by comprising the following steps: a) carrying out at least one stage of crushing the hardened concrete waste until a material is obtained whose size is close to the maximum size of the aggregates constituting the concrete waste, between 20-32 mm; b) screening the crushed material resulting from step a) with separation of material smaller than 1 mm; c) fragmenting the material larger than 1 mm resulting from step b) until at least 90% of the material smaller than 1 mm is obtained; d) screening the material resulting from step c) less than 1 mm in size in granulometric fractions between 150 μm and 1 mm; e) high intensity magnetic separation of material larger than 150 μm by at least one pass through the magnetic separator; f) grinding the cementitious fraction resulting from the magnetic separation of the previous step to a size that enables its efficient thermoactivation; g) thermoactivation of the material obtained in the previous steps to obtain recycled cement.

2. Separation process according to the preceding claim characterised in that the material in step c) is fragmented by mechanical treatment through at least one cycle of grinding until a yield of more than 90% of the material smaller than 1 mm is achieved.

3. Separation process according to claim 1 characterised in that the material in step c) is fragmented by means of a heat treatment, such that after cooling, the material subjected to the heat treatment is subjected to at least one grinding cycle.

4. Separation process according to claim 3 characterised in that the heat treatment takes place at about 400° C. for 2 hours and the grinding is autogenous and carried out in a horizontal axis mill for about 30 minutes.

5. Separation process according to claim 1 characterised in that the material screened in step d) is subjected to a further wet screening cycle followed by drying.

6. Separation process according to claim 1 characterised in that the material resulting from the screening of step d) is magnetically separated by means of a high intensity magnetic separator.

7. Separation process according to claim 1 characterised in that after at least one step of crushing the concrete waste according to step a), magnetic separation of ceramic waste with paramagnetic characteristics occurs.

8. Separation process according to claim 1 characterised in that the screened material resulting from the fragmentation in step c) is subjected to magnetic separation of ceramic residues by at least one pass through the magnetic separator.

9. Separation process according to claim 1 characterised in that the magnetic separation is performed dry using permanent rolls (4) made from rare-earth magnets.

10. Separation process according to claim 9 characterised in that the permanent roll (4) are sequentially arranged and comprise discs or rings of permanent magnets interrupted at intervals with steel discs, the rings being positioned with faces of the same polarity side by side.

11. Separation process according to claim 9 characterised in that the diameter of the permanent rolls (4) is greater than 76 mm, preferably 300 mm.

12. Separation process according to claim 9 characterised in that the magnetic separation is performed through a conveyor screen (3) with a thickness of less than 1-2 mm and with a minimum thickness of 0.12 mm.

13. Separation process according to claim 9 characterised in that the rare-earth magnets are made of Neodymium-iron-boron.

14. Separation process according to claim 1 claim characterised in that the crushing of the concrete is performed in a jaw crusher, in cone crushers or by other mechanical means of release.

15. Separation process according to claim 1 characterised in that the magnetic separation in step e) is performed by induced rollers, by high intensity and gradient wet magnetic separators or by other means of magnetic separation.

16. Recycled cement obtained by the process described in claim 1 characterised by having a potential mechanical strength that can be similar to that of ordinary cements of strength class 32.5.

Description

DESCRIPTION OF THE FIGURES

[0068] FIG. 1—schematic representation of the permanent magnetic roll separation equipment consisting of a feeder (1), a vibrating tray (2) that deposits the material on a conveyor screen (3), which is then separated by means of a permanent magnetic roll (4) and a splitter (5).

[0069] FIG. 2—generalized schematic representation of the separation of constituents of concrete waste.

[0070] FIG. 3—schematic representation of the separation of the constituents of concrete waste with a view to recovering the cementitious fraction, as described in Example 1 of the detailed description of the invention.

[0071] FIG. 4—schematic representation of the separation of the constituents of concrete waste with a view to recovering the aggregates fraction, in addition to the cementitious fraction, as described in example 2 of the detailed description of the invention.

[0072] FIG. 5—graphical representation of the compressive strength at 3, 7 and 28 days of age of: mortars with 100% recycled cement from pure pastes (RCPASTE) of similar hydration level to concrete subjected to recycling; mortars with 100% recycled cement from concrete waste (RCCONC) of similar hydration level subjected to the separation procedure according to Example 1 and having identical workability to mortars with RCPASTE.

[0073] FIG. 6 graphical representation of thermogravimetry (TG) and differential thermogravimetry (DTG) of recycled cement from pastes (RCPASTE) and recycled cement from concrete waste (RCCONC)—fraction 250-500 μm.

[0074] FIG. 7—representative graph of the X-ray diffraction analysis (XRD) of: a) recycled cement from pastes (RCPASTE); b) recycled cement from concrete waste (RCCONC); c) non-magnetic material (aggregate). Open ball-ettringite; Closed ball-portlandite; Open hexagon calcite; star-quartz; triangle-hydrated calcium silicates.

DETAILED DESCRIPTION OF THE INVENTION

[0075] The various configurations described below are applicable to different types of concrete waste, resulting from concretes of different compositions containing various types of aggregate of a distinct nature, calcareous and/or siliceous. During magnetic separation, some contamination of other magnetic materials that may be present in the aggregates can always occur, such as ferromagnesian minerals, for example, amphiboles or pyroxenes, but in general their content is not significant.

[0076] The crushing stages in the present invention can be carried out in a jaw crusher or in conventional cone crushers, and can also be carried out by other mechanical means that aim at better optimization of the process of releasing the different constituents of the concrete.

[0077] In an advantageous form of the invention, high-intensity magnetic separation is performed dry with permanent rolls made from rare-earth magnets and allowing field intensities at their surface exceeding 1 T. The selected rare-earth magnets could be neodymium-iron-boron, as they exhibit excellent magnetic properties, high productivity and energy efficiency.

[0078] For a better understanding of the invention it is important to explain the configuration and operation of a magnetic separator with permanent rolls. According to FIG. 1, the magnetic separator used in a form to implement the present invention includes a feeder (1) and a vibrating tray (2) that deposits the material on a conveyor screen (3), which conducts the material to a permanent magnetic roll (4) that will exert on the paramagnetic particles of the cementitious fraction a magnetic force opposing the centrifugal force in order to deflect their trajectory. The trajectory of the diamagnetic particles is not altered, however, which allows the separation of the magnetic material from the non-magnetic material through the adjustable divider (5) located at the base of the roll (4). The divider (5) opening and the feed speed are adjustable depending on the thickness of the conveyor screen (3), the characteristics of the magnetic roll (4) and the granulometry and magnetic susceptibility of the material.

[0079] In permanent magnetic roll separation, the high gradients required for higher magnetic forces and efficient separation are ensured by optimising the configuration of the separation rolls to allow the creation of a non-uniform field.

[0080] For example, the rolls can consist of discs or rings of permanent magnets interrupted at intervals with steel discs. The arrangement of the poles is optimised so that alternating polarity patterns are generated. The permanent rings are positioned so that the faces with the same polarity are side by side.

[0081] With regard to the diameter of the rolls (4) these can have a diameter greater than 76 mm. With the increase of the diameter of the roll (4), the separation efficiency increases, since the particles spend a longer time in the magnetic field. Thus, in a preferred form of the invention the said rolls (4) have a diameter of 300 mm since they enable greater efficiency.

[0082] Since there is a sharp drop in field strength with distance to the roll (4) surface, low thicknesses of less than 1-2 mm are recommended for the conveyor screen (3) for a minimum thickness of about 0.12 mm.

[0083] Depending on the particle size and the characteristics of the roll, the yield is variable. For example, for particles smaller than about 1 mm in size, yields of 15-30 kg/h per metre of screen width of 1.5 mm thickness have been achieved on a laboratory machine with a permanent roll 76 mm in diameter and about 1 T of field intensity at its surface. However, magnetic rolls can reach up to about 300 mm diameter and 2 T field strengths at their surface, thereby improving the separation yield.

[0084] In permanent magnetic roll separation, the particle size must be reduced and the particle granulometric spectrum narrowed because magnetic separation is affected by the shape and size of the particles. Therefore, the material must be pre-screened into different particle size fractions before proceeding with magnetic separation. To make sure that the various particles are equally affected by the magnetic force induced by the roll, it is important that the feeding is carried out in a monolayer, which should be ensured by a vibrating feeding system with adjustable opening.

[0085] In another embodiment of the invention, the magnetic separation can be performed by induced rollers, in which a metal alloy laminated rotor rotates between the polar parts of an electromagnet, inducing high magnetic field gradients and intensities, and thus bringing about the separation of paramagnetic materials of very weak magnetic susceptibility. A device with a capacity of up to 2 T has been successfully tried by the inventors of the present invention in the separation of concrete waste, for particles between 250 μm and 1 mm.

[0086] In another form of implementation of the invention the magnetic separation can be performed by high intensity, wet gradient magnetic separators, which use a matrix of ferromagnetic material of varied geometric shapes in order to produce high gradients. In order to have high performance and efficiency, ensuring continuous operation, only carousel separators should be of interest. However, the wet process may also have the advantage of protecting the ground material from eventual carbonation, which tends to decrease the reactivity of the final product (recycled cement).

[0087] High intensity magnetic separation can also be performed by another type of magnetic separator, following the same philosophy underlying the present invention.

[0088] Finally, the grinding of the paramagnetic fraction obtained from the magnetic separation; that is, the cementitious fraction should be ground until an average particle size lower than about 125 μm is obtained, which allows a more efficient reactivation of the cementitious fraction.

[0089] Some examples are described below that have been implemented in the separation of concrete constituents, according to the essential features of the method of the present invention.

[0090] The method of the present invention starts after a first crushing and sorting, in which the construction and demolition waste essentially consists of concrete debris and ceramic materials up to about 250 mm in size.

Example 1

[0091] The example described below is shown in FIG. 3 and aims to make use of the cementitious fraction of concrete waste.

[0092] In a first stage, the materials are subjected to a crushing process in jaw crushers, where the material is reduced to dimensions below about 20 mm. The material is then screened so that the material below 1 mm is separated from the rest of the waste. Any material larger than 1 mm is then crushed in a smaller jaw crusher, allowing the final waste to be smaller than 12 mm.

[0093] In order to eliminate ceramic waste, the material is subjected to a first stage of high intensity magnetic separation. For this, a permanent roll with about 1 T on the roll surface is used and it is found that the separation is very effective for debris up to 8 mm in size (over 97% separation), though it becomes slightly more contaminated between 8 and 12 mm (over 90% separation). This step can also be carried out at a later stage, after grinding the material in the roller mill. For very fine particles, the magnetic separation will have to be selective, isolating the ceramic material from the cementitious material, which are associated with different magnetic susceptibilities.

[0094] After the crushing and separation of the ceramic material, the debris above 1 mm is passed at least twice through a roller mill system. It has been ascertained that 2 passes ensure about 93% of the material below 1 mm, increasing to levels of 98% separation if another pass is run (the efficiency depends on the adjustment of the rollers and the feed speed). In brief, for 3 passes the material wastage is less than 2%. At the end, the material below 1 mm yielded by the various mechanical separation processes is screened into particle granulometric fractions of <150 μm; 150-250 μm; 250-500 μm and 500 μm−1 mm. This separation into different fractions is important for magnetic separation. Subsequently, the material above 150 μm, divided into the different fractions, is subjected to the magnetic separation procedure by high intensity permanent rolls. Since magnetic separation is affected by the presence of very fine particles and by any dust on their surface, left there by the previous mechanical crushing and grinding processes, the material undergoes a wet sieving followed by drying at 80° C. before this procedure is carried out.

[0095] At this stage, taking advantage of the paramagnetic characteristics of cement, the cementitious matrix is isolated from the other constituents of the concrete. In the presence of the magnetic roll, the cement particles are affected by the magnetic force that competes with the centrifugal force, and so the trajectory of the particles suffers a deflection. Meanwhile, the non-magnetic aggregate particles tend to present a more open trajectory, which makes it possible to separate these constituents. At this stage, other diamagnetic particles, such as the glass in C&DW are also separated from the cement, which further highlights the efficiency of the proposed separation method since it is very hard for gravitational methods to extract this fraction.

[0096] As the concrete waste is mostly composed of aggregate (non-magnetic material), an inverse methodology is chosen, where the first pass aims at eliminating the non-magnetic material (greater opening in the splitter and greater speed in the material dosing system, FIG. 1).

[0097] At a later stage, the material retained as magnetic is successively purified, involving at least one further pass on the magnetic roll for a smaller opening and slower feed speed. These rolls can be arranged sequentially, which is quite easy to implement at industrial level.

[0098] As a reference, for a roll of 76 mm diameter and about 1 T of field intensity on its surface and a screen 1.5 mm thick, the following approximate separation yields (mass of material separated (kg) per unit of time (h) and per metre of conveyor screen width) were obtained: fraction 150-250 μm−16 kg/(h.m); 250-500 μm−24 kg/(h.m); 500 μm−1 mm−32 kg/(h.m). The fraction below 150 μm, which cannot be separated, means about 10-15% of the mass of the concrete, depending on the fine-tuning in crushing and grinding. Since this material can contain about 15-20% cement in its composition, and the amount of cementitious matrix in the source concrete is about 18%, the cement content in this fraction could amount to about 10-15% by mass of the initial cement in the concrete. By adopting wet magnetic separation, it is possible to cover minimum particle size up to about 75 μm, reducing the content of material not subject to purification (about 4% of material has a fraction below 75 μm). Finally, after the magnetic separation, the material mostly consisting of cement is collected and stored for subsequent forwarding to the thermoactivation stage, to obtain recycled cement. Before thermoactivation, the material is ground in a ball mill until its size is reduced to less than 125 μm, in order to enhance its cementitious capacity.

[0099] The splitter opening and feed speed can be easily adjusted according to the particle size fraction, conveyor and feeder characteristics, and the size and capacity of the roll used.

Example 2

[0100] The example described below is shown in FIG. 4 and aims to exploit the aggregates fraction of concrete waste, i.e. it envisages obtaining recycled coarse and fine concrete aggregates in addition to the hardened cement fraction, thus enabling the production of fully recycled concrete at the aggregate and binder level.

[0101] The material is therefore initially subjected to a crushing stage, so as to present particles smaller than about 20 mm. Subsequently, if there are residues of ceramic materials, their magnetic separation can be carried out, as performed in the procedure of Example 1. It is also possible to separate the ceramics only at a later stage, as also done in Example 1. After crushing and eventual removal of any ceramic materials, the concrete debris larger than 1 mm is heated at 400° C., for about 2 hours in a rotary kiln (or other type, though the residence time will have to be adjusted to achieve similar efficiency), where the aggregates and the cement matrix are subjected to differential stresses that lead to their separation after cooling. The material obtained is subsequently subjected to autogenous grinding in a horizontal axis mill (without the use of grinding balls), for about 30 minutes, and is then screened in different granulometric fractions, adding the material smaller than 1 mm produced by the crushing: <150 μm; 150-250 μm; 250-500 μm; 500 μm−1 mm; 1-12 mm; >12 mm. It has been found that after thermal treatment and grinding, waste above 12 mm tends to have a lower percentage of particles with little cement contamination, and aggregate cleanliness levels similar to those of the lower fractions are not achieved. In this case, it may be advisable to reduce the aggregate to fractions smaller than 12 mm, or to perform an extra grinding stage in a ball mill in order to achieve higher levels of purification. The fraction larger than 12 mm represents about 30% of the sample. Regarding the fractions between 1 and 12 mm, the aggregate cleaning was quite effective for most of the particles. For the fractions between 2 and 8 mm, the percentage of cement adhered to the aggregates was less than 1% in 84%, 84% and 72% of the particles, in fractions 2-4, 4-6.3 and 6.3-8 mm, respectively. For the 8-12 mm fraction, the percentage of clean particles with less than 1% paste was 62%. Thus, it can at least be assured that more than 80% of the aggregate particles between 1 and 6 mm, more than 70% of those between 6 and 8 mm, and more than 60% of those between 8 and 12 mm, have high quality, and practically no adhered paste (less than 1%).

[0102] After this procedure, all material smaller than 1 mm is subjected to the same processes as in Example 1, namely wet screening, magnetic separation, grinding below 125 μm and thermoactivation. The non-magnetic material and any remaining material above 1 mm can be reused as high-quality recycled aggregate. The fraction below 150 μm is smaller in the second procedure, being about 7-8% of the mass of the concrete. Taking into account that this material could contain about 30% cement in its composition, and the amount of cementitious matrix in the original concrete is about 18%, the cement in this non-separated fraction may amount to approximately 10-15% (by mass) of the initial cement in the concrete.

Example 3

[0103] In order to confirm the suitability of the proposed procedure, several thermogravimetry (TG), X-ray diffraction (XRD), and microscopic tests were performed to evaluate the yield obtained during the various separation processes. In the 150-500 μm fractions, the separation led to a shift from mixtures with about 20-30% cement to final solutions with about 75-80% cement. In the mixtures with 500-1000 μm the yield was lower, shifting from about 15-20% to about 50-60%, as the maximum desired release did not occur. In this case, two options are suggested: additional grinding of the material above 500 μm to trigger the effective release of the various constituents of the concrete; adopting a more effective release methodology that allows a better yield in the 500-1000 μm fraction. Regarding the material below 150 μm, as mentioned, this cannot be purified by magnetic separation, but it can be used as a mineral addition of lower reactivity (filler).

[0104] FIG. 5 shows the compressive strength results at 3, 7 and 28 days of age of mortars of identical workability produced with 100% recycled cement from concrete debris of similar hydration level, subjected to the separation procedure according to example 1 (RCCONC), and produced with 100% recycled cement from pure paste of similar hydration level to the concrete subjected to recycling (RCPASTE). Both mortars were produced with the same weight ratio of 1:3 (binder:sand). Due to the higher water demand of the mortars with RCPASTE, associated with higher recycled cement content, the water/binder ratio (w/b) was 0.73 and 0.90 in the mixtures with RCCONC and RCPASTE, respectively.

[0105] The strength results shown in FIG. 5 demonstrate that the separation process will yield binders with 28-days strength of about 78% of that found for the reference mortars with 100% of RCPASTE. The difference in strength is in keeping with the percentage of cement identified in the samples (75-80%). Additionally, it was observed that the thermoactivation of recycled cement allows mechanical strength values at 28 days of about 15 MPa, for poor mortars with w/b of 0.9. In other tests performed for lower w/b ratios, strengths were estimated close to those verified for standard cements of class 32.5, according to EN 197-1 of 2011, defined for standard mortars with w/b of 0.5. This demonstrates the great effectiveness of the present invention, which leads to the production of eco-efficient cements of reasonable performance.

[0106] FIG. 6 presents the thermogravimetry (TG) and differential thermogravimetry (DTG) of the two cements analysed, RCPASTE and RCCONC. Analysing the combined amount of water between 120 and 400° C., it is estimated that RCCONC contains about 76% of cementitious matrix by mass, while the rest of the material was composed of fine aggregate that was not released or separated.

[0107] FIG. 7, meanwhile, presents the XRD of the two types of cement, confirming the presence of the same types of majority phases (portlandite, hydrated calcium silicates). The XRD of the non-magnetic separated material is also presented, where the presence of the aggregate crystalline phases (quartz and calcite) and a weak peak corresponding to portlandite are clearly identified. These results help demonstrate the effectiveness of the separation. Compared to the RCPASTE, the RCCONC also exhibits some quartz and slightly higher calcite content, which are associated with aggregate contamination and a higher level of carbonation of the product, arising from the separation procedures.

REFERENCES

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