Method of obtaining calcium rich particles from a construction demolition waste

Abstract

A method of obtaining calcium rich particles from concrete demolition waste, said concrete demolition waste comprising a plurality of entangled solid domains comprising at least one silicon dioxide rich domain and at least one calcium rich domain, comprising the steps of: a) grinding the concrete demolition waste by applying attrition and / or shearing stress to obtain particles having a particle size of < 300 µm, preferably < 150 µm, the particles comprising calcium rich particles and silicon dioxide rich particles, b) separating the calcium rich particles from the silicon dioxide rich particles by means of a separator.

Description

[0001] Method of obtaining calcium rich particles from a construction demolition waste

[0002] The invention refers to a method of obtaining calcium rich particles from a concrete demolition waste , said concrete demolition waste comprising a plurality of entangled solid domains comprising at least one silicon dioxide rich domain and at least one calcium rich domain . In particular, the invention refers to the use of the calcium rich particles as a component in Portland cements , or as a component of a raw meal mixture for the preparation of Portland clinker, and for the sequestration of carbon dioxide by minerali zation .

[0003] In the past few years , the recycling of construction demolition wastes for being re-used in the production of cement of concrete compositions has gradually been increasing . In its ef forts to reduce the use of virgin materials for the production of cement and concrete , the construction materials industry has developed several solutions that attempt to maximi ze the substitution of Portland clinker in cement compositions , or the substitution of limestone and clay in clinker raw meal compositions .

[0004] Incorporating construction demolition wastes in cement production can signi ficantly reduce CO2 emissions associated with the cement manufacturing process . This is achieved through reduced limestone calcination, as well as the use of an industrial byproduct , which would otherwise require disposal or long-term storage .

[0005] Construction demolition wastes are a mixture of di f ferent materials that come from the demolition site of an old construction . It is typically composed of concrete demolition wastes , plasterboards , bricks , steel , wood, tiles or glass . The common practice is to sort these materials in recycling centers , so as to separate the materials listed above to re-

[0006] SUBSTITUTE SHEET (RULE 26) use them in applications for which their chemical composition is most suitable.

[0007] These demolition wastes are classified under different waste codes of the European Waste Catalogue (EWC) depending on its composition and potential contamination. The EWC is a hierarchical list of waste descriptions established by EU Commission decision 2000 / 532 / EC2. Construction and demolition wastes fall under chapter 17 01 of the EWC and comprise the following waste codes:

[0008] • 17 01 01: concrete

[0009] • 17 01 02 : bricks

[0010] • 17 01 03: tiles and ceramics

[0011] • 17 01 06: mixtures of, or separate fractions of concrete, bricks, tiles and ceramics containing hazardous substances

[0012] • 17 01 07: mixtures of concrete, bricks, tiles and ceramics other than those mentioned in 17 01 06

[0013] Other sub-chapters comprise wood, glass and plastic (17 02) , bituminous mixtures, coal tar and tarred products (17 03) , metals (17 04) , soil (17 05) , and other fractions (17 06 - 17 09) .

[0014] For the purpose of the instant invention, concrete demolition waste according to EWC waste code 17 01, in particular 17 01 01 and / or 17 01 07, is used for obtaining calcium rich particles .

[0015] Concrete demolition wastes typically contain calcium oxides, silicon dioxide, aluminium oxides, and oxides that are present in a plurality of entangled solid domains within the construction demolition waste . Di f ferent solid domains would have di f ferent contents of calcium oxides , silicon dioxide , aluminium oxides , or calcium silicates , depending mainly on their origin . These solid domains can indeed come from the sand or aggregates initially present in the concrete material , or in the hardened cement paste .

[0016] One limitation to the recycling of concrete demolition wastes is that the total amount of calcium, coming from calcium silicates or calcium oxides , is too low, and the amount of silicon is too high for being used in the production of Portland cements . Indeed, when Portland clinker is produced, a raw meal composition is prepared in such a way that the ratios between calcium oxide , aluminium oxides , silicon dioxides and iron oxides are suitable for the formation of the mineral phases of Portland clinker in the cement kiln, namely alite , belite , calcium aluminates and alumino- ferrites . The lower amount of calcium in construction demolition wastes also limits their use as mineral components in Portland cement compositions , as it is well known that mineral components with higher total calcium oxide compositions have higher reactivity .

[0017] The purpose of the present invention is to provide a method to obtain calcium rich particles from concrete demolition waste , in order to enrich the content of calcium in materials that are destined to be used in cement and concrete compositions .

[0018] Therefore , the instant invention aims at providing an improved method, which on the one side is more ef fective in enriching the amount of calcium components of the calcium rich particles obtained from concrete demolition waste and involves less costs . In particular, at least in Europe , the cost shall be proportional to the fee which would be paid i f an equivalent quantity of carbon dioxide was emitted at the stack of the cement manufacturing plant in case no calcium enriched materials are added to the cement composition . More generally, the instant invention aims at providing an improved method, which ef fectively enriches the amount of at least one calcium component of a calcium oxide rich material

[0019] In order to solve this obj ect , the invention provides a method of obtaining calcium rich particles from concrete demolition waste , said concrete demolition waste comprising a plurality of entangled solid domains comprising at least one silicon dioxide rich domain and at least one calcium rich domain, comprising the steps of : a ) grinding the construction demolition waste by applying attrition and / or shearing stress to obtain particles having a particle si ze of < 300 pm, preferably < 150 pm, even more preferably < 75 pm, the particles comprising calcium rich particles and silicon dioxide rich particles , b ) separating the calcium rich particles from the silicon dioxide rich particles by means of a separator .

[0020] The invention may be applied to any concrete demolition waste that contains crystallised calcium based mineral phases , the calcium component of which shall be recovered in order to make the calcium based mineral phases suitable for being recycled, reused or further processed, such as for being recycled as raw material in a raw material mix for producing cement clinker . A particularly useful and commercially relevant embodiment of the invention refers to the use of the calcium rich particles as the input material for the production of cement or clinker . Alternatively, the calcium rich particles are more suitable to capture carbon dioxide , as increasing amounts of calcium rich solid domains increase the capacity of the material to react with carbon dioxide to form calcium carbonate .

[0021] The invention is based on the observation made by means of scanning electron microscopy, that concrete demolition wastes display domains of cement paste , calcium carbonate , and silicon dioxide . By applying attrition and / or shearing stress during the grinding step a fragmentation of the concrete demolition waste along the phase boundaries can be achieved so as to obtain silicon dioxide rich particles on the one hand, and calcium rich particles on the other hand . This allows an ef ficient separation of the calcium components from the other particles by means of a separator .

[0022] In the calcium rich domains , the calcium may be present in the form of calcium oxide and calcium silicates , aluminates , and / or sorosilicates .

[0023] The calcium rich domains are thus defined as domains that have a maximum si ze of 150-200 pm, measured by microscopy, and that contain at least 30 wt . -% of calcium oxide , measured by X-ray fluorescence .

[0024] In the silicon dioxide rich domains , the silicon dioxide may be present in the form of crystalline or amorphous silicon dioxide .

[0025] The silicon dioxide rich domains are thus defined as domains that have a maximum si ze of 4000 pm, measured by microscopy, and that contain at least 30 wt . -% of silicon dioxide , measured by X-ray fluorescence .

[0026] Applying attrition and / or shearing stress allows one to take advantage of the fact that the individual phases of the input material have di f ferent compression strength resistance . Generally speaking, when the material is composed of two phases (A) and (B ) , fragmentation first occurs within the most brittle phase (A) , generating grains composed of the phase (A) on one side , and grains composed by a harder phase core (B ) surrounded by the remaining (A) phase on the other side . When using attrition and / or shearing stress , the shearing forces involved (particle-particle contacts or grinding part-particle contacts ) "peel" of f the remaining brittle phase (A) from the (B ) core when grinding is undertaken . In concrete demolition waste or other input material , the calcium rich domains tend to be comminuted more easily . Silicates most often have a crystalline structure , which makes them more prone to fracture and comminution during the grinding process . The calcium rich domains in concrete demolition waste contrast are relatively ductile materials . As a result , the grinding process achieves a high selectivity between calcium rich domains and silicon dioxide rich domains , generating calcium rich particles and silicon dioxide rich particles . Only a limited number of composite grains remains at the grinding equipment output , favouring the ef ficiency of the downstream separating process .

[0027] As mentioned above , the grinding process yields particles that have a higher content of calcium, and particles that have a higher content of silicon dioxide . In a preferred embodiment , the 0-4 mm fraction of concrete demolition waste can first be separated by sieving, this fraction would then be subj ect to the process of grinding . In this embodiment , the material ground has a higher concentration in cement paste , making the separation of calcium rich and silica rich particles more ef ficient .

[0028] Calcium rich particles are understood to have a higher content of calcium oxide than the concrete demolition wastes , but still contain other components . The calcium rich particles may preferably have a calcium oxide content by weight that is 110% or more , preferably 120% or more , of the calcium oxide content by weight of the concrete demolition waste .

[0029] Further, the calcium rich particles may preferably have a silicon dioxide content by weight that is 90% or less , preferably 80% or less , of the silicon dioxide content by weight of the concrete demolition waste .

[0030] Silicon dioxide rich particles are understood to have a higher content of silicon dioxide than the concrete demolition wastes , but still contain other components . The silicon dioxide rich particles may preferably have a silicon dioxide content by weight that is 110% or more , preferably 120% or more , of the silicon dioxide content by weight of the concrete demolition waste .

[0031] Further, the silicon dioxide rich particles may preferably have a calcium oxide content by weight that is 90% or less , preferably 80% or less , of the calcium oxide content by weight of the concrete demolition waste . For example , the calcium rich particles may have a calcium oxide content of > 20 wt . -% and the silicon dioxide rich particles have a silicon dioxide content of > 30 wt . -% , preferably > 50 wt . -% , determined by X-ray fluorescence .

[0032] The amount of calcium oxide and silicon dioxide can be measured using a high performance energy dispersive X-ray fluorescence spectrometer (EDXRF) , with a polari zed X-ray excitation geometry (HE XEPOS , Ametek) . The instrument is equipped with a 50 W Pd-Co tube (maximum 60kV, 2mA) . The samples are dried and finely ground (<100 pm) before being analyzed in sample holders covered with a 4p polypropylene film ( Film Velope , Fluxana ) .

[0033] Concrete demolition waste is composed of various phases which need di f ferent levels of energy to get fragmented . Since calcium silicates present in hydrated cement paste need less energy to get fragmented, calcium-rich particles are usually obtained after a shorter grinding time and with smaller particle si zes . In contrast , silicon dioxide phases need more energy to get fragmented, so that they are present in the grinding stock with a larger mean particle si ze . One can take advantage of this fact by removing calcium rich particles from the grinding stock even before feeding the ground particles to the separating step b ) . In particular, a particle si ze based separation may be conducted by removing particles having a particle si ze above a given threshold value from the grinding stock by means of a suitable separating technique . Alternatively or in addition, a particle si ze based separation may be conducted by separating particles having a particle si ze below a given threshold value from the grinding stock by means of a suitable separating technique . The coarser fraction has a higher portion of silica rich particles , so that removing the coarser fraction from the grinding stock avoids the necessity of grinding the entire feedstock of input material to the final particle si ze that is required for the separating step b ) .

[0034] In this connection, a preferred embodiment of the invention provides that the step of grinding the concrete demolition waste is carried out to obtain at least a fine fraction of particles and a coarse fraction of particles , wherein only the fine fraction of particles having a maximum particle si ze of < 300 pm, preferably < 150 pm, more preferably < 75 pm, is provided to the separating step b ) . The maximum particle si ze is determined by means of sieving, using for example ASTM El l test sieves .

[0035] In a preferred embodiment , the fine fraction of particles have a minimum si ze of > 1 pm, preferably > 5 pm . It was indeed found that the process of the present invention is most ef ficient when the particles are ground to a level that maximises the separation of the mineralogical phases of the input material , without further grinding .

[0036] The minimum particle si ze corresponds to a D90 value (volume based) , measured by means of laser di f fraction, for example with a Mastersi zer 2000 provided by Malvern . The measurement is here carried out by dispersing beforehand the particles in water and subj ecting this dispersion to ultrasounds so as to de-agglomerate the particles .

[0037] Further, a fine fraction may at least partially be withdrawn from the grinding device by means of an air separator . This avoids overgrinding of the said fine fraction . Turning now to the separation step b ) of the method of the invention, any separation technique may be used that is able to separate the calcium rich particles from the silica rich particles by means of a separator . Separation has to be seen here as a tool exploiting di f ferences in terms of chemical and / or physical properties , such as si ze , shape , mass , density, or chemical af finity, between the mineralogical phases composing the input material , i . e . between the calcium rich particles and the silicon dioxide rich particles . Preferably, the step of separating said particles comprises a separation by a separation by flocculation, a triboelectric separation and / or an electrostatic separation, or a separation by si ze .

[0038] The step of separating the calcium rich particles from the silicon dioxide rich particles may comprise one separation process or at least two consecutive separation processes . In the case of at least two consecutive separation processes , the separation processes may be configured to operate according to di f ferent separation techniques or according to the same separation technique . In the case of at least two successive separation processes using the same separating technique , the successive steps may preferably be operated with di f ferent parameters of the same separation technique . For example , in case of magnetic separation, the successive steps may be operated with increasing magnetic field intensity .

[0039] Preferably, the magnetic separator is designed as a wet drum magnetic separator, wherein a rotating drum is partly immersed into the slurry, the slurry being fed through a gap formed between the drum and a cylindrical housing either in concurrent or counter current direction with the direction of rotation . In doing so , the particles with a higher magnetic susceptibility are attracted by the magnetic field, collected on the drum surface , and rotated out of the slurry flow .

[0040] According to an alternative embodiment , the calcium rich particles are separated from the silicon dioxide rich particles by triboelectrostatic separation . Triboelectrostatic separation is based on the di f ferences in surface properties of granular materials that charge with opposite polarity by surface contact or triboelectric charging . When two materials are in contact , material with a higher af finity for electrons gains electrons and thus charges negative , while material with lower electron af finity charges positive . This contact exchange of charge is universally observed for all materials .

[0041] As to the nature of the input material , a preferred embodiment of the invention provides that the concrete demolition waste is concrete demolition waste .

[0042] Where the description of the invention refers to concrete demolition waste , such description equally applies to any concrete material that contains crystallised calcium based mineral phases .

[0043] In this connection, a preferred embodiment of the invention provides that the concrete demolition waste comprises :

[0044] > 90 wt . -% of concrete , concrete products , mortar, concrete masonry units , unbound aggregate , natural stone , and / or hydraulically bound aggregate ; and - < 10 wt.-% of clay masonry units (i.e., bricks and tiles) , calcium silicate masonry units, and / or aerated non-floating concrete. Example:

[0045] A 0-4mm sample of concrete demolition waste (CDW) was tested and separated according to the present invention. The raw 0-4mm sample is sieved into four finer fractions, each of them being characterised (by means of) :

[0046] Calculation allows the following conclusion: - the finer the fraction, the more the cement paste is concentrated (~27 wt.-% in the <0, 063mm fraction of the exampled material)

[0047] - Two thirds of the cement paste is still in the [0,25; 4] mm fraction. Mechanical treatment has to be applied to liberate the cement paste from the coarse grains.

[0048] In another test, the 0 / 4 mm CDW sample was ground by attrition in a ball mill equipment with:

[0049] - rotation speed set up at 70% of the critical speed - balls of 20 mm diameter

[0050] - 1 / 10 CDW / ball mass ratio

[0051] - equipment filling factor = 25% - Number of equipment revolution = 123

[0052] In such a grinding process , the grinding energy is low and done without steel balls impact and crushing the CDW particles by using a low speed of rotation of the ball mill , by using balls of a smaller diameter and a short grinding time . This process ensures that the grinding is by attrition and shearing only .

[0053] The recovered material is subsequently sieved and each fraction characteri zed by means of XRF analysis :

[0054] As can be seen, the finer fraction has an enriched relative amount of calcium oxide , and the larger fractions have a higher relative amount of silica . The finer fractions , i . e . below 0 . 25 mm, are the calcium rich particles according to the present invention and have a calcium oxide content of 23

Claims

Claims :1 . A method of obtaining calcium rich particles from concrete demolition waste , said concrete demolition waste comprising a plurality of entangled solid domains comprising at least one silicon dioxide rich domain and at least one calcium rich domain comprising the steps of : a ) grinding the concrete demolition waste by applying attrition and / or shearing stress to obtain particles having a particle si ze of < 300 pm, preferably < 150 pm, the particles comprising calcium rich particles and silicon dioxide rich particles , b ) separating the calcium rich particles from the silicon dioxide rich particles by means of a separator .2 . The method according to claim 1 , wherein an additional step of selecting the 0-4 mm fraction of the concrete demolition waste is done prior to step a ) , and is subsequently used in steps a ) and b ) .3 . The method according to claim 1 or 2 , wherein the particles obtained by said grinding step have a particle si ze distribution having a D10 of > 1 pm, preferably > 5 pm, measured by laser di f fraction .4 . The method according to any one of claims 1 to 3 , wherein the calcium rich particles have a calcium oxide content by weight that is 110% or more , preferably 120% or more , of the calcium oxide content by weight of the concrete demolition waste .5 . The method according to any one of claims 1 to 4 , wherein the calcium rich particles have a calcium oxidecontent of > 15 wt.-%, preferably > 20 wt.-%, determined by X ray fluorescence.

6. The method according to any one of claims 1 to 5, wherein said step of grinding is carried out by electrofragmentation, a ball mill, or autogenous grinding.

7. The method according to any one of claims 1 to 6, wherein the ground concrete demolition waste obtained in step a) undergoes a further separation process so as to obtain a ground concrete demolition material having a maximum size of 500 pm, preferentially 300 pm, prior to step b) .

8. The method according to any one of claims 1 to 7, wherein said step b) of separating the calcium rich particles from the silicon dioxide rich particles comprises at least two consecutive separating processes.

9. The method according to any one of claims 1 to 8, wherein said step of separating the calcium rich particles from the silicon dioxide rich particles comprises a triboelectric separation and / or an electrostatic separation.

10. Use of the calcium rich particles obtained from any one of claims 1 to 9 as a component in Portland cements, or as a component of a raw meal mixture for the preparation of Portland clinker.

11. Use of the calcium rich particles obtained from any one of claims 1 to 9 for the sequestration of carbon dioxide by mineralization .

Images