FIELD OF THE INVENTION

The present invention relates to a cement waste recy cling device and a method of recycling cement waste

BACKGROUND OF THE INVENTION

Cement's major constituents are calciumsilicates . Af ter its use as a binder in concrete, by addition of water and gravel and/or sand, it is present in a hydrated form in a con centration of approximately 20% (w/w) . If the hydrated calci umsilicates in waste concrete can be liberated from the gravel and/or sand, it can be heated again to produce new cement . The hydrated calciumsilicates can be liberated by heating the waste concrete to a temperature over 900 °C at which point it falls apart into gravel and/or sand and calciumsilicates. As the gravel and/or sand and the calciumsilicates are initially attached to one another, the gravel and/or sand act as a para sitic heat capacity.

German patent application DE 10 2006 049 836 Al dis closes a process for the production of a hydraulic binder from calcium silicates containing building rubble, wherein the building rubble is first reduced to particles with a size smaller than 10 mm, optionally enriching the binder phase of the building rubble by sieving or sorting, and subsequently heating the rubble at a temperature of 600°C to 800°C during 0.25 to 10 hours.

Obviously, such heating for extended periods requires a lot of energy, resulting in an industrially irrelevant pro cess.

Alternatively, a two-step process can be applied wherein concrete waste is first heated to a lower temperature whereby the bonds between the gravel and/or sand and the cal ciumsilicates weaken. Thereafter, the heated concrete waste is milled and the bonds are broken.

A cement waste recycling device that makes use of such a two-step process is known from "H. Shima et al, An Ad vanced Concrete Recycling Technology and its Applicability As sessment through Input-Output Analysis, Journal of Advanced Concrete Technology, 2005". Concrete rubble is crushed to a size under 50 mm and heated to 300°C in a vertical kerosene fueled furnace. The heated concrete is thereafter sent to rub bing equipment .

In this batch-type process the cement waste is heated while it is stationary. This implies that heat is not distrib uted evenly throughout the cement waste resulting in tempera ture gradients and waste of energy.

The objective of the present invention is to provide a cement recycling device and a method for recycling cement which have a lower energy requirement as compared to the prior art devices and methods. Within the scope of this invention and for the avoidance of doubt it is remarked that cement waste includes concrete fines.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method of recycling cement waste, comprising con tacting cement waste and heated gas with each other and ob taining heated cement waste having a temperature between 400 °C and 600°C, wherein the cement waste and the heated gas move in opposite directions to each other. This counter-current move ment of cement waste and heated gas is advantageous because it promotes that the cement waste and the heated gas thoroughly mix with each other. This leads to an efficient transfer of heat from the heated gas to the cement waste, as the contact surfaces of both phases are as large as possible. Moreover, the countercurrent movement ensures that a temperature differ ence between both phases is as large as possible at every point in the device, resulting in a most optimal transfer of heat .

In accordance with the present invention, there is further provided a cement waste recycling device which is par ticularly suited for carrying out such a method, comprising a heater with an inlet for cement waste and an outlet for pro cessed cement waste, the heater being configured for trans porting cement waste in a cement waste transportation direc tion from the inlet to the outlet while transporting heated gas in counter-current with the cement waste transportation direction, wherein at least one screen is located between the inlet and the outlet of the heater and wherein at least part of the screen is vibratable. The screen between the inlet and the outlet of the heater acts to distribute the cement waste more evenly throughout the heater, thereby improving heat transfer from the heated gas phase to the cement waste phase. In addition, the screen acts as a barrier for the cement waste phase and slows down the transportation of the cement waste, resulting in a longer residence time of the cement waste in the heater and thus a better transfer of heat from the heated gas to the cement waste. The screen may be constructed from a sturdy material, such as steel or other metals or metal al loys .

At least part of the screen is vibratable. This pre vents the screen from getting clogged and ensures no cement waste remains on top of the elongate bars that could be heated far above the desired temperature as the residence time of that particular cement waste would be much longer as other ce ment waste, resulting in unnecessary waste of energy.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment the screen may comprise a plurality of layers having multiple elongate bars, wherein the elongate bars within each layer are spaced apart and arranged approximately parallel to each other. An even distribution of parallel elongate bars ensures an even distribution of cement waste throughout the heater. By having gaps with equal dis tance between the bars, the velocity at which the cement waste passes through these gaps is more constant, resulting in a more evenly mixture of heated gas and cement waste.

In a preferred embodiment each of the plurality of layers may be arranged approximately perpendicular to the ce ment waste transportation direction. Such an arrangement im proves the homogenous distribution of cement waste throughout the heater. A direction of each of the plurality of layers other than a direction perpendicular to the cement waste transportation direction would lead to a more uneven distribu- tion of cement waste throughout the heater and the residence time of the cement waste would possess a larger variability.

In a preferred embodiment a first layer of the plu rality of layers may be stacked in a staggered position on top of a second layer of the plurality of layers, wherein the elongate bars within the first layer are approximately paral lel to the elongate bars within the second layer. A staggered configuration of layers of the screen increases the average distance a cement waste particle has to travel across the screen. Therefore, the average residence time is increased.

The layers may be staggered such that the distances between a first bar in a first layer and two other bars in a second lay er on top of said first layer are equal. In other words, the offset of a first layer in relation to a second layer on top of said first layer equals half the distance between the elon gate bars within a layer.

In a preferred embodiment the elongate bars may be spaced apart at a regular first distance from each other with in at least one layer of the plurality of layers. This ensures that a flux of cement waste through the heater from the inlet to the outlet is more constant in a cross-sectional plane per pendicular to the cement waste transportation direction, which leads to a more homogenous distribution of cement waste throughout the heater and thus a better transfer of heat from the heated gas to the cement waste.

In a preferred embodiment a width of each spacing be tween the elongate bars within at least one layer of the plu rality of layers may be 5-15 mm, preferably 8-12 mm, most preferably approximately 10 mm. The spacing between elongate bars in a layer of the screen is chosen dependent on the ce ment waste particle size distribution. When feeding the cement waste recycling device with cement waste having a size between 0-4 mm, an optimal width of the spacing between elongate bars is found to be approximately 10 mm. This spacing width repre sents a good trade-off between on the one hand slowing down the cement waste, thereby increasing the average residence time for efficient heat transfer, and on the other hand pre venting the screen from getting clogged by larger cement waste particles. Cement waste may be milled until a size of the ce- ment waste particles reaches 0-4 mm. The cement waste material can be passed over a coarse protection screen, such as a 10 mm grizzly before being fed to the cement waste recycling device, to avoid accidental large particles blocking the internal space of the cement waste recycling device.

In a preferred embodiment each layer may be spaced apart at a regular distance from each other. This provides gaps between elongate bars within the screen that are very similar. Transporting cement waste across such a screen re sults in a more constant velocity and a homogenous distribu tion of the cement waste across the screen and therefore a more constant and predictable transfer of heat from the heated gas to the cement waste.

In a preferred embodiment the elongate bars may have an at least partly circular cross-section, such as an arc-like cross-section. An arc-like cross-section of an elongate bar is on the one hand efficient in deflecting cement waste towards either side of the elongate bar and on the other hand effi cient in slowing down the cement waste. Another advantage is that an arc-like cross-section makes more efficient use of ma terials that constitute the elongate bars as compared to, for example, elongate bars having a circular or square cross- section. In other words they are cheaper.

In a preferred embodiment the elongate bars may have a radius of curvature of 10-20 mm, preferably 12.5 to 17.5 mm, and most preferably a radius of curvature of 15 mm. This ena bles the elongate bars to have sufficient width to slow down the cement waste stream, while having a curvature that allows the cement waste to easily slide of to either side of the elongate bars, preventing the screen from becoming clogged with cement waste.

In a preferred embodiment at least part of the screen is vibratable at a frequency between 20-30 Hz, preferably at about 25 Hz. This frequency ensures a good throughput of ce ment waste through the heater.

In a preferred embodiment at least part of the screen is vibratable in a vibration direction perpendicular to the cement waste transportation direction. Such a vibration direc tion is preferred, as it prevents the screen from getting clogged, while minimizing wear on the elongate bars by imping ing cement waste.

In a preferred embodiment the vibration direction may be parallel to a longitudinal axis of the elongate bars. This vibration direction prevents cement waste particles from fly ing around inside the screen and thus minimizing wearing down the elongate bars. In addition, a steady flow of cement waste through the screen is realized.

In a preferred embodiment the cement waste recycling device may comprise a dampening device, preferably a bellow. This bellow acts as a vibration absorber and can be connected between the screen and the remainder of the cement waste recy cling device to efficiently absorb the vibrations of the vi brating screen and prevent unwanted transmission of these vi brations to other stationary parts of the cement waste recy cling device.

In a preferred embodiment the length of the screen in the cement transportation direction is approximately 1-2 m, preferably 1.25-1.75 m, most preferably about 1.5 m. This length allows a residence time of the cement waste in the heater of over 30 seconds, which is sufficient to heat the ce ment waste up to 520°C. The residence time can be adjusted by changing the size of the elongate bars in the screen, changing the distance between these elongate bars, changing the vibra tion direction and/or vibration frequency of the screen, changing a temperature of the heated gas introduced by the heater, changing the length and/or width of the screen, chang ing the number of layers inside the screen, and other varia bles .

In a preferred embodiment the cement waste recycling device comprises a first heater and a second heater, each hav ing an inlet and an outlet, wherein the outlet of the first heater is connected to the inlet of the second heater. Such an arrangement is particularly advantageous, as it allows heating of the cement in the first heater, up to a temperature of ap proximately 520°C to fracture the bonds between the

sand/gravel and calcium silicates. Thereafter, in the second heater, the cement waste is cooled to a temperature below 250°C and the recovered heat is directly fed into the first heater. By stacking both heaters above one another, a constant flow of cement waste and heated gas throughout the cement waste recycling device is ensured.

In a preferred embodiment the cement waste recycling device comprises a heated gas recycling conduit at one end connected near the outlet of a heater to the device and at an other end thereof connected near the inlet of a heater to the device. The heated gas temperature near the inlet of the heat er, or near the inlet of the first heater, if the cement waste recycling device comprises two heaters, is approximately 120°C if the heated gas temperature near the outlet of the heater is approximately 600°C with a countercurrent heated gas flow of approximately 1.3 kg/s for every kg/s throughput of cement waste. The exiting heated gas flow having a temperature of ap proximately 120°C represents a significant amount of energy and as such, is a valuable source to recycle via a recycling conduit. Moreover, if the cement recycling device comprises two heaters, as mentioned above, heat exiting the inlet of the first heater can be guided to the outlet of the second heater, wherein the outlet of the first heater is connected to the in let of the second heater. As such, cooled cement waste having a temperature below 250°C exits the outlet of the second heat er, via for example a rotary sluice. The calcium silicates contained within this cooled cement waste can thereafter be liberated from the cement waste by a suitable mechanical pro cess, such as milling.

In a preferred embodiment the heated gas recycling conduit comprises a separator, preferably a cyclone. The exit ing heated gas flow having a temperature of approximately 120°C contains moisture, as well as a very fine fraction (250 micron) of the fine material in the cement waste. The flow of this very fine fraction represents about 10% of the total flow of fine material in the cement waste. As such, it is useful to remove this material from the exiting heated gas flow when this gas flow is recycled, to avoid accumulation of this mate rial inside the cement waste recycling device. A cyclone can be used to remove this material from the exiting heated gas flow before this flow is recycled. In a preferred embodiment the cement waste recycling device comprises a feeding unit connected to the inlet for feeding cement waste to the heater, wherein the feeding unit comprises a conveyor. Cement waste is relatively dense and a pilot cement waste recycling device is already capable of pro cessing 3 tons of fine cement waste per hour. A conveyer ena bles the constant feeding of the cement waste recycling de vice, which is required to maintain its countercurrent opera tion .

In a preferred embodiment the conveyor is a screw conveyor. A screw conveyor can provide a constant flow of ce ment waste material and is suited to transport hard and heavy material. If desired, other conveyors can be used as well, such as a belt conveyor.

In a preferred embodiment the method may comprise moving the cement waste and the heated gas across a screen.

The cement waste is dried while moving across the screen and is heated from ambient temperature up to a temperature of ap proximately 520°C by contact with the heated gas whose temper ature is lowered from approximately 600°C to 120°C. The screen acts to slow down and disperse both the heated gas as well as the cement waste, to allow sufficient contact area and team for heat exchange.

In a preferred embodiment the method may comprise providing the screen with a plurality of layers having multi ple elongate bars, wherein the elongate bars within each layer are spaced apart and arranged approximately parallel to each other. If the plurality of layers are arranged approximately perpendicular to the direction of movement of the cement waste and the heated gas, an optimal distribution of cement waste over the heater is achieved. Moreover, this allows the cement waste to be slowed down efficiently.

In a preferred embodiment the method may comprise stacking a first layer of the plurality of layers in a stag gered position on top of a second layer of the plurality of layers, wherein the elongate bars within the first layer are approximately parallel to the elongate bars within the second layer. This method of stacking the layers within a screen re sults in a more efficient slowdown of the cement waste across the layers of the screen and enhances the homogenous distribu tion of cement waste throughout the screen.

In a preferred embodiment the method may comprise ar ranging the elongate bars spaced apart at a regular first dis tance from each other within at least one layer of the plural ity of layers. This spacing apart can be such that a width of each spacing between the elongate bars is 5-15 mm, preferably 8-12 mm, most preferably approximately 10 mm.

In a preferred embodiment the method may additionally comprise arranging each layer such that each layer is spaced apart at a regular second distance from each other. Having even spacing of the elongate bars throughout the screen pro vides for an even and homogenous distribution of cement waste throughout the screen during operation of the cement waste re cycling device.

In a preferred embodiment the method may comprise providing the elongate bars with an at least partly circular cross-section, such as an arc-like cross-section. Such an arc like shape represents a good trade-off between slowing down the cement waste to promote efficient heat transfer, and pre venting the screen from getting clogged. Arcs are additionally resilient and strong shapes, that are suited to withstand the impact of falling cement waste.

In a preferred embodiment the method may comprise providing the elongate bars with a radius of curvature of 10- 20 mm, preferably 12.5 to 17.5 mm, and most preferably a radi us of curvature of 15 mm. As such the elongate bars are curved enough so that cement waste cannot accumulate on the elongate bars .

In a preferred embodiment the method may comprise providing that at least part of the screen is vibratable. Vi brational movements further enhance the fluid flow of cement waste throughout the screen and heater.

In a preferred embodiment the method may comprise providing that at least part of the screen is vibratable at a frequency between 20-30 Hz, preferably at about 25 Hz.

In a preferred embodiment the method may comprise providing that at least part of the screen is vibratable in a vibration direction perpendicular to the direction of movement of the cement waste and the heated gas. This vibration direc tion makes sure that cement waste does not bounce off of the elongate bars as much as it would with another vibration di rection .

In a preferred embodiment the vibration direction may be parallel to the elongate bars. In addition to being perpen dicular to the cement waste transportation direction, this parallel direction further reduces the chance that cement waste bounces off of the elongate bars, and reduces wear of the elongate bars and the screen itself.

In a preferred embodiment the method may comprise providing the screen with a dampening device, preferably a bellow. This bellow, stops vibrational motion from being transmitted to stationary parts of the cement waste recycling device. Multiple dampening devices may be provided within the cement recycling device, depending on the amount of vibratable screens present.

In a preferred embodiment the method may comprise providing the screen with a length in the direction of move ment of the cement waste and the heated gas of approximately 1-2 m, preferably 1.25-1.75 m, most preferably about 1.5 m. This length allows sufficient residence time to heat the ce ment waste up to a temperature of about 520°C, although dif ferent lengths can be chosen dependent on the configuration of the screen and its purpose (heating or cooling) .

In a preferred embodiment the method may comprise cool-ing the cement waste to a temperature between 50°C and 200°C after heating the cement waste to a temperature between 400°C and 600°C. Cooling the cement waste after heating is an efficient way to reuse energy. As energy costs represent a large fraction of total costs required to recycle cement waste, regaining the energy spent on heating is very advanta geous .

In a preferred embodiment the method may comprise re cycling heated gas. Heated gas contains usable energy, and as such, recycling this gas results in a more energy efficient process, leading to lower operating costs. The heated gas stream can be captured at the inlet of a first heater and guided to an outlet of a second heater, wherein the first heater is stacked upon the second heater, and the outlet of the first heater is connected to the inlet of the second heat er. The first heater as such effectively heats cement waste up to a temperature of about 450 - 550°C and effectively cools heated gas down to a temperature of about 50 - 200°C. While in the second heater, the cement waste is cooled down below 250°C and the recycled gas is heated to over 350°C. This very effi cient arrangement of heaters leads to a cement waste recycling process that requires little energy as compared to prior art cement waste recycling processes.

In a preferred embodiment the method may comprise separating particles comprised in the heated gas with a sepa rator, preferably a cyclone. If particles are removed from re cycled gas, they cannot act as an energy sink anymore, further optimizing the overall energy efficiency of the cement waste recycling method.

In a preferred embodiment the method may comprise feed-ing the cement waste with a feeding unit before contact ing the cement waste and the heated gas with each other, wherein the feeding unit comprises a conveyor. This results in a steady feed of cement waste, enabling effective heat trans fer between the heated gas and the cement waste.

In a preferred embodiment the conveyor may be a screw conveyor. A screw conveyor is well equipped to transport hard and dense cement waste to an inlet of a heater at a steady flow rate, further optimizing heat transfer between the heated gas and the cement waste.

The invention will hereinafter be further elucidated with reference to the drawing of an exemplary embodiment of the device and its method of operation, which is not limiting as to the appended claims.

In the drawing:

Fig. 1 is a front view of a cement recycling device in accordance with a preferred embodiment of the present in vention;

Fig. 2 is a side view of the cement recycling device shown in Fig. 1; Fig. 3 is a front view of a cement recycling device having two heaters stacked above each other, a cyclone and a heated gas recycling conduit; and

Fig. 4 is a side view of the cement recycling device shown in Fig. 3.

DETAILED DESCRIPTION OF THE FIGURES

Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts. In the figures, the following reference numerals are applied that de note the parts mentioned thereafter:

1 cement recycling device

2 heater

3 inlet

4 outlet

5 screen

6 layer

7 elongate bar

8 first distance

9 spacing

10 second distance

11 dampening device

12 heated gas recycling conduit

13 separator

Referring to Figs. 1-4, a cement recycling device 1, in accordance with a preferred embodiment of the present in vention, comprises a heater 2 with an inlet 3 for cement waste and an outlet 4 for processed cement waste, wherein the heater 2 is configured for transporting cement waste in a cement waste transportation direction from the inlet 3 to the outlet 4 while transporting heated gas in countercurrent with the ce ment waste transportation direction.

At least one screen 5 may be located between the in let 3 and the outlet 4 of the heater 2. This screen 5 may com prise a plurality of layers 6 having multiple elongate bars 7, wherein the elongate bars 7 within each layer 6 are spaced apart and arranged approximately parallel to each other. Each of the plurality of layers 6 may be arranged ap proximately perpendicular to the cement waste transportation direction. A first layer 6 of the plurality of layers 6 may be stacked in a staggered position on top of a second layer 6 of the plurality of layers 6, wherein the elongate bars 7 within the first layer 6 are approximately parallel to the elongate bars 7 within the second layer 6.

The elongate bars 7 may be spaced apart at a regular first distance 8 from each other within at least one layer 6 of the plurality of layers 6. A width of each spacing 9 be tween the elongate bars within at least one layer 6 of the plurality of layers 6 may be 5-15 mm, preferably 8-12 mm, most preferably approximately 10 mm.

Each layer 6 may be spaced apart at a regular second distance 10 from each other. The elongate bars 7 may have an at least partly circular cross-section, such as an arc-like cross-section, having a radius of curvature of 10-20 mm, pref erably 12.5 to 17.5 mm, and most preferably a radius of curva ture of 15 mm.

At least part of the screen 5 may be vibratable at a frequency between 20-30 Hz, preferably at about 25 Hz. The di rection of this vibration may be perpendicular to the cement waste transportation direction or parallel to a longitudinal axis of the elongate bars 7.

The cement waste recycling device 1 may further com prise a dampening device 11, preferably a bellow.

The length of the screen in the cement transportation direction may be approximately 1-2 m, preferably 1.25-1.75 m, most preferably about 1.5 m.

The cement waste recycling device 1 may comprise a first heater 2 and a second heater 2, each having an inlet 3 and an outlet 4, wherein the outlet 4 of the first heater 2 is connected to the inlet 3 of the second heater 2. Heat can be recycled in the cement waste recycling device 1 which may com prise a heated gas recycling conduit 12 at one end connected near the outlet 4 of a heater 2 to the device 1 and at another end thereof connected near the inlet 3 of a heater 2 to the device 1. This heated gas recycling conduit 12 may comprise a separator 13, preferably a cyclone. The cement waste recycling device 1 may comprise a feeding unit connected to the inlet 3 for feeding cement waste to the heater 2, wherein the feeding unit comprises a convey or. This conveyor can be a screw conveyor.

Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the de vice and method of the invention, the invention is not re stricted to this particular embodiment which can be varied in many ways without departing from the invention. The discussed exemplary embodiment shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claims to this exemplary embodiment. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exem plary embodiment.