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Separation studies of concrete and brick from construction and demolition waste
Waste Management 85 (2019) 396–404
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Separation studies of concrete and brick from construction and demolition waste Kui Hu a,b,⇑, Yujing Chen a, Falak Naz a, Changnv Zeng a,b, Shihao Cao a a b
College of Civil Engineering and Architecture, Henan University of Technology, Zhengzhou 450001, China National Engineering Laboratory for Wheat & Corn Further Processing, Henan University of Technology, Zhengzhou 450001, China
a r t i c l e
i n f o
Article history: Received 13 December 2018 Revised 3 January 2019 Accepted 6 January 2019
Keywords: Construction and demolition waste (CDW) Recycling technology Concrete/brick separation Air jigging
a b s t r a c t The quality of recycled aggregates from construction and demolition waste (CDW) is strictly related to the content of porous and low strength phases and is specifically related to the high content of brick particles, despite representing approximately 50 wt.% of the total recycled aggregates. This paper focus on air jigging separation studies for removing brick particles from recycled construction and demolition waste aggregates. The operational parameters were achieved by studying the aggregate movement trajectories based on the small specific density differences of 2.52 g/cm3 and 1.97 g/cm3. Separation tests were conducted with a binary mixture of concrete and brick particles ranging from 5 to 10 mm for three operational parameters. The attained results confirmed that the brick fraction increases the water absorption and compromises the consistency and strength of the recycled aggregates. The proposed air jigging separation method was effective at reducing brick particle content and producing significant recycled concrete aggregates with a purity of 95 wt.%, paving the way for greater use of recycled aggregates in high grade applications, such as concrete and pavement layers. Ó 2019 Published by Elsevier Ltd.
1. Introduction A stream of construction and demolition waste (CDW) generally results from infrastructure construction, renovation and demolition of buildings, roads, bridges and other structures (Wu et al., 2014). Overall construction and demolition waste generation in 40 countries worldwide has reached more than 3.0 billion tons annually in 2016 and is increasing (Akhtar and Sarmah, 2018). Developing countries, including India and China, produced the largest amount of CDW in the world (Lu, 2014). In big cities, CDW is growing dramatically due to economic development and urbanization. Consequently, CDW production has reached an amount that most landfill sites near large cities are overflowing as a result of having to dispose of these materials (Ooshaksaraie et al., 2011). The portions of each element depend on the building location, age and pretreatment technology (Bazaz et al., 2006; Jankovic et al., 2012). Some usable fractions (e.g., steel and wood) are presorted and recycled, but a huge amount of crumbed materials are dumped uselessly. Typical components in CDW are inert materials (concrete and brick), which are generally believed to not significantly damage the environment and have been proven to be a sub⇑ Corresponding author at: College of Civil Engineering and Architecture, Henan University of Technology, Zhengzhou 450001, China. E-mail address: [email protected] (K. Hu). https://doi.org/10.1016/j.wasman.2019.01.007 0956-053X/Ó 2019 Published by Elsevier Ltd.
stitute for virgin natural aggregate (Fatemi and Imaninasab, 2016; Khabiri, 2010; Nazemi et al., 2014). However, the demand for pavement and concrete aggregate is expected to increase in the next 50 years, especially in developing countries (Akhtar and Sarmah, 2018). CDW recycling has environmental and economic advantages, as it reduces the consumption of virgin nature aggregates, CDW recycling for paving, concrete manufacturing and footpaths has been proven to be economically viable and have a positive impact on the environment (Ossa et al., 2016; Ozalp et al., 2016; Pasandin and Perez, 2013; Al-Fakih et al., 2019). Currently, most CDW recycling technology is based on automated crushing mobile machinery consisting of wheel loaders, vibration feeders, crushing machines, magnetic separators, and winnower machines, along with various screens and conveyor belts. In the framework of the use mixed recycled aggregates from CDW for high grade applications, more rigorous separation and sorting techniques are needed to meet the required purity of the recycled concrete aggregates (RCA) (Vegas et al., 2015; Medina et al., 2015). The challenge lies in finding the right combination of onsite presorting during demolition, inexpensive separation techniques and further advanced automated sorting techniques. Advanced automated separation and sorting techniques by gravity concentration, color, X-ray, near infrared and spectral parameters have been successfully researched and developed for upgrading the quality of mixed recycled CDW (Vegas et al., 2015;
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Ambros et al., 2017; Di Maria et al., 2016; Ulsen et al., 2013; Feil et al., 2012). Gravity concentration is usually performed in the presence of water or air. Jigs are versatile gravity concentrators that work efficiently with coarse particles based on the vertical pulsation of the particle bed (Boylu et al., 2014). It is believed that air jigging separation is more technically suitable (inexpensive) for RCA purity (Cazacliu et al., 2014; Sampaio et al., 2016; Wang et al., 2015). Hori et al. (2009) and Xing et al. (2004) have shown the effectiveness of density separation methods for removing plastic and brick particles from recycled CDW materials. Cazacliu et al. (2014), Sampaio et al. (2016), and Ambros et al. (2017) showed the stratification of concrete/brick/gypsum particles in a selfdesigned air jigging system, as well as pointed out its potential to separate recycled CDW aggregates automatically. It is expected that in the near future, air jigs can be efficiently used in the treatment of CDW (Sampaio et al., 2016). This paper reports an investigation into automatic concrete/ brick particles separation via continuous feeding using air jigging. Based on the physical indexes for CDW components, an automatic separation system was proposed, and its particle trajectories were studied. Frequency, airstream velocity and horizontal angle of the sieve were adopted for optimization. 2. Materials and methods 2.1. CDW materials The CDW materials were collected from one recycling plant in Shaanxi, China, as shown in Fig. 1a. The manufacturing process of CDW included the following: removal of oversized individual frag-
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ments, primary screening, decreasing the size of bulky parts (impact crushing), removal of ferrous materials (magnetic belt conveyor), classification of reclaimed aggregates (secondary screening) and removal of light particles (air blower). The constituents of the recycled CDW included brick, concrete and a small amount of gypsum, tile, and dust, as shown in Fig. 1b. To achieve better experiment results and avoid the dust becoming entrained in the air when testing by airstream, the materials were washed in water and dried, and the gypsum particles and tiles were picked out manually. Approximately 50 wt.% Concrete and 50 wt.% brick particles in size range from 5 to 10 mm were prepared. To ensure the materials can be tested in a separation bed for 8–10 min, roughly 3.5 tons of mixture was prepared in this research. 2.2. Air jigging-based stratification Jigging separation is generally used to upgrade the quality of grain (Pielot, 2010), coal (Tripathy et al., 2016) and plastic particles (Hori et al., 2009). Jigs are widely used mainly due to the low cost and high efficiency compared with other sorting technologies (Sampaio et al., 2008). Fig. 2 shows a computer generated stratification process from air jigging. The jigging process consists of dispersing to a proper thickness and contraction to stratification via the action of an airstream and bed vibration. In a jig, the separation of materials with different densities is accomplished in a bed that is rendered fluid by a pulsating current of air. The intermittent upward air generates significant turbulence upon passing through several layers of differently sized particles. The particle’s shape and size distribution significantly affect the separation results in a batch jig (Pita and Castilho, 2016; Brozek and Surowiak, 2007). When the airstream is put under pressure by a blower, the particles fall with different speeds to the bottom, depending on their densities. The heavy particles will sink first and be discharged at the bottom. The concentration criterion by jigging depends on the ratio of the specific densities:
q ¼ ðqconctete qair Þ=ðqbrick qair Þ
ð1Þ
where qconctete is the specific density of the concrete particles, qbrick is the specific density of the brick particles, and qair is the specific density of the airflow (set at 0 g/cm3). The separation efficiency of sorting equipment using an air jigging pattern is lower than when using water (Hori et al., 2009). The bigger of concentration criterion q values, the easier for jig stratification and separation of brick/concrete materials (Pita and Castilho, 2016). 2.3. Air jigging-based separation A concrete/brick separation method based on air jigging was put forward and described as follows.
Fig. 1. Materials collected from the CDW plant onsite.
(1) Two fraction regime stratification. Material turbulence has an important influence on particle stratification contributing to the vibration of the bed. The size and shape of the materials determine the amplitude of oscillation and affects the stratification results (Voigt and Twala, 2012). Good stratification can be obtained if the size and shape of the particles are within a close range (Huo et al., 2013). As shown in Figs. 2 and 3a, the brick particles float on the upper layer of the mixture, whereas the concrete particles sink into the bottom layer due to the airflow and vibration of the sorting bed. As the airstream flow from the bottom passes through several layers of aggregate, the gap between the aggregate increases and the extrusion force and friction resistance of the aggregate decreases. Then, the entire mixture on the sifting bed enters a state of fluidization and is conducive to stratification.
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Fig. 2. Stratification process in a jig.
Fig. 3. Stair-like sieve bed (a) and motion curve of the concrete (b).
(2) Two fraction regime moving in the opposite direction. Fig. 3 shows the separation process generated by a horizontal angle b, bars on the sieve, and the airstream and vibration. The motion curve of the concrete in Fig. 3b was obtained in the proposed method. The concrete moves forward on the jig bed due to the concrete particles moving forward one step over one loop in Fig. 3b. Because of the pressure from the continuous mixture feeding and the branched gravity from angle b, the brick particles on the upper layer are pushed downwards. (3) Important parameters. To obtain good results, the critical operational parameters that must be optimized are listed as follows. Particle size: This parameter determines the sieve mesh and the amplitude of oscillation and influences the separation result. A good separation result can be obtained if the size and shape are within a close range (Pita and Castilho, 2016; Paul and Bhattacharya, 2018). In contrast to mineral particles, CDW recycled aggregate has irregular shapes and is distributed unevenly. The influence of particle size is less evident than the influence of the shape due to the lamellar characteristic shape of the brick particles. Airflow rate: The airflow should not be so large as to blow away materials on the sieve bed and not so small that stratification does not take place. Within a proper airflow rate range, the separation will be more productive as the airflow rate increases (Kurosaka et al., 2013). Vibration mode: The specific frequency and trajectory of the pulsation allow concentration for good separation results (Cierpisz and Joostberens, 2016; Abd Aziz et al., 2017). A fast frequency was applied widely for fine particle separation and a slower frequency was used to separate larger particles (Feil et al., 2012; Riazi, 2015). However, a larger amplitude always obtained good separation results for large sized materials.
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In general, the concrete/brick separation process was achieved in two stages: (1) two fraction regime stratification and (2) the two fraction regime moving in the opposite direction. The intermittent upward air causes significant turbulence upon passing through several layers of particles, which results in stratification due to vibrations of the sieve bed. In the second stage, the proposed bars welded on the sieve move the concrete particles forward after stratification. As a result, the concrete particles are pushed by the bars when the jig bed vibrates from quadrant II to quadrant III. 2.4. Separation test Air jigging tests were carried out at the National Engineering Laboratory for Wheat & Corn Further Processing at the Henan University of Technology. The equipment was designed and manufactured in the Henan province in Sept. 2015. It was possible to set some jigging parameters during the tests including (1) horizontal angle of the sieve bed, (2) airstream velocity, and (3) frequency. The horizontal angle of the sieve can be set from 0 to 30 degrees. The airstream velocity was measured via a TESTO425TM anemometer assembled inside the equipment. The vibration frequency of the sieve can be set from 30 to 150 Hz. The dimensions of the jig bed adopted in this research were 220 cm 160 cm. The separation performance was accomplished using one lift truck, and the materials were fed into the jig bed manually, the
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separation process was shown in Fig. 4. The purity of the RCA was tested by inputting the product from the upper outlet. Samples of about 3.5 tons of CDW mixture with approximately 50 wt. % concrete and 50 wt.% brick particles in size range from 5 to 10 mm, in mass. The separation process in Fig. 4 can be divided into three stages. The first stage, as indicated in Fig. 4a and c, mixed particles flow into a flat layer on the sieve gradually. The second stage, as indicated in Fig. 4c and d, the particles stratified on the jigging bed and two layers forms as indicated by Fig. 3a. Meanwhile, the heavy fraction (concrete particles) was motivated by vibration, air flow and pushed by the pressure from mixture feeding (Fig. 4e), especially owing to the bars designed on the sieve, and jumping forward step by step as indicated in Fig. 3b. The photos clearly demonstrate a distinct behavior for the recycled separated concrete/brick particles. As the separated RCA was the desired product (Fig. 4f), the separation result was evaluated by the concrete purity (Pc), which is given by:
Pc ðwt:%Þ ¼
masstotal massbrick 100 masstotal
ð2Þ
where massbrick is the weight of the brick particles picked up manually from the upper outlet product, massbrick is the weight of the brick particles from the bottom outlet, and masstotal is the weight of the product discharged from the upper outlet.
Fig. 4. Video snapshot of the air jig separation process.
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Table 1 Physical indexes of the three construction materials. Physical indexes
True density (g/cm3)
Apparent density (g/cm3)
Bulk density (g/cm3)
Water absorption (%)
2.641 2.520 1.973
2.602 2.421 1.630
15 3.95 18
AASHTO T 19M/T 19 basalt concrete brick
2.709 2.678 2.480
3. Results and discussion 3.1. Characterization of the recycled CDW aggregates Specific densities, absorption, soundness (Standard Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate) and crushing tests of the brick and concrete aggregate were carried out to analyze any property differences. One widely used construction material, basalt, was adopted as a control material to evaluate the strength and durability properties of the recycled CDW, as shown in Table 1. As indicted in Table 1, the properties from the crushing and soundness tests for recycled concrete are close to the virgin aggregate (basalt). The results from the brick particle analysis show that there are 74.7 wt.% weak portions of the total weight, corresponding to 18 wt.% water absorption and 31.6 wt.% soundness test in terms of physical indexes. As a consequence, the water absorption and crushing performance indicated that the brick particles are undesirable for use in pavement or concrete. In the mineral processing industry, generally the concentration criterion q in Eq. (1) was used to estimate the degree of separation difficulty (Cazacliu et al., 2014; Xing et al., 2004). However, there are three density definitions for each aggregate. It is believed that the open pores were also influencing the air jigging result, as some particles were suspended in the airstream (Boylu et al., 2014; Sampaio et al., 2008). Jigging is a stratification process based on vertical pulsation of the particle bed through the movement of a medium (air or water, generally water). A pack of particles is converted into a airflow bed by lifting action of air passing through it. During this period, the porosity increases and the pressure drop rise more slowly than before due to the net effect of increased porosity and velocity. Repeated expansions (dilatation) and contractions (compression) of the particle bed promote the stratification of the bed, which corresponds to the separation of particles in layers of increasing densities from the top to the base. Therefore, the apparent density was adopted for use in this case. To develop a precise apparent density result, the recycled concrete and brick aggregate were tested using the wire-basket method described in AASHTO T 85. Their apparent densities change
Fig. 5. Apparent densities and crushing strength test.
Crushing test (wt.%)
Soundness test (wt.%)
JTG E42-2005
ASTM C88-2013
16.1 23.1 74.7
4.2 9.2 31.6
underwater with time and reach a constant value after 20 min, as shown in Fig. 5. From the beginning of water absorption to the water saturated state of the tested aggregates, there is always a distinguishable difference in the apparent densities of virgin aggregate (basalt), recycled concrete and recycled brick. For apparent density, the recycled concrete was close to basalt aggregate, which was different from recycled brick. The dramatic increase in the initial five minutes also indicated large air voids and water absorption, which is characteristic of brick particles. To separate concrete (qconctete =2.52 g/cm3) and brick (qbrick =1.97 g/cm3) by jigging, the concentration criterion was obtained from the result q = (2.52 0)/(1.97 0) = 1.28. Therefore, the separation of concrete and brick by air jigging is much more difficult than for the sorting technologies applied for grain, coal and plastic particles (Xing et al., 2004; Sampaio et al., 2008). The recycled concrete/brick mixtures with concrete purities (Pc) of 0 wt.%, 20 wt.%, 40 wt.%, 60 wt.%, 80 wt.% and 100 wt.% were studied for strength and durability properties using soundness and water absorption indexes, as shown in Fig. 6. The ratio of the concrete/brick greatly influences the strength and durability properties of the mixtures. The soundness of the recycled mixtures was improved as Pc increased. The water absorption decreased dramatically as a result of decreasing the brick proportion.
Fig. 6. Soundness (a) and water absorption (b) of the concrete/brick mixtures.
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From the point view of pavement construction, recycled aggregates may be applied to pavement if Pc reaches 80 wt.% or more. Due to the large air voids and low strength characteristics, the brick fraction was the undesirable aggregate in CDW. The behavior indicates as greater proportions of brick particles were removed, better recycled CDW aggregate quality was obtained. This research agrees with the fact that the concrete/brick separation resolution is of great significance for CDW reclamation. 3.2. Technology aspects: particle trajectories Due to the extreme closeness of the two apparent density fractions, stair-shaped bars with a length of 3 mm and an angle of 20 degrees welded to the sieve bed were proposed as shown in Fig. 3. The aim of this small modification is to promote separation by changing the motion trajectory of the concrete on the gradient jig bed. The forces distribution for one particle in a jig bed is provided in Fig. 7, where a is the angle of the driving and sieve bed and b is the angle of the planes between the sieve bed and the horizon. For the harmonic vibration mode adopted in Fig. 3b, the motion trajectories can be classified into four quadrants (I–IV) as shown in the bottom right corner of Fig. 7. For a harmonic vibration, the well-known motion trajectory relations are described in Eq. (3):
s ¼ rcoswt; v ¼ wrsinwt; a ¼ w2 rcoswt;
2
mx2 rcosxtsinða þ b þ hÞ P mgsinðb þ UÞ kfv
frictional force of the particles is F ¼ Ntanh, where k is the coefficient of air resistance, f is the air blowing surface area, and v is the airstream velocity. Turbulence has a large influence on the particle stratification along the Y-axis and the balance of gravity and airstream force in the Y-axis can be obtained using the condition P Y ¼ 0:
ð4Þ
For the vibration mode of the sieve bed in Eq. (4), the accelera2
tion for the X-axis is defined as: RX ¼ m ddt2x,
2
c 2g
sinh:
ð7Þ
Since jcosxtjmax ¼ 1:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c g sin ðb þ hÞ k f v 2 2G sinh xP : r sinða þ b þ hÞ
ð8Þ
As the sieve bed moves to quadrant II and quadrant III in Fig. 7, the acceleration direction of the particles is upward, the inertial force is downward, and the brick particles were directed on a general downward separation trend in the sieve bed due to the pressure from the continuous feed mixture. The concrete particles are between the sieve bed and brick particles in the turbulence jig in this study. Due to the close D-value of the brick and concrete, the bars on the stair shaped sieve kept the ‘‘heavy” concrete on the jig bed. The concrete particles should also not move backward when the sieve bed moves to quadrant II–III. 2
The expression RX ¼ M ddt2x was analyzed as follows: 2
M
ð3Þ
where s is the displacement, v is the velocity, a is the acceleration, r is the amplitude of the sieve bed, and w is the angular velocity. A force analysis for a single particle in Fig. 7 can be described using the following components: gravity G = mg, the inertial force c , and the is P ¼ mx2 r cosxt, the airstream force is W ¼ kf v 2 2g
N þ W þ Psinð90 a bÞ G cos b ¼ 0:
To promote the concrete particle forward motion, the acceleration should larger than zero (RX ¼ m ddt2x P 0). Combining Eqs. (4) and (5), we arrive at:
d x ¼ F 2 þ F 0 sinða þ bÞ F 1 P sinða þ bÞ G sinb P 0; dt2 ð9Þ
where M is the weight of the concrete particles, F1 is the fraction force from the other particles, and F 1 in Eq. (9) is approximately to N. The variable F2 is the fraction force from the sieve bed, F 2 ¼ N tanu, and F’ is the pushing force from the bars. As jcosxt jmax ¼ 1 in Eq. (7), the concrete particles remain on or move forward in the jig based on the precondition:
x6
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c M g sinb þ k f v 2 2g r sinða þ bÞ
:
ð10Þ
As indicated by Eqs. (8) and (10), as the concrete particles move forward on the sieve bed, the angular velocity varies from quadrant I to IV. Therefore, the trajectory of the vibration exciter is elliptical rather than circular. The airstream force is approximately equal to the force of gravity on the brick particles in the separation pattern proposed in this paper.
2
m
d x ¼ P cosð90 a bÞ G sin b N tanh; dt 2
d x 1 c tan h: ¼ ½mx2 rcos x tsinða þ b þ hÞ mg sinðb þ hÞ þ kf v 2 2g dt2 cosh ð6Þ 2
m
ð5Þ
Fig. 7. Force analysis of particles on the jigging bed.
3.3. Operation parameters From the particle trajectories described in the equations, the important operational parameters influencing the separation results are the angle b, the airstream velocity v , and frequency of the sieve bed. To determine the proper variation range for these parameters, calculations were carried out in EXCEL, MicrosoftTM. Here, b ranged from 10 to 40 using a calculation step size of 1. Afterwards, the appropriate airstream velocity v and angular velocity x were determined. The amplitude was determined by the average size of the aggregates using an empirical approach. One automated concrete/brick separation testing device was designed and assembled based on the parameters and air jiggingbased separation methods, as shown in Fig. 8. The air jig was designed as an open structure to be able to adjust the thickness of the materials on the jig bed. Particle separation in the pneumatic jig resulted from an air pulsing flow producing stratification of the feedstock. The equipment is composed of two air stream inputs using blowers and one exciter acting as a bed vibration input. To obtain good separation results, the feed speed was controlled to achieve a proper stratification thickness. The input mixture is evenly fed onto the jig bed, and the concrete and brick were
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distributed on the jig bed at a height of 2–4 mixture layers. Fig. 4 shows a visual observation of the uniform mixture distribution as stratification gradually occurred on the jig bed.
3.4. Separation performance The goal of the separation experiments was to optimize the pulsation frequency for the concrete/brick particle separation. An incorrect frequency cannot provide enough time for turbulence stratification (Feil et al., 2012). Because some parameters adopted in this proposed method are outside the control of the testing equipment in Fig. 4, the angular velocity x, angle a, and amplitude were not tested under this condition. Figs. 9 and 10 show the concrete particle purities after the separation tests at various b angles, airstream velocities, and variation frequencies. 3.5 tons of recycled CDW material contains 50 wt.% Concrete and 50 wt.% brick particles in size range from 5 to 10 mm were prepared tested for the separation performance tests.
As indicated in Fig. 9, the desired RCA purity at a 95 wt.% yield occurred at a frequency of 50 r/min, an airstream velocity of 9 m/s and b = 30 . As the airstream increased to 11 m/s, the aggregates blew away from the jig bed. All the eight sample mixtures in Figs. 9 and 10 could not be completed due to the nonoptimized 11 m/s airstream velocity. By increasing the frequency and airstream velocity and setting b = 30, we see that the best separation results are obtained for a 7–9 m/s airstream velocity and a 50–70 r/min frequency. In this case, the RCA purity may be 95 wt.% or larger. It is also suggested that to obtain good performance for recycled CDW beneficiation, a jig bed with an airstream and vibration exciter are vital for this proposed method. Fig. 10 shows the result of increasing the angle b and thus decreasing the airstream velocity for the RCA purity yield at an angle of b = 30. The separability columns indicate that for larger values of the angle b in the sieve bed, higher purity RCA was obtained (from 50 wt.% to 85 wt.%) using a traditional jig bed.
Fig. 8. A sketch of the design (a) and the product discussed in this work (b).
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Fig. 9. Separation results for various frequencies and airstream velocities.
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alternative for obtaining RCA in the low-profit CDW recycling market and achieving environmental benefits. The strength and durability properties of the recycled CDW aggregates were dramatically upgraded from the purified RCA. The separation studies performed based on the theoretical approach at a laboratory scale confirmed that air jigging-based separation was effective for CDW recycling. The RCA product reached as high as 95 wt.% purity. These results prompt the greater use of recycled aggregates in applications such as pavement construction and concrete manufacturing. The concrete/brick separation process was achieved in two stages: two fraction regime stratification with intermittent upward air to induce turbulence and bars welded on the sieve to move the concrete particles forward after stratification of the mixture via a vibration trajectory from quadrant II to quadrant III. Operational parameters including a 60 r/min frequency, 9 m/s airstream velocity and 30 degree sieve horizontal angle b provided the best separation result. A suggestion for future studies is to evaluate the separability performance of the recycled CDW in pilot or semi-industrial scale operations. The separation should consider aggregates larger than 10 mm. Particle trajectories on a jig could be conducted using computational fluid dynamics to simulate the air stream flow and discrete elements to resolve the particle motion. Moreover, the optimization and application of recycled red brick particles deserves further investigation. Acknowledgements This research was supported by funds from the National Natural Science Foundation of China (No. 51608045) and the Foundation for Distinguished Young Talents of Henan University of Technology (No. 2017BS035 and 2018QNJH09). The work was carried out with the help of Prof. Han at Chang’an University, engineer Liu and entrepreneur Li. Air jigging tests were carried out at the National Engineering Laboratory for Wheat & Corn Further Processing at the Henan University of Technology. References
Fig. 10. Separation results for various angles b and airstream velocities.
The desired products were not obtained at angles of 20 and 25 degrees due to a large amount of mixture congestion in the center of the jig bed. Furthermore, trials examining four airstream velocities were conducted. We obtained the highest RCA purity at b = 30 and a 7 m/s airstream velocity. The testing with an angle b = 20 or an airstream velocity of 11 m/s were unsatisfactory, suggesting that the horizontal angle of the sieve bed is also an important parameter for concrete/brick separation. Thus, an air jigging-based separation method appears to be an alternative for separating recycled CDW particles with concrete/ brick contents. This method has the advantage of being a dry separation method. Compared to other separation methods (such as by color, X-ray, near infrared and spectral parameters), the purity of the RCA was not 100 wt.%, indicating that air jigging-based separation is less selective than other separation methods. 4. Conclusions The production of RCA from recycled CDW aggregates using air jigging-based concrete/brick separation represents a potential
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Hori, K., Tsunekawa, M., Hiroyoshi, N., Ito, M., 2009. Optimum water pulsation of jig separation for crushed plastic particles. Int. J. Miner. Process. 92, 103–108. Huo, S.H., Wang, F.S., Yuan, Z., Yue, Z.F., 2013. Mesh regeneration method for jig-shape optimization design of the high-aspect-ratio wing. Adv. Mech. Eng. 5, 5–13. Jankovic, Ksenija, Nikolic, Dragan, Bojovic, Dragan, 2012. Concrete paving blocks and flags made with crushed brick as aggregate. Constr. Build. Mater. 28, 659– 663. Khabiri, Mohammad M., 2010. The effect of stabilized subbase containing waste construction materials on reduction of pavement rutting depth. Electron. J. Geotech. Eng. 15, 1211–1219. Kurosaka, K., Ochi, Y., Inada, H., Arimoto, T., Sakai, H., 2013. Strategy for controlling the hauling speed of the jigging machine for reducing the catch loss of neon flying squid Ommastrephes bartramii. Nippon Suisan Gakkaishi 79, 327–336. Lu, Weisheng, 2014. Estimating the Amount of Building-Related Construction and Demolition Waste in China. Springer, Berlin Heidelberg. Medina, C., Zhu, W., Howind, T., Frias, M., de Rojas, M.I.S., 2015. Effect of the constituents (asphalt, clay materials, floating particles and fines) of construction and demolition waste on the properties of recycled concretes. Constr. Build. Mater. 79, 22–33. Nazemi, Saeid, Aliakbar, Roudbari, Kamyar, Yaghmaeian, 2014. Design and implementation of integrated solid wastes management pattern in industrial zones, case study of Shahroud, Iran. J. Environ. Health Sci. Eng. 12 (1) (2014-0114), 6: 32–32. Ooshaksaraie, Leila, Mardookhpour, Alireza, Basri, Noor Ezlin Ahmad, Aghaee, Azam, 2011. An Expert System for Construction Sites Best Management Practices. Springer, Berlin Heidelberg. Ossa, A., Garcia, J.L., Botero, E., 2016. Use of recycled construction and demolition waste (CDW) aggregates: a sustainable alternative for the pavement construction industry. J. Clean. Prod. 135, 379–386. Ozalp, F., Yilmaz, H.D., Kara, M., Kaya, O., Sahin, A., 2016. Effects of recycled aggregates from construction and demolition wastes on mechanical and permeability properties of paving stone, kerb and concrete pipes. Constr. Build. Mater. 110, 17–23.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Separation studies of concrete and brick from construction and demolition waste Kui Hu a,b,⇑, Yujing Chen a, Falak Naz a, Changnv Zeng a,b, Shihao Cao a a b
College of Civil Engineering and Architecture, Henan University of Technology, Zhengzhou 450001, China National Engineering Laboratory for Wheat & Corn Further Processing, Henan University of Technology, Zhengzhou 450001, China
a r t i c l e
i n f o
Article history: Received 13 December 2018 Revised 3 January 2019 Accepted 6 January 2019
Keywords: Construction and demolition waste (CDW) Recycling technology Concrete/brick separation Air jigging
a b s t r a c t The quality of recycled aggregates from construction and demolition waste (CDW) is strictly related to the content of porous and low strength phases and is specifically related to the high content of brick particles, despite representing approximately 50 wt.% of the total recycled aggregates. This paper focus on air jigging separation studies for removing brick particles from recycled construction and demolition waste aggregates. The operational parameters were achieved by studying the aggregate movement trajectories based on the small specific density differences of 2.52 g/cm3 and 1.97 g/cm3. Separation tests were conducted with a binary mixture of concrete and brick particles ranging from 5 to 10 mm for three operational parameters. The attained results confirmed that the brick fraction increases the water absorption and compromises the consistency and strength of the recycled aggregates. The proposed air jigging separation method was effective at reducing brick particle content and producing significant recycled concrete aggregates with a purity of 95 wt.%, paving the way for greater use of recycled aggregates in high grade applications, such as concrete and pavement layers. Ó 2019 Published by Elsevier Ltd.
1. Introduction A stream of construction and demolition waste (CDW) generally results from infrastructure construction, renovation and demolition of buildings, roads, bridges and other structures (Wu et al., 2014). Overall construction and demolition waste generation in 40 countries worldwide has reached more than 3.0 billion tons annually in 2016 and is increasing (Akhtar and Sarmah, 2018). Developing countries, including India and China, produced the largest amount of CDW in the world (Lu, 2014). In big cities, CDW is growing dramatically due to economic development and urbanization. Consequently, CDW production has reached an amount that most landfill sites near large cities are overflowing as a result of having to dispose of these materials (Ooshaksaraie et al., 2011). The portions of each element depend on the building location, age and pretreatment technology (Bazaz et al., 2006; Jankovic et al., 2012). Some usable fractions (e.g., steel and wood) are presorted and recycled, but a huge amount of crumbed materials are dumped uselessly. Typical components in CDW are inert materials (concrete and brick), which are generally believed to not significantly damage the environment and have been proven to be a sub⇑ Corresponding author at: College of Civil Engineering and Architecture, Henan University of Technology, Zhengzhou 450001, China. E-mail address: [email protected] (K. Hu). https://doi.org/10.1016/j.wasman.2019.01.007 0956-053X/Ó 2019 Published by Elsevier Ltd.
stitute for virgin natural aggregate (Fatemi and Imaninasab, 2016; Khabiri, 2010; Nazemi et al., 2014). However, the demand for pavement and concrete aggregate is expected to increase in the next 50 years, especially in developing countries (Akhtar and Sarmah, 2018). CDW recycling has environmental and economic advantages, as it reduces the consumption of virgin nature aggregates, CDW recycling for paving, concrete manufacturing and footpaths has been proven to be economically viable and have a positive impact on the environment (Ossa et al., 2016; Ozalp et al., 2016; Pasandin and Perez, 2013; Al-Fakih et al., 2019). Currently, most CDW recycling technology is based on automated crushing mobile machinery consisting of wheel loaders, vibration feeders, crushing machines, magnetic separators, and winnower machines, along with various screens and conveyor belts. In the framework of the use mixed recycled aggregates from CDW for high grade applications, more rigorous separation and sorting techniques are needed to meet the required purity of the recycled concrete aggregates (RCA) (Vegas et al., 2015; Medina et al., 2015). The challenge lies in finding the right combination of onsite presorting during demolition, inexpensive separation techniques and further advanced automated sorting techniques. Advanced automated separation and sorting techniques by gravity concentration, color, X-ray, near infrared and spectral parameters have been successfully researched and developed for upgrading the quality of mixed recycled CDW (Vegas et al., 2015;
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Ambros et al., 2017; Di Maria et al., 2016; Ulsen et al., 2013; Feil et al., 2012). Gravity concentration is usually performed in the presence of water or air. Jigs are versatile gravity concentrators that work efficiently with coarse particles based on the vertical pulsation of the particle bed (Boylu et al., 2014). It is believed that air jigging separation is more technically suitable (inexpensive) for RCA purity (Cazacliu et al., 2014; Sampaio et al., 2016; Wang et al., 2015). Hori et al. (2009) and Xing et al. (2004) have shown the effectiveness of density separation methods for removing plastic and brick particles from recycled CDW materials. Cazacliu et al. (2014), Sampaio et al. (2016), and Ambros et al. (2017) showed the stratification of concrete/brick/gypsum particles in a selfdesigned air jigging system, as well as pointed out its potential to separate recycled CDW aggregates automatically. It is expected that in the near future, air jigs can be efficiently used in the treatment of CDW (Sampaio et al., 2016). This paper reports an investigation into automatic concrete/ brick particles separation via continuous feeding using air jigging. Based on the physical indexes for CDW components, an automatic separation system was proposed, and its particle trajectories were studied. Frequency, airstream velocity and horizontal angle of the sieve were adopted for optimization. 2. Materials and methods 2.1. CDW materials The CDW materials were collected from one recycling plant in Shaanxi, China, as shown in Fig. 1a. The manufacturing process of CDW included the following: removal of oversized individual frag-
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ments, primary screening, decreasing the size of bulky parts (impact crushing), removal of ferrous materials (magnetic belt conveyor), classification of reclaimed aggregates (secondary screening) and removal of light particles (air blower). The constituents of the recycled CDW included brick, concrete and a small amount of gypsum, tile, and dust, as shown in Fig. 1b. To achieve better experiment results and avoid the dust becoming entrained in the air when testing by airstream, the materials were washed in water and dried, and the gypsum particles and tiles were picked out manually. Approximately 50 wt.% Concrete and 50 wt.% brick particles in size range from 5 to 10 mm were prepared. To ensure the materials can be tested in a separation bed for 8–10 min, roughly 3.5 tons of mixture was prepared in this research. 2.2. Air jigging-based stratification Jigging separation is generally used to upgrade the quality of grain (Pielot, 2010), coal (Tripathy et al., 2016) and plastic particles (Hori et al., 2009). Jigs are widely used mainly due to the low cost and high efficiency compared with other sorting technologies (Sampaio et al., 2008). Fig. 2 shows a computer generated stratification process from air jigging. The jigging process consists of dispersing to a proper thickness and contraction to stratification via the action of an airstream and bed vibration. In a jig, the separation of materials with different densities is accomplished in a bed that is rendered fluid by a pulsating current of air. The intermittent upward air generates significant turbulence upon passing through several layers of differently sized particles. The particle’s shape and size distribution significantly affect the separation results in a batch jig (Pita and Castilho, 2016; Brozek and Surowiak, 2007). When the airstream is put under pressure by a blower, the particles fall with different speeds to the bottom, depending on their densities. The heavy particles will sink first and be discharged at the bottom. The concentration criterion by jigging depends on the ratio of the specific densities:
q ¼ ðqconctete qair Þ=ðqbrick qair Þ
ð1Þ
where qconctete is the specific density of the concrete particles, qbrick is the specific density of the brick particles, and qair is the specific density of the airflow (set at 0 g/cm3). The separation efficiency of sorting equipment using an air jigging pattern is lower than when using water (Hori et al., 2009). The bigger of concentration criterion q values, the easier for jig stratification and separation of brick/concrete materials (Pita and Castilho, 2016). 2.3. Air jigging-based separation A concrete/brick separation method based on air jigging was put forward and described as follows.
Fig. 1. Materials collected from the CDW plant onsite.
(1) Two fraction regime stratification. Material turbulence has an important influence on particle stratification contributing to the vibration of the bed. The size and shape of the materials determine the amplitude of oscillation and affects the stratification results (Voigt and Twala, 2012). Good stratification can be obtained if the size and shape of the particles are within a close range (Huo et al., 2013). As shown in Figs. 2 and 3a, the brick particles float on the upper layer of the mixture, whereas the concrete particles sink into the bottom layer due to the airflow and vibration of the sorting bed. As the airstream flow from the bottom passes through several layers of aggregate, the gap between the aggregate increases and the extrusion force and friction resistance of the aggregate decreases. Then, the entire mixture on the sifting bed enters a state of fluidization and is conducive to stratification.
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Fig. 2. Stratification process in a jig.
Fig. 3. Stair-like sieve bed (a) and motion curve of the concrete (b).
(2) Two fraction regime moving in the opposite direction. Fig. 3 shows the separation process generated by a horizontal angle b, bars on the sieve, and the airstream and vibration. The motion curve of the concrete in Fig. 3b was obtained in the proposed method. The concrete moves forward on the jig bed due to the concrete particles moving forward one step over one loop in Fig. 3b. Because of the pressure from the continuous mixture feeding and the branched gravity from angle b, the brick particles on the upper layer are pushed downwards. (3) Important parameters. To obtain good results, the critical operational parameters that must be optimized are listed as follows. Particle size: This parameter determines the sieve mesh and the amplitude of oscillation and influences the separation result. A good separation result can be obtained if the size and shape are within a close range (Pita and Castilho, 2016; Paul and Bhattacharya, 2018). In contrast to mineral particles, CDW recycled aggregate has irregular shapes and is distributed unevenly. The influence of particle size is less evident than the influence of the shape due to the lamellar characteristic shape of the brick particles. Airflow rate: The airflow should not be so large as to blow away materials on the sieve bed and not so small that stratification does not take place. Within a proper airflow rate range, the separation will be more productive as the airflow rate increases (Kurosaka et al., 2013). Vibration mode: The specific frequency and trajectory of the pulsation allow concentration for good separation results (Cierpisz and Joostberens, 2016; Abd Aziz et al., 2017). A fast frequency was applied widely for fine particle separation and a slower frequency was used to separate larger particles (Feil et al., 2012; Riazi, 2015). However, a larger amplitude always obtained good separation results for large sized materials.
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In general, the concrete/brick separation process was achieved in two stages: (1) two fraction regime stratification and (2) the two fraction regime moving in the opposite direction. The intermittent upward air causes significant turbulence upon passing through several layers of particles, which results in stratification due to vibrations of the sieve bed. In the second stage, the proposed bars welded on the sieve move the concrete particles forward after stratification. As a result, the concrete particles are pushed by the bars when the jig bed vibrates from quadrant II to quadrant III. 2.4. Separation test Air jigging tests were carried out at the National Engineering Laboratory for Wheat & Corn Further Processing at the Henan University of Technology. The equipment was designed and manufactured in the Henan province in Sept. 2015. It was possible to set some jigging parameters during the tests including (1) horizontal angle of the sieve bed, (2) airstream velocity, and (3) frequency. The horizontal angle of the sieve can be set from 0 to 30 degrees. The airstream velocity was measured via a TESTO425TM anemometer assembled inside the equipment. The vibration frequency of the sieve can be set from 30 to 150 Hz. The dimensions of the jig bed adopted in this research were 220 cm 160 cm. The separation performance was accomplished using one lift truck, and the materials were fed into the jig bed manually, the
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separation process was shown in Fig. 4. The purity of the RCA was tested by inputting the product from the upper outlet. Samples of about 3.5 tons of CDW mixture with approximately 50 wt. % concrete and 50 wt.% brick particles in size range from 5 to 10 mm, in mass. The separation process in Fig. 4 can be divided into three stages. The first stage, as indicated in Fig. 4a and c, mixed particles flow into a flat layer on the sieve gradually. The second stage, as indicated in Fig. 4c and d, the particles stratified on the jigging bed and two layers forms as indicated by Fig. 3a. Meanwhile, the heavy fraction (concrete particles) was motivated by vibration, air flow and pushed by the pressure from mixture feeding (Fig. 4e), especially owing to the bars designed on the sieve, and jumping forward step by step as indicated in Fig. 3b. The photos clearly demonstrate a distinct behavior for the recycled separated concrete/brick particles. As the separated RCA was the desired product (Fig. 4f), the separation result was evaluated by the concrete purity (Pc), which is given by:
Pc ðwt:%Þ ¼
masstotal massbrick 100 masstotal
ð2Þ
where massbrick is the weight of the brick particles picked up manually from the upper outlet product, massbrick is the weight of the brick particles from the bottom outlet, and masstotal is the weight of the product discharged from the upper outlet.
Fig. 4. Video snapshot of the air jig separation process.
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Table 1 Physical indexes of the three construction materials. Physical indexes
True density (g/cm3)
Apparent density (g/cm3)
Bulk density (g/cm3)
Water absorption (%)
2.641 2.520 1.973
2.602 2.421 1.630
15 3.95 18
AASHTO T 19M/T 19 basalt concrete brick
2.709 2.678 2.480
3. Results and discussion 3.1. Characterization of the recycled CDW aggregates Specific densities, absorption, soundness (Standard Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate) and crushing tests of the brick and concrete aggregate were carried out to analyze any property differences. One widely used construction material, basalt, was adopted as a control material to evaluate the strength and durability properties of the recycled CDW, as shown in Table 1. As indicted in Table 1, the properties from the crushing and soundness tests for recycled concrete are close to the virgin aggregate (basalt). The results from the brick particle analysis show that there are 74.7 wt.% weak portions of the total weight, corresponding to 18 wt.% water absorption and 31.6 wt.% soundness test in terms of physical indexes. As a consequence, the water absorption and crushing performance indicated that the brick particles are undesirable for use in pavement or concrete. In the mineral processing industry, generally the concentration criterion q in Eq. (1) was used to estimate the degree of separation difficulty (Cazacliu et al., 2014; Xing et al., 2004). However, there are three density definitions for each aggregate. It is believed that the open pores were also influencing the air jigging result, as some particles were suspended in the airstream (Boylu et al., 2014; Sampaio et al., 2008). Jigging is a stratification process based on vertical pulsation of the particle bed through the movement of a medium (air or water, generally water). A pack of particles is converted into a airflow bed by lifting action of air passing through it. During this period, the porosity increases and the pressure drop rise more slowly than before due to the net effect of increased porosity and velocity. Repeated expansions (dilatation) and contractions (compression) of the particle bed promote the stratification of the bed, which corresponds to the separation of particles in layers of increasing densities from the top to the base. Therefore, the apparent density was adopted for use in this case. To develop a precise apparent density result, the recycled concrete and brick aggregate were tested using the wire-basket method described in AASHTO T 85. Their apparent densities change
Fig. 5. Apparent densities and crushing strength test.
Crushing test (wt.%)
Soundness test (wt.%)
JTG E42-2005
ASTM C88-2013
16.1 23.1 74.7
4.2 9.2 31.6
underwater with time and reach a constant value after 20 min, as shown in Fig. 5. From the beginning of water absorption to the water saturated state of the tested aggregates, there is always a distinguishable difference in the apparent densities of virgin aggregate (basalt), recycled concrete and recycled brick. For apparent density, the recycled concrete was close to basalt aggregate, which was different from recycled brick. The dramatic increase in the initial five minutes also indicated large air voids and water absorption, which is characteristic of brick particles. To separate concrete (qconctete =2.52 g/cm3) and brick (qbrick =1.97 g/cm3) by jigging, the concentration criterion was obtained from the result q = (2.52 0)/(1.97 0) = 1.28. Therefore, the separation of concrete and brick by air jigging is much more difficult than for the sorting technologies applied for grain, coal and plastic particles (Xing et al., 2004; Sampaio et al., 2008). The recycled concrete/brick mixtures with concrete purities (Pc) of 0 wt.%, 20 wt.%, 40 wt.%, 60 wt.%, 80 wt.% and 100 wt.% were studied for strength and durability properties using soundness and water absorption indexes, as shown in Fig. 6. The ratio of the concrete/brick greatly influences the strength and durability properties of the mixtures. The soundness of the recycled mixtures was improved as Pc increased. The water absorption decreased dramatically as a result of decreasing the brick proportion.
Fig. 6. Soundness (a) and water absorption (b) of the concrete/brick mixtures.
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From the point view of pavement construction, recycled aggregates may be applied to pavement if Pc reaches 80 wt.% or more. Due to the large air voids and low strength characteristics, the brick fraction was the undesirable aggregate in CDW. The behavior indicates as greater proportions of brick particles were removed, better recycled CDW aggregate quality was obtained. This research agrees with the fact that the concrete/brick separation resolution is of great significance for CDW reclamation. 3.2. Technology aspects: particle trajectories Due to the extreme closeness of the two apparent density fractions, stair-shaped bars with a length of 3 mm and an angle of 20 degrees welded to the sieve bed were proposed as shown in Fig. 3. The aim of this small modification is to promote separation by changing the motion trajectory of the concrete on the gradient jig bed. The forces distribution for one particle in a jig bed is provided in Fig. 7, where a is the angle of the driving and sieve bed and b is the angle of the planes between the sieve bed and the horizon. For the harmonic vibration mode adopted in Fig. 3b, the motion trajectories can be classified into four quadrants (I–IV) as shown in the bottom right corner of Fig. 7. For a harmonic vibration, the well-known motion trajectory relations are described in Eq. (3):
s ¼ rcoswt; v ¼ wrsinwt; a ¼ w2 rcoswt;
2
mx2 rcosxtsinða þ b þ hÞ P mgsinðb þ UÞ kfv
frictional force of the particles is F ¼ Ntanh, where k is the coefficient of air resistance, f is the air blowing surface area, and v is the airstream velocity. Turbulence has a large influence on the particle stratification along the Y-axis and the balance of gravity and airstream force in the Y-axis can be obtained using the condition P Y ¼ 0:
ð4Þ
For the vibration mode of the sieve bed in Eq. (4), the accelera2
tion for the X-axis is defined as: RX ¼ m ddt2x,
2
c 2g
sinh:
ð7Þ
Since jcosxtjmax ¼ 1:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c g sin ðb þ hÞ k f v 2 2G sinh xP : r sinða þ b þ hÞ
ð8Þ
As the sieve bed moves to quadrant II and quadrant III in Fig. 7, the acceleration direction of the particles is upward, the inertial force is downward, and the brick particles were directed on a general downward separation trend in the sieve bed due to the pressure from the continuous feed mixture. The concrete particles are between the sieve bed and brick particles in the turbulence jig in this study. Due to the close D-value of the brick and concrete, the bars on the stair shaped sieve kept the ‘‘heavy” concrete on the jig bed. The concrete particles should also not move backward when the sieve bed moves to quadrant II–III. 2
The expression RX ¼ M ddt2x was analyzed as follows: 2
M
ð3Þ
where s is the displacement, v is the velocity, a is the acceleration, r is the amplitude of the sieve bed, and w is the angular velocity. A force analysis for a single particle in Fig. 7 can be described using the following components: gravity G = mg, the inertial force c , and the is P ¼ mx2 r cosxt, the airstream force is W ¼ kf v 2 2g
N þ W þ Psinð90 a bÞ G cos b ¼ 0:
To promote the concrete particle forward motion, the acceleration should larger than zero (RX ¼ m ddt2x P 0). Combining Eqs. (4) and (5), we arrive at:
d x ¼ F 2 þ F 0 sinða þ bÞ F 1 P sinða þ bÞ G sinb P 0; dt2 ð9Þ
where M is the weight of the concrete particles, F1 is the fraction force from the other particles, and F 1 in Eq. (9) is approximately to N. The variable F2 is the fraction force from the sieve bed, F 2 ¼ N tanu, and F’ is the pushing force from the bars. As jcosxt jmax ¼ 1 in Eq. (7), the concrete particles remain on or move forward in the jig based on the precondition:
x6
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c M g sinb þ k f v 2 2g r sinða þ bÞ
:
ð10Þ
As indicated by Eqs. (8) and (10), as the concrete particles move forward on the sieve bed, the angular velocity varies from quadrant I to IV. Therefore, the trajectory of the vibration exciter is elliptical rather than circular. The airstream force is approximately equal to the force of gravity on the brick particles in the separation pattern proposed in this paper.
2
m
d x ¼ P cosð90 a bÞ G sin b N tanh; dt 2
d x 1 c tan h: ¼ ½mx2 rcos x tsinða þ b þ hÞ mg sinðb þ hÞ þ kf v 2 2g dt2 cosh ð6Þ 2
m
ð5Þ
Fig. 7. Force analysis of particles on the jigging bed.
3.3. Operation parameters From the particle trajectories described in the equations, the important operational parameters influencing the separation results are the angle b, the airstream velocity v , and frequency of the sieve bed. To determine the proper variation range for these parameters, calculations were carried out in EXCEL, MicrosoftTM. Here, b ranged from 10 to 40 using a calculation step size of 1. Afterwards, the appropriate airstream velocity v and angular velocity x were determined. The amplitude was determined by the average size of the aggregates using an empirical approach. One automated concrete/brick separation testing device was designed and assembled based on the parameters and air jiggingbased separation methods, as shown in Fig. 8. The air jig was designed as an open structure to be able to adjust the thickness of the materials on the jig bed. Particle separation in the pneumatic jig resulted from an air pulsing flow producing stratification of the feedstock. The equipment is composed of two air stream inputs using blowers and one exciter acting as a bed vibration input. To obtain good separation results, the feed speed was controlled to achieve a proper stratification thickness. The input mixture is evenly fed onto the jig bed, and the concrete and brick were
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distributed on the jig bed at a height of 2–4 mixture layers. Fig. 4 shows a visual observation of the uniform mixture distribution as stratification gradually occurred on the jig bed.
3.4. Separation performance The goal of the separation experiments was to optimize the pulsation frequency for the concrete/brick particle separation. An incorrect frequency cannot provide enough time for turbulence stratification (Feil et al., 2012). Because some parameters adopted in this proposed method are outside the control of the testing equipment in Fig. 4, the angular velocity x, angle a, and amplitude were not tested under this condition. Figs. 9 and 10 show the concrete particle purities after the separation tests at various b angles, airstream velocities, and variation frequencies. 3.5 tons of recycled CDW material contains 50 wt.% Concrete and 50 wt.% brick particles in size range from 5 to 10 mm were prepared tested for the separation performance tests.
As indicated in Fig. 9, the desired RCA purity at a 95 wt.% yield occurred at a frequency of 50 r/min, an airstream velocity of 9 m/s and b = 30 . As the airstream increased to 11 m/s, the aggregates blew away from the jig bed. All the eight sample mixtures in Figs. 9 and 10 could not be completed due to the nonoptimized 11 m/s airstream velocity. By increasing the frequency and airstream velocity and setting b = 30, we see that the best separation results are obtained for a 7–9 m/s airstream velocity and a 50–70 r/min frequency. In this case, the RCA purity may be 95 wt.% or larger. It is also suggested that to obtain good performance for recycled CDW beneficiation, a jig bed with an airstream and vibration exciter are vital for this proposed method. Fig. 10 shows the result of increasing the angle b and thus decreasing the airstream velocity for the RCA purity yield at an angle of b = 30. The separability columns indicate that for larger values of the angle b in the sieve bed, higher purity RCA was obtained (from 50 wt.% to 85 wt.%) using a traditional jig bed.
Fig. 8. A sketch of the design (a) and the product discussed in this work (b).
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Fig. 9. Separation results for various frequencies and airstream velocities.
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alternative for obtaining RCA in the low-profit CDW recycling market and achieving environmental benefits. The strength and durability properties of the recycled CDW aggregates were dramatically upgraded from the purified RCA. The separation studies performed based on the theoretical approach at a laboratory scale confirmed that air jigging-based separation was effective for CDW recycling. The RCA product reached as high as 95 wt.% purity. These results prompt the greater use of recycled aggregates in applications such as pavement construction and concrete manufacturing. The concrete/brick separation process was achieved in two stages: two fraction regime stratification with intermittent upward air to induce turbulence and bars welded on the sieve to move the concrete particles forward after stratification of the mixture via a vibration trajectory from quadrant II to quadrant III. Operational parameters including a 60 r/min frequency, 9 m/s airstream velocity and 30 degree sieve horizontal angle b provided the best separation result. A suggestion for future studies is to evaluate the separability performance of the recycled CDW in pilot or semi-industrial scale operations. The separation should consider aggregates larger than 10 mm. Particle trajectories on a jig could be conducted using computational fluid dynamics to simulate the air stream flow and discrete elements to resolve the particle motion. Moreover, the optimization and application of recycled red brick particles deserves further investigation. Acknowledgements This research was supported by funds from the National Natural Science Foundation of China (No. 51608045) and the Foundation for Distinguished Young Talents of Henan University of Technology (No. 2017BS035 and 2018QNJH09). The work was carried out with the help of Prof. Han at Chang’an University, engineer Liu and entrepreneur Li. Air jigging tests were carried out at the National Engineering Laboratory for Wheat & Corn Further Processing at the Henan University of Technology. References
Fig. 10. Separation results for various angles b and airstream velocities.
The desired products were not obtained at angles of 20 and 25 degrees due to a large amount of mixture congestion in the center of the jig bed. Furthermore, trials examining four airstream velocities were conducted. We obtained the highest RCA purity at b = 30 and a 7 m/s airstream velocity. The testing with an angle b = 20 or an airstream velocity of 11 m/s were unsatisfactory, suggesting that the horizontal angle of the sieve bed is also an important parameter for concrete/brick separation. Thus, an air jigging-based separation method appears to be an alternative for separating recycled CDW particles with concrete/ brick contents. This method has the advantage of being a dry separation method. Compared to other separation methods (such as by color, X-ray, near infrared and spectral parameters), the purity of the RCA was not 100 wt.%, indicating that air jigging-based separation is less selective than other separation methods. 4. Conclusions The production of RCA from recycled CDW aggregates using air jigging-based concrete/brick separation represents a potential
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