Transformation of hazardous residues from medical laboratory waste treatment plant into permeable brick

doi:10.21203/rs.3.rs-4703259/v1

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Transformation of hazardous residues from medical laboratory waste treatment plant into permeable brick

https://doi.org/10.21203/rs.3.rs-4703259/v1

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In this study, the transformation of hazardous residues (fly ash, silica gel, bottom ash, and sludge) from a medical laboratory waste incineration system of Sustainable Environment Research Center into permeable bricks was investigated. The fly ash was elutriated using water twice to remove the Cl⁻. The elutriated fly ash was mixed with silica gel, bottom ash, and sludge with different formulas which were designed using response surface methodology (RSM) in Design Expert 8.0.6 software. The mixed materials were melted at 1450 ℃. The RSM analysis results showed that the most optimal effect was achieved when silica gel constituted 60% of the total sample and the elutriated fly ash constituted 10% of the encapsulated phase (elutriated fly ash, bottom ash, and sludge), in terms of minimizing the amount of unmelted materials and maximizing the percentage of mass reduction.

The slag with optimal melting effect was mixed with cullet, clay, cement, and water, molded, cured, and transformed into permeable bricks. The physical properties met the regulations for grade-A bricks, and the Toxicity Characteristic Leaching Procedure (TCLP) results showed that metals were well encapsulated in the permeable bricks. Overall, the permeable bricks will offer sufficient physical strength and will not contribute to environmental pollution when recycled.

Fly ash

Slag

Response surface methodology

Permeable brick

Melting

In school education and research, laboratory wastes (LW) are inevitably generated during experiments. LW is difficult to dispose of because it is often diverse and in small amounts. To address this issue, the Sustainable Environment Research Center (SERC) of National Cheng Kung University established a LW treatment plant in 2005. This plant featured three different treatment systems: an incineration system, a physicochemical treatment system, and a plasma melting system. Combustible medical laboratory wastes (MLW) and plastic containers from medical laboratories (rather than hospitals) were directed to the incineration system. Liquid LW was classified based on their characteristics, such as organic LW, acid LW, or LW containing heavy metals, and then sent to various units within the physicochemical treatment system.

During the two treatment processes mentioned above, residues such as fly ash, bottom ash, and sludge were generated and sent to the plasma melting system. Plasma melting has been applied to melt a wide variety of materials, such as glass mixtures or waste incinerator fly ash [1, 2] (Gonterman and Weinstein, 2009; Chen et al., 2009). The mixture of residue was combined with silica gel or cullet containing rich SiO₂ and heated to 1400 ℃ using the plasma torch. The high temperature decomposed the organic materials, and the SiO₂ acted as the glassy matrix to encapsulate the hazardous materials [3](Kuo et al., 2010). The molten materials were then quenched with water to form glassy slag. After the melting and vitrification processes, the slag was transformed into an inert form, and the hazardous materials were decomposed or stabilized. According to previous studies, the slag could be recycled as composite materials or building materials [4, 5] (Malhotra et al., 1996; Kuo et al., 2011).

However, the combustible medical LW from the medical or biochemical experiments usually contained a high concentration of Cl. The Cl would cause the formation of PCDD/Fs and form HCl during the incineration process [6, 7](Hatanaka et al., 2005; Cai et al., 2022). The HCl was neutralized by NaCl in the quenching tower and existed in the fly ash as NaCl [8] (Kuo et al., 2019). Therefore, the fly ash contained a high level of Cl and PCDD/Fs and needed to be treated by the plasma melting furnace. During the melting process, the Cl ions would elevate the melting temperature and retard the formation of slag. Therefore, the Cl might need to be removed before the melting process to promote the effect of stabilization. An elutriation process was proven to effectively eliminate chloride [9] (Wang et al., 2016).

After the melting process, the waste was transformed into slag, and the recycling process was another crucial issue. Various methods are employed to recycle stabilized waste, such as blast furnace slag, fly ash, or gangue, into bricks. To achieve sufficient strength, heating or high pressure may be necessary during the manufacturing process [4, 10, 11](Malhotra and Tehri, 1996; Li et al., 2021; Zhu et al., 2017).

In this study, the fly ash from the incineration system was washed with water to remove chloride ions. The washed fly ash was then combined with silica gel (from the LW treatment plant), bottom ash (from the incineration system), and sludge (from the physicochemical treatment system) in various mass ratios. During the melting process, silica gel acted as the glassy matrix to encapsulate hazardous materials, referred to as the encapsulating phase. The other three materials functioned as hazardous materials to be encapsulated within the glassy matrix, known as the encapsulated phase. The mixtures were melted at 1400 ℃ in a lab-scale heating furnace and then cooled to form slags. The composition and leaching of metals in the slags were examined to assess the vitrification effect. The slag exhibiting the most effective vitrification was reused to create permeable bricks. The chemical properties and physical strength of the bricks were assessed to determine the feasibility of slag recycling in an industrial setting.

2.1 Preparation of raw materials

The flow chart diagram of the LW incineration system was provided in our previous study [12] (Liao et al., 2014). The fly ash and bottom ash were collected from the primary quenching tower and the bottom ash pit of the incineration system, respectively. The sludge was sampled from the filter press plate of the heavy-metal-based liquid LW treatment unit. The silica gel and the cullet were collected from the temporary storage area of the LW treatment plant. All the specimens were dried and ground to a powder smaller than 150 mm (100 mesh) to ensure uniformity for further experiments.

2.2 Procedure of the elutriation, melting, and permeable brick making processes

Figure 1 illustrates the process flow chart of the experiments. The raw fly ash (RFA) was mixed with a mass ratio of fly ash to water = 1:5. The mixture was stirred for 10 minutes and filtered using a mixed cellulose ester filter (average pore size = 1 mm) to remove suspended solids. The elutriated fly ash was then dried, weighed, and analyzed. The first elutriated fly ash (FEFA) underwent the same elutriation, filtration, drying, and analysis process as the second elutriated fly ash (SEFA). The elutriating wastewater was also filtered for further analysis.

The melting effect of various materials was assessed using response surface methodology (RSM), which was used to investigate the melting behavior of a fluidized bed in a previous report [13] (Kukec et al., 2012). The mixing formula for melted materials (SEFA, silica gel, bottom ash, and sludge) for melting was designed using Design Expert 8.0.6 software (Stat-Ease Inc. Com Ltd.). To evaluate how the mixing formula of wastes governs the effect of melting processes, we used two expected yields as response values for the control factors. The first yield was the remaining mass of input materials that was not transformed into slag (defined as unmelted materials). The second yield was the reduced mass percentage of input materials during the thermal melting process.

Among the melted materials, the silica gel was considered the encapsulating phase, while the sum of SEFA, bottom ash, and sludge was considered the encapsulated phase. Two operational parameters, the mass fraction of the encapsulating phase and the mass fraction of SEFA, were used for the experiments and were defined as follows. The mass fraction of the encapsulating phase was defined as ranging from 50–70%. The mass fraction of SEFA was defined as ranging from 5–15%. The mass ratio of sludge to bottom ash was set at 5:3. According to the initial design, a total of 13 sets of test runs were planned for the experiment. Subsequently, the analyzed data will be further processed and imported into the software. By plotting a contour plot on a plane with independent variables as axes, connecting points with the same function value will yield contour lines. These contour lines are used to illustrate the relationship between the response and independent variables, creating a contour plot of a three-dimensional surface. This contour plot will facilitate the analysis of the optimal operating conditions for this research.

The total mass of the raw materials was 40 g, and the formulas for 13 test runs are shown in Table 1. The specimens were held in graphite crucibles and heated using an electrical heating furnace. The operating voltage was 220 V, and the maximum operating temperature was 1500 ℃. The heating programs were as follows: room temperature to 800 ℃ at 10 ℃/min, 800 ℃ to 1200 ℃ at 6 ℃/min, 1200 ℃ to 1450 ℃ at 4 ℃/min, and held at 1450 ℃ for 1 hr. The melted specimens were then cooled without any forced ventilation to room temperature and formed slags. The slag was labeled with an S-number according to the experimental batch number it belonged to.

Table 1

Formula of raw materials for melting
Run	Encapsulating phase	Encapsulated phase			Total mass
Run	Silica gel	SEFA	Sludge	Bottom ash	Total mass
R1	24	1.6	9	5.4	40
R2	28	0.6	7.13	4.27	40
R3	24	1.6	9	5.4	40
R4	20	2	11.25	6.75	40
R5	20	3	10.63	6.37	40
R6	24	0.8	9.5	5.7	40
R7	28	1.8	6.375	3.825	40
R8	24	2.4	8.5	5.1	40
R9	20	1	11.88	7.12	40
R10	24	1.6	9	5.4	40
R11	28	1.2	6.75	4.05	40
R12	24	1.6	9	5.4	40
R13	24	1.6	9	5.4	40
Unit: g

The slags were pulverized and analyzed for their physical and chemical properties to evaluate the effect of the thermal melting process. Previous studies have shown that the recovered materials (fly ash or slag) can be transformed into permeable bricks through compression and sintering [10, 11](Zhu et al., 2017; Li et al., 2021).

The slag powder was then mixed with cullet, clay, cement, and water according to the formulas presented in Table 2. Two rounds of permeable brick making tests were conducted. Each permeable brick was identified with a PB-number corresponding to the experimental batch number it was part of. The compressive strength of the permeable bricks was measured at various curing times (ranging from 7 to 28 days) to assess their physical properties. A previous study indicated that the curing process enhanced the physical strength [14](Haffar et al., 2021).

Table 2

Formula of raw materials of bricks (in%)
Run	Slag	Glassy powdery	Clay	Water	Cement
1	13.25	13.25	27.5	30	15
2	27.5	0	27.5	30	15

2.3 Analysis of element composition and metal leaching behavior

To analyze the metal composition of solid specimens, a procedure modified from the standard method outlined in NIEA R317.11C was conducted [15] (TME, 2014). The solid specimens were pulverized and ground to a particle size smaller than 150 mm (100 mesh) to ensure uniformity. The ground samples (approximately 0.1 g) were mixed with a concentrated acid mixture: 3 mL HCl (32 wt%), 5 mL HNO₃ (67 wt%), and 0.5% HF (32%), and then placed in sealed Teflon tubes. These tubes were heated using a microwave digester (MARS Xpress, CEM) following a heating program: from room temperature to 180°C at a rate of 5°C min^-1, from 180°C to 210°C at a rate of 3°C min^-1, held isothermally for 15 minutes, and cooled down to room temperature with forced ventilation. The digested solutions were diluted with deionized water to 50 mL and filtered using mixed cellulose ester filters to remove suspended solids for further analysis. The toxicity characteristic leaching procedure (TCLP) was employed to simulate the leaching behavior of heavy metals from the slag in an acidic environment (pH = 5). The procedures for preparing the leaching solution, determining the leaching parameters, diluting the solution, digesting the sample, and filtering the mixture all adhered to the standard method outlined in the NIEA R201.14C [16] (TME, 2009). The concentrations of Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mn, Mg, Na, Ni, Pb, Si, and Zn in the digested solutions, elutriating wastewater, and TCLP extracts were measured using atomic absorption spectroscopy (AAS, Agilent Technologies 55AA).

2.4 Evaluation of physical property

The solid specimens were pulverized and ground to a particle size smaller than 150 µm (100 mesh) to ensure uniformity. The crystalline phases in the slags were determined using X-ray diffraction (XRD) analysis. The analysis was conducted using a powder diffractometer (Geigerflex 3063) with Ni-filtered Cu Kα radiation on powders, with a particle size smaller than 20 µm, at a rate of 4° min^-1, in the 2θ = 10–60° range. Crystalline phases were identified by comparing intensities and positions of Bragg peaks with those listed in the Joint Committee on Powder Diffraction Standards (JCPDS) data files. Specimens pulverized to < 74 µm particles were affixed to a metallic plate, coated with a gold film, and examined qualitatively for the microstructure of the slags using scanning electron microscopy (SEM, Jeol JXA-840).

The compressive strength of permeable bricks was measured following the standard method outlined in CNS 8905 [17] (CNS, 2018). The permeable bricks were made in the size of 5 cm × 5 cm × 5 cm. Without affecting the specimen, the load was increased at a rate of 0.2–0.3 N mm^-² s^-2 until reaching the load at which the specimen ruptured. Then, the compressive strength can be calculated by dividing the load at rupture (P) by the cross-sectional area of the specimen.

3.1 Metal migration of fly ashes during the washing processes

Table 3 presents the metal composition of the RFA and the fly ashes after the first and second washing processes. In RFA, the major species were Na (133,000 mg kg^− 1) and Cl (112,000 mg kg^− 1), followed by Fe (25,400 mg kg^− 1) and Si (22,400 mg kg^− 1). The levels of other metals were all below 10,000 mg kg^− 1. After the first and second washing processes, the remaining mass percentages of FEFA and SEFA decreased to 80.7% and 72.1%, respectively. During the washing processes, some substances were washed out, leading to a significant decrease in the content of most elements. The most significant reductions were observed in Na and Cl levels. The levels of Na and Cl in fly ash decreased from 133,000 mg kg^− 1 and 112,000 mg kg^− 1 to 4,190 mg kg^− 1 and 8,530 mg kg^− 1, respectively.

Table 3

Element composition of raw ash and two elutriated ashes
Item	RFA		FEFA		SEFA
Item	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)
Al	1,280	57.1	1,070	45.4	570	41.1
Ca	6,900	67.03	3,350	16.8	1,670	8.5
Cd	38.1	2.4	39.3	15.0	37.0	3.4
Co	57.2	4.4	34.4	27.7	15.5	2.25
Cr	4,759	18.5	5,180	51.1	3,510	27.0
Cu	1250	12.3	888	12.2	657	3.5
Fe	25,400	21.8	20,800	17.6	24,900	33.3
K	2,110	57.0	495	31.1	125	12.8
Mn	270	7.9	155	27.6	87.2	3.97
Mg	1,850	4.5	1,170	43.9	338	12.4
Na	133,000	1.3	47,400	48.7	4,190	5.3
Ni	1,620	6.3	840	39.5	305	34.8
Pb	3,150	23.4	2,123	3.87	2,150	6.4
Si	22,400	21.1	17500	14.2	14,500	15.3
Zn	1,770	2.2	1,140	40.9	537	4.7
Cl	112,000	19.4	36,500	13.5	8,530	16.2
RMP (%)	100	--	80.7	12.3	72.1	14.5
RMP (Remained mass percentage, %) = remained mass ¸ RFA × 100%

Table 4 displays the mass fraction of elements between ash and wastewater during the two washing processes. The ratio of output mass to input mass (O/I ratio) ranged from 0.5 to 1.5. The differences between output mass and input mass could be attributed to sampling and analysis errors. The sampling error can be explained by the extreme inhomogeneity of fly ash, which is an aggregation of residue. However, the total output mass balance can serve as a check for the precision of the analysis. The mass balance of the metal species exceeded 200% or more. The analysis of samples and calculation of output mass balance must be verified to ensure accuracy in determining the metal fates. Among these elements, those with a total washing percentage above 70% were Ca (78.2%), Co (75.6%), K (94.7%), Mg (83.6%), Na (97.2%), Ni (81.6%), Zn (72.7%), and Cl (93.1%). These elements primarily remained in the washing wastewater, contributing significantly to the reduction in the mass of fly ash.

Table 4

Washing behavior of the two washing processes
Item	First washing process			Secondary washing process			Total washing percentage (%)
Item	FEFA	wastewater	O/I ratio	SEFA	wastewater	O/I ratio
Al	61.8	38.2	1.08	71.0	29.0	0.84	59.9
Ca	27.6	72.4	1.41	45.0	55.0	1.25	78.2
Cd	96.1	3.9	0.85	95.0	5.0	1.11	8.6
Co	40.2	59.8	1.19	64.9	35.1	0.76	75.6
Cr	77.1	22.9	1.13	90.3	9.7	0.84	33.6
Cu	51.1	48.9	1.11	60.9	39.1	1.37	52.7
Fe	83.3	16.7	0.79	94.0	6.0	1.44	11.8
K	14.2	85.8	1.33	24.1	75.9	1.18	94.7
Mn	36.6	63.4	1.26	49.5	50.5	1.28	70.9
Mg	35.3	64.7	1.43	51.2	48.8	0.63	83.6
Na	23.0	77.0	1.24	12.5	87.5	0.80	97.2
Ni	29.3	70.7	1.42	2.4	99.9	0.58	81.2
Pb	80.7	19.3	0.67	95.9	4.1	1.19	38.6
Si	46.8	53.2	1.33	83.4	16.6	1.12	41.7
Zn	47.9	52.1	1.07	40.0	60.0	1.32	72.7
Cl	21.2	78.8	1.23	30.5	69.5	0.86	93.1

Figures 2 and 3 show the XRD analysis results and SEM images of the fly ash and elutriated ashes. In Fig. 2a, NaCl was the main crystalline phase, and its intensity was much higher than that of CaCO₃. After the first washing process, the intensity of NaCl was greatly reduced, and the baseline intensity was significantly elevated. After the second washing process, the NaCl crystals were no longer visible in the XRD diagram. The XRD analysis results indicated that a substantial amount of NaCl was leached from the fly ash during the elutriation process, consistent with the findings in Table 3.

In Fig. 3a, numerous porous crystals were observed on the surface of RFA. According to our previous study, the porous crystals were confirmed to be NaCl [8](Kuo et al., 2019). As the water washing process continued (in Fig. 3b and 3c), the porous crystals gradually decreased, and a block-like structure emerged. Both the XRD analysis results and the SEM images aligned with the analysis results of element contents (in Table 3).

3.2 Evaluation of melting effect

Table 5 shows the metal composition of input materials for the thermal melting process. The SEFA contained Fe (24,900 mg kg^− 1), Si (14,500 mg kg^− 1), Cl (8,530 mg kg^− 1), Na (4,190 mg kg^− 1), and other trace elements. Silica gel primarily consisted of Si (286,000 mg kg^− 1) with trace amounts of other elements. The sludge was predominantly composed of Fe (41,500 mg kg^− 1), Na (39,100 mg kg^− 1), Ca (26,100 mg kg^− 1), Cu (24,100 mg kg^− 1), and Si (15,200 mg kg^− 1). The bottom ash contained mainly Fe (70,800 mg kg^− 1), Si (37,800 mg kg^− 1), Al (31,300 mg kg^− 1), Na (22,500 mg kg^− 1), and Ca (10,500 mg kg^− 1).

Table 5

Element content of input-material of thermal melting process
Item	SEFA		Silica gel		Sludge		Bottom ash
Item	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)
Al	570	41.1	N.D.	N.A.	8,660	2.83	31,300	9.2
Ca	1,670	8.5	146	25.1	26,100	5.91	10,500	27.9
Cd	37.0	3.4	N.D.	N.A.	354	3.33	0.39	49.5
Co	15.5	2.25	N.D.	N.A.	700	4.7	518	3.65
Cr	3,510	27.0	13.3	69.6	8,220	5.00	2,800	8.43
Cu	657	3.5	N.D.	N.A.	24,100	2.86	7,080	7.51
Fe	24,900	33.3	352	15.9	41,500	10.03	70,800	7.18
K	125	12.8	16.8	14.0	6,380	7.42	2,690	8.74
Mn	87.2	3.97	N.D.	N.A.	1,960	4.28	2,190	26.42
Mg	338	12.4	1,510	8.12	956	11.62	8,973	18.9
Na	4,190	5.3	1,890	7.2	39,100	7.96	22,500	20.3
Ni	305	34.8	N.D.	N.A.	5,230	4.55	2,015	5.3
Pb	2,150	6.4	N.D.	N.A.	3,190	3.8	1,492	5.61
Si	14,500	15.3	286,000	21.8	15,200	31.1	37,800	28.4
Zn	537	4.7	49	17.8	7,260	3.76	2,477	5.27
Cl	8,530	16.2	790	15.4	9,350	28.5	1,420	25.4

Figure 4 illustrates the mass percentage of unmelted materials with various mixing formulas. The highest proportion of unmelted slag, at 1.82%, was observed when the SEFA content in the encapsulated material was higher and the silicone gel content was lower. In other test processes, the proportion of unmelted materials ranged from 0.21–0.87%. Conversely, an increase in the proportion of silicone gel led to a decrease in the amount of unmelted materials during the melting process.

Figure 5 illustrates the percentage of mass reduction with different mixing formulas during the melting process. The experimental results indicated that as the proportion of SEFA and silica gel additives increased, the mass reduction percentage also increased. The intricate composition of SEFA was the primary reason for this, as when heated to 1,450°C during the melting process, volatile elements (mainly chloride) evaporated into the flue gas. Moreover, a higher proportion of silica gel additives enhanced the treatment effect on the melted materials. However, adding more silica gel did not necessarily lead to better effects. Overall, the experimental results of the RSM indicated that the most optimal effect was achieved when silica gel constituted 60% of the total sample and SEFA constituted 10% of the encapsulated phase by mass, in terms of minimizing the amount of unmelted materials and maximizing the percentage of mass reduction.

Table 6 presents the slag compositions under three different conditions. The elemental compositions of these slags were generally similar, except for silicon. In the case of S-5, incorporating the maximum amount of SEFA and the minimum quantity of silica gel resulted in a decrease in silicon content (42,700 mg kg^− 1). Consequently, the mass percentage of unformed material was relatively high. Conversely, adding the maximum amount of SEFA and silica gel in Run-7 led to a significant increase in silicon content and a decrease in the mass percentage of S-7. The reduction in mass percentage was due to the volatilization of SEFA, while the increase in silica gel amount reduced the basicity (mass ratio of CaO to SiO₂), leading to a more complete slag formation. In comparison to S-7, S-3 had a lower mass fraction of encapsulating phase and a relatively low mass percentage of unformed material. Therefore, S-3 was considered to be the slag with the most optimal thermal melting effect.

Table 6

Element composition of slags
Item	S-3		S-5		S-7
Item	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)
Al	7,640	5.84	8,850	18.9	5,430	8.03
Ca	18,800	2.46	16,700	7.73	12,600	11.4
Cd	N.D	N.A.	N.D	N.A.	N.D	N.A.
Co	280	5.75	420	4.79	200	7.05
Cr	2,450	6.04	4,430	3.11	1,630	7.74
Cu	8,620	2.06	11,800	5.52	5,970	5.27
Fe	18,800	6.89	26,300	6.18	16,500	5.49
K	1,950	4.4	2,530	9.8	1,490	1.41
Mg	4,770	4.18	2,930	1.75	4,020	13.3
Mn	820	4.37	910	7.25	540	7.31
Na	12,400	15.2	19,200	10.3	16,700	8.71
Ni	1,950	2.37	2,700	2.81	1,600	5.79
Pb	N.D	N.A.	N.D	N.A.	N.D	N.A.
Si	84,700	8.95	42,700	12.1	108,000	10.8
Zn	45	2.52	40	2.2	20	7.41
Cl	490	-	430	-	525

3.3 Transformation of slags into permeable bricks

Due to the optimal effect of thermal melting, S-3 was chosen as the raw material for permeable bricks. Table 7 displays the elemental composition of the raw materials used for making permeable bricks. The cullet primarily consisted of Si (376,000 mg kg^− 1), with additional amounts of Al (54,100 mg kg^− 1) and Ca (29,000 mg kg^− 1). The clay was predominantly composed of Si (185,000 mg kg^− 1), Al (92,600 mg kg^− 1), and Fe (17,800 mg kg^− 1). The cement's elemental composition was mainly Ca (282,000 mg kg^− 1) and Si (254,000 mg kg^− 1). The raw materials from two different formulas are mixed with water, shaped, cured, and then transformed into permeable bricks.

Table 7

Element content of raw materials of bricks
Item	Cullet		Clay		Cement		S-3
Item	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)	Average (mg kg^− 1)	RSD(%)
Al	54,100	3.7	92,600	22.8	7,460	23.1	7,640	5.84
Ca	29,000	4.7	1,680	7.97	282,000	16.6	18,800	2.46
Cd	N.D.	N.A.	8.5	16.3	N.D.	N.A.	N.D	N.A.
Co	6.35	37.1	17.6	12.9	11.2	13.9	280	5.75
Cr	19.4	43.9	320	21.4	75	12.4	2,450	6.04
Cu	66.6	12.6	3,910	8.63	3.88	24.7	8,620	2.06
Fe	590	29.3	17,800	10.7	19,100	5.89	18,800	6.89
K	80.2	8.61	4.5	74.2	1,800	8.4	1,950	4.4
Mn	N.D.	N.A.	560	17.1	350	2.96	4,770	4.18
Mg	6,310	7.16	495	56.2	29,800	15.5	820	4.37
Na	2,170	4.01	1,210	13.3	3,540	3.7	12,400	15.2
Ni	N.D.	N.A.	390	15.1	34	2.83	1,950	2.37
Pb	N.D.	N.A.	335	7.73	61.6	11.7	N.D	-
Si	376,000	36.4	185,000	25.8	254,000	32.1	84,700	-
Zn	95.6	29.3	3,530	2.34	56.6	15.6	45	2.52
Cl	824	19.6	251	31.5	15.8	38.5	490	-

Figure 6 illustrates the compressive strength of permeable bricks at various curing days. The compressive strength of permeable bricks, PB-1 and PB-2, increased gradually during the curing period. PB-2 exhibited slightly higher strength than PB-1, approximately 30% more. The physical strength of PB-2 met the grade-A brick criteria (CNS, 2018). PB-1 met the grade-B brick standard on the 7th curing day, and by the 21st day, it met the grade-A brick regulation. Comparing PB-1 and PB-2, substituting glassy powdery for slag did not enhance the physical strength of permeable brick. This could be attributed to slag having superior physical properties compared to glassy powdery and being more suitable for blending with clay and cement.

Table 8 displays the TCLP results of the permeable bricks. The metal leaching concentrations of both PB-1 and PB-2 were lower than the regulated standard for hazardous industrial waste by at least one order [18](TME, 2020). The results indicate that metals were effectively encapsulated in the permeable bricks, preventing leaching. Therefore, the TCLP results suggest that recycling the permeable bricks will not lead to environmental pollution.

Table 8

TCLP results of permeable bricks
Item	PB-1	PB-2	Regulated standard
Cd	N.D.	N.D.	1.0
Cr	0.314	0.373	5.0
Cu	0.047	0.043	15.0
Pb	0.376	0.394	5.0

3.4 Crystalline characteristics and surface structure of slags and bricks

Figure 7 shows the SEM images of slags. In Fig. 7b, there were numerous fine powders on the surface of S-5, and the structure was porous. This is due to the excessive addition of SEFA, resulting in a high Cl content. The vaporization of Cl caused the porous structure and retarded the mixture from being well vitrified. For S-3 (in Fig. 7a), the small powders disappeared, and a glassy and amorphous structure was found. This can be explained by the increased addition of silica gel and decreased addition of SEFA promoting the vitrification effect. In Fig. 7(c), the structure (S-7) was similar to that of S-3. There were some fine powdery grains on the amorphous structure. Compared to the input materials in Run-3, those in Run-7 had a higher amount of silica gel and SEFA added. Due to the partial volatilization of silica gel and SEFA, R7 has the highest mass reduction among all slags.

Figure 8 shows the XRD analysis of slags. The main crystalline phase in S-3, S-5, and S-7 was all SiO₂, which was contributed by the melting and reformation of silica gel. Additionally, Fe₂O₃ was also found in the three slags. In Fig. 8b, the baseline intensity of S-5 was relatively higher than that of S-3 and S-7, implying that the quantity of SiO₂ in S-5 was less than that in S-3 and S-7. This resulted in the glassy matrix of the slag not being well-formed, leading to an increase in unmelted materials, which supported the results of RSM.

The primary composition of RFA was sodium (133,000 mg kg^-1) and Cl (112,000 mg kg^-1), which were mostly removed during the two-stage elutriation process, resulting in reduced levels of sodium (4,190 mg kg^-1) and chlorine (8,530 mg kg^-1). By mixing with silica gel, sludge, and bottom ash using various formulas, the mixture was melted at 1450 ℃. The findings indicate that the amount of unmelted slag peaked at 1.82% when the proportion of SEFA in the encapsulated material was higher and the amount of silica gel was lower. Moreover, the presence of silica gel decreased the quantity of unmelted materials during the melting process. The inclusion of SEFA and silica gel increased the percentage of mass reduction. While silica gel enhanced the melting process, increasing its amount did not always result in better outcomes. According to the RSM experimental data, the most optimal results were achieved when silica gel made up 60% of the total sample and SEFA accounted for 10% of the encapsulated phase mass. The S-3 with the best melting effect was transformed into permeable bricks. The physical strength and metal leaching concentrations of these bricks met grade-A standards and were at least one order of magnitude lower than the regulated limit for hazardous industrial waste. These results demonstrate that the permeable bricks possess satisfactory physical strength and do not pose a risk of environmental pollution when recycled.

Availability of data and materials

All data generated or analyzed during this study are contained within the article.

Competing interests

The authors declare they have no competing interests.

Funding

This work was supported by the National Science and Technology Council of Taiwan (grant no. MOST 105-2221-E-273-002 and MOST 111-2221-E-273-003).

Authors' contributions

HHW and KLH conceptualized this study; LCC and YMK conducted investigation; LCC, and SWH performed formal analysis; YMK undertook visualization and methodology, and wrote the first draft; HHW and KLH reviewed and edited the article. All authors have read and agreed to the published version of the manuscript.

Acknowledgements

The authors wish to thank all colleagues and students who contributed to this study. It would not have been possible without their help. We thank the editor for constructive criticisms of an earlier version of this paper. We would also like to thank the National Science and Technology Council of Taiwan for funding this study.

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Transformation of hazardous residues from medical laboratory waste treatment plant into permeable brick

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Version 1

Abstract

Figures

1 Introduction

2 Experimental sections

2.1 Preparation of raw materials

2.2 Procedure of the elutriation, melting, and permeable brick making processes

2.3 Analysis of element composition and metal leaching behavior

2.4 Evaluation of physical property

3 Results and discussions

3.1 Metal migration of fly ashes during the washing processes

3.2 Evaluation of melting effect

3.3 Transformation of slags into permeable bricks

3.4 Crystalline characteristics and surface structure of slags and bricks

4 Conclusions

Declarations

References

Status:

Version 1