Study on the Effect of Mineral Compounds on the Behavior of Heavy Metals During Oily Sludge Incineration

Abstract

Incineration is a highly effective method for treating oily sludge. However, during the incineration process, heavy metals may either be released into the air via flue gas or remain in the bottom ash in an unstable form, posing significant environmental threats. Mineral compounds can provide adsorption sites for heavy metals and promote the stable existence of heavy metals in incineration bottom ash. In this paper, the incineration experiments of oily sludge with CaO, Fe₂O₃, Al₂O₃ and MgO were conducted using a horizontal tube furnace. The total amount, leaching characteristics, and morphological distribution of heavy metals in the obtained incineration bottom ash were analyzed. The results showed that CaO had a significant adsorption effect on Cu, Cr, Pb, As and Cd. Al₂O₃ exhibited the best leaching inhibition effect on Cr, Zn, Pb, As, and Cd. The influence of mineral compounds on the morphological distribution of heavy metals during incineration was highly dependent on the type of metal. This work will provide crucial theoretical support for the source control of heavy metals during oily sludge incineration and hold important practical significance for achieving the harmless treatment of oily sludge and promoting the development of oily sludge incineration technology.

1. Introduction

Oil is a vital pillar of the national economy and an indispensable strategic resource for the survival and development of a country. In the processes of oil exploration, transportation, and refining, large amounts of oily sludge are generated [1]. In China, the annual production of oily sludge exceeds 6 million tons, resulting in a historical accumulation of approximately 143 million tons. Oily sludge contains harmful substances such as benzene compounds, phenols, and heavy metals and is classified as hazardous waste under Chinese law. If not treated properly and in a timely manner, it can cause severe pollution to soil, water, and the atmosphere, posing significant risks to human health [2,3].

Incineration technology cannot only carbonize all harmful substances but also significantly reduces the volume of oily sludge and shortens its treatment time [4,5,6]. In addition, the heat generated from incineration can be harnessed for power generation, making it the fastest and most effective method to achieve the reduction, harmlessness and recycling of oily sludge. Due to the high content of heavy metals in oily sludge, during the incineration process, heavy metals are released into the air along with fly ash, or will exist in the incineration bottom ash in unstable form, and eventually be converted into more toxic compounds, which contain lead, cadmium, and zinc [1]. If not properly managed, these heavy metals will easily precipitate to the surface and infiltrate into groundwater, posing a great threat to the environment, which seriously restricts the resource utilization of oily sludge. Therefore, there is an urgent need for effective methods to enhance the application and development of incineration technology.

By adding mineral compounds (lime, kaolin, iron oxides, etc.) to the combustion process of solid fuels, the release of heavy metals can be reduced, and the capture and solidification of heavy metals can be achieved [7,8,9,10]. This method has been proven effective for managing heavy metals during the incineration of solid fuels. Specifically, these mineral compounds function as adsorbents, which have high activity at high temperatures, enabling them to physically adsorb heavy metal vapors on their surfaces or within their pores [11]. Thus, heavy metal vapor is captured before forming particles, which reduces the formation of heavy metal particles in fly ash and promotes the residue of heavy metals in incineration bottom ash. Moreover, these minerals possess highly chemically active sites that can react with heavy metals in the incineration flue gas or on particles to form stable compounds, thus achieving capture [12]. For instance, Zn reacts with SiO₂ and Al₂O₃ to produce Zn₂SiO₄ and ZnAl₂O₄ with a high melting point. This dual mechanism of physical adsorption and chemical reaction helps to immobilize heavy metals, reducing their environmental mobility and toxicity [13].

Researchers both domestically and internationally have conducted extensive studies on the influence of mineral compounds on the migration and transformation of heavy metals during the combustion of solid fuels. Maierdan [14] incorporated river sludge (CRS) into the production of unfired green bricks and found that the hemihydrate phosphogypsum exhibited an excellent fixing capacity for heavy metals. Zhang [15] investigated the effect of CaO and montmorillonite on the behavior of heavy metals during sludge combustion. The results showed that CaO inhibited the volatilization of Cr, Zn, and Cu but promoted As volatilization. Ke [16] studied the effect of calcium-based sorbents on heavy metals during the oxy-fuel combustion of food waste. The results showed that nature and modified CaO had excellent performance for Al capture, while CaCO₃ could not absorb Al. Yu [17] evaluated the risk during the combustion of oily sludge in industrial scale. Qin [5] pointed out that there may be synergistic effects between different heavy metals during the incineration process, and the mechanisms of their mutual interactions are still not well understood. Currently, the majority of research on the migration characteristics of heavy metals during the incineration process focuses on urban sludge or biochar, with relatively few studies dedicated to oily sludge. This research gap is significant because oily sludge, often containing high concentrations of heavy metals, poses unique challenges and environmental risks during thermal treatment processes.

In this work, CaO, Fe₂O₃, Al₂O₃ and MgO were introduced into the incineration process of oily sludge. The total amount, leaching characteristics, and morphological distribution of heavy metals in the incineration bottom ash were analyzed by microwave digestion, the acetic acid buffer solution method, and the Bureau Community of Reference (BCR) continuous extraction. This research provides the theoretical support for the source control of heavy metals during the incineration of oily sludge and contributes to the advancement of oily sludge incineration technology.

2. Materials and Methods

2.1. Sample Collection and Analysis

The oily sludge samples utilized in this experiment were sourced from the tank bottom of Shengli Oilfield. The samples underwent initial drying by being placed in an oven set at 105 °C for a duration of 24 h. Subsequently, the dried samples were ground and passed through an 80-mesh sieve to obtain a uniform particle size of less than 0.2 mm. The results of the proximate and ultimate analyses of these prepared samples are presented in

Table 1. Proximate and ultimate analysis of oily sludge.

Sample	Proximate Analysis (wt %)				HHV a (MJ·kg−1)	Ultimate Analysis (wt %)
	M a	A a	V a	FC a,b		C	H	Ob	N	S
Oily sludge	0	42.82	51.74	5.44	22.58	44.24	5.67	4.60	0.42	2.25

M, A, V, FC, and HHV refer to moisture, ash, volatile, fixed carbon and high heating value, respectively. ^a as air-dried basis. ^b FC and O, calculated by difference.

.

The experimental reagents and equipment used in this work are presented in Tables S1 and S2. CaO, Fe₂O₃, Al₂O₃, and MgO used in the experiment were produced by the Chemical Reagent Co., Ltd. of Guoyao Group (Shanghai, China), with a particle size of 0.1~1 μm.

2.2. Experimental and Analytical Methods

The incineration experiments of oily sludge with and without mineral compounds were carried out, respectively, in a horizontal tube furnace. The horizontal tube furnace has been described in detail in the literature [18].

The experimental operation steps are as follows.

Step A: At the beginning of the incineration process, adjust the temperature of the furnace to the set value (900 °C in this case) at a heating rate of 20 °C/min. When the furnace temperature approaches the set value, open the air valve, and regulate the airflow to 0.2 m³/h.

Step B: Once the temperature is stable at the set value and the furnace is fully ventilated with air, quickly push the porcelain boat containing the samples into the heating area using the push rod.

Step C: Allow the samples to incinerate for 40 min. After 40 min, push the porcelain boat to the cooling area and turn off the power to the heating furnace. Label the collected oily sludge incineration bottom ash as OSA, and store it in a dry container.

CaO, Fe₂O₃, Al₂O₃, and MgO with a mass ratio of 4% to oily sludge were added, respectively, after the oily sludge was dried, and, then, incineration experiments were carried out according to the above experimental steps. The incineration bottom ash was labeled as OSA-CaO, OSA-Fe₂O₃, OSA-Al₂O₃, and OSA-MgO, respectively.

The total amount, leaching characteristics, and the morphological distribution of heavy metals in the incineration bottom ash were analyzed using microwave digestion, the toxicity characteristic leaching procedure (TCLP), and the Bureau Community of Reference (BCR) continuous extraction, respectively. The detailed procedures for these analyses are provided in attached Step S1, Step S2, and Step S3, respectively. To ensure reproducibility, each measurement of heavy metal content in different samples was performed in triplicate, and the results are presented as the average of these three measurements.

3. Results and Discussions

3.1. Effect of Mineral Compounds on the Total Amount of Heavy Metals

The concentrations of heavy metals in different samples are shown in

Table 2. Heavy metal concentrations in different incineration bottom ashes.

Samples	Weight (g)	Cr	Cd	Cu	Zn	As	Pb
		mg/kg
Oily sludge	100	92.25	24.24	65.64	725.14	84.24	128.25
OSA	42.54	173.62	8.25	113.55	488.11	16.43	65.12
OSA-CaO	41.62	186.22	15.37	145.24	451.78	60.11	202.14
OSA-Fe2O3	40.08	173.21	13.73	134.37	434.98	57.21	63.32
OSA-Al2O3	43.25	168.37	11.12	140.14	496.32	75.37	198.32
OSA-MgO	43.12	170.26	12.37	115.21	505.31	16.21	64.32
Limited value a	—	1000	20	1500	4000	75	300

^a as types of agricultural land allowed to be used for class B sludge include garden land, pasture land, and cultivated land without edible crops (data from GB 4284-2018 Control Standard for Agricultural Sludge Contamination [19]).

. In order to more accurately represent the residue of heavy metals after oily sludge incineration, the concept of residual rate R is introduced, as shown in Formula (1), which represents the percentage share of the absolute content of heavy metals in oily sludge transferred to the incineration bottom ash. The results are shown in Figure 1.

$$R={\displaystyle \frac{C_2\times M_2}{C_1\times M_1}}\times 100\%$$

(1)

where C₁ is the concentration of heavy metals in oily sludge (mg/kg). C₂ is the concentration of heavy metals in incineration bottom ash (mg/kg). M₁ is the weight of oily sludge (g). M₂ is the weight of incineration bottom ash (g).

It can be seen that the concentration of Cu in OSA increased significantly, indicating that CaO, Fe₂O₃, Al₂O₃, and MgO all promote the retention of Cu in the incineration bottom ash. Compared with OSA, the residual rate of Cu in OSA-Fe₂O₃ increased from 73.02% to 86.41%. This is because when the incineration temperature is low, Cu mainly exists in the form of solid CuO. As the temperature rises, Cu reacts with Fe₂O₃ to generate CuO-Fe₂O₃, indicating that Fe₂O₃ promotes the residue of Cu in the incineration bottom ash. At the same time, some scholars found that CuO-Fe₂O₃ is relatively stable, and only when the temperature is higher than 1300 °C can it be decomposed. Therefore, Cu can stably exist in the incineration bottom ash at 800 °C. The residual rate of Cu in OSA-Al₂O₃ is increased to 90.12%. This is attributed to the reaction between CuO in oily sludge and Al₂O₃, forming aluminate CuO-Al₂O₃ at high temperature. CuO-Al₂O₃ has a high melting and boiling point and is not easy to volatilize, making it easier for Cu to remain in the bottom ash. Additionally, Al₂O₃ can form a eutectic mixture with adsorbed Cu during melting, encapsulating the heavy metals and preventing their escape. Therefore, Al₂O₃ can effectively reduce the volatilization of Cu in flue gas. In addition, it can be found that the promotion effect of different mineral compounds on Cu residue in the bottom ash is in the order of CaO > Al₂O₃ > Fe₂O₃ > MgO.

The residual rate of Cr in OSA reached 79.44%, which is due to the low volatility and high boiling point of Cr, so it is more likely to remain in the bottom ash. At the same time, it was found that the residual rate of Cr in OSA-CaO reached 85.21%, while the residual rate of Cr in OSA-Fe₂O₃, OSA-Al₂O₃, and OSA-MgO had little change. The order of promoting effects of different mineral compounds on Cr residue in bottom ash was CaO > Fe₂O₃ > MgO > Al₂O₃.

The concentration of Zn in OSA significantly decreased, and the residue rate is only 28.41%. It can be seen that most of Zn is volatilized into the flue gas, because the boiling point of oxides and sulfides of Zn is low, which is a volatile element. The residual rate of Zn in OSA-Al₂O₃ was 28.89%, which is higher. This is because ZnO reacts with SiO₂ and Al₂O₃ in oily sludge during incineration to generate stable Zn₂SiO₄ and ZnAl₂O₄, thus inhibiting the evaporation of Zn. The order of the promoting effect of different mineral compounds on Zn residue in the bottom ash is MgO > Al₂O₃ > CaO > Fe₂O₃.

The concentration of Pb in OSA is low, which was because Pb is a volatile element, which was more likely to volatilize into the flue gas during incineration. The residual rate of Pb in OSA-Al₂O₃ was 65.27%, which was significantly higher than that in OSA. This is because, after adding Al₂O₃ during the incineration of oily sludge, Pb and Si react with Al₂O₃ to generate PbO-Al₂O₃-2SiO₂, which promotes the enrichment of Pb into bottom ash [16]. It can be seen that the order of the effect of different mineral compounds on promoting Pb residue in the bottom ash is CaO > Al₂O₃ > MgO > Fe₂O₃.

The residual rate of As in OSA was 8.23%, indicating that most of As has volatilized into the flue gas. The residue rates of As in OSA-Fe₂O₃ and OSA-Al₂O₃ were 28.67% and 37.77%, respectively, which were higher than those in OSA. This is because As mainly exists in the form of gaseous As₂O₃ during incineration. However, when Fe₂O₃ or Al₂O₃ was added to oily sludge, gaseous As₂O₃ reacted with inorganic compounds to generate arsenate, which has a good ability to capture As and improves the residual rate of As in incineration bottom ash. The order of promoting effects of different mineral compounds on As residue in bottom ash is Al₂O₃ > CaO > Fe₂O₃ > MgO.

The residual rate of Cd in OSA was 21.61%. The boiling point of Cd, its oxides, and sulfides are low, belonging to high volatile elements. The residual rate of Cd in OSA-CaO increased significantly, reaching 50.02%, indicating that CaO has little adsorption effect on Cd. The order of the promoting effect of different mineral compounds on Cd residue in the bottom ash is CaO > Al₂O₃ > MgO > Fe₂O₃.

In general, the adsorption capacities of different mineral compounds for heavy metals vary significantly. Among them, CaO has a significant adsorption effect on Cu, Cr, Pb, As, Cd, while Fe₂O₃ has a great adsorption effect on Cu, As, and Cd but a weak adsorption effect on Cr, Zn, and Pb, while Al₂O₃ has a great adsorption effect on other heavy metals except Cr, especially on Pb, As, and other volatile heavy metals. MgO has a significant adsorption effect on Cd, while its adsorption capacity for other heavy metals is relatively moderate.

3.2. Effect of Mineral Compounds on the Leaching Characteristics of Heavy Metals

The leaching characteristics of heavy metals in the obtained incineration bottom ash were analyzed, with the leaching concentrations presented in

Table 3. Leaching concentrations of heavy metal in different incineration bottom ashes.

Samples	Cr	Cd	Cu	Zn	As	Pb
	mg/L
Oily sludge	19.46	6.99	19.93	111.11	11.29	10.93
OSA	13.27	2.03	16.17	35.19	0.53	3.66
OSA-CaO	10.77	0.29	8.11	26.27	1.41	4.72
OSA-Fe2O3	10.97	0.22	16.17	26.7	1.51	3.75
OSA-Al2O3	11.88	0.42	18.49	22.06	1.11	4.02
OSA-MgO	10.06	0.61	18.57	25.06	1.25	3.37

. To more accurately represent the leaching of heavy metals in the sample, the leaching rate L of heavy metals is introduced, as shown in Formula (2), which is defined as the percentage of the leaching concentration and the total concentration of heavy metals, and the change trend of the leaching rate of different heavy metals with the incineration temperature is shown in Figure 2.

$$L={\displaystyle \frac{LC}{TC}}\times 100\%$$

(2)

where LC is the leaching concentration of heavy metals in samples (mg/L). TC is the total concentration of heavy metals in samples (mg/L).

Compared with oily sludge, the leaching rate of Cu in OSA reduced from 60.73% to 28.47%, indicating that the incineration treatment can reduce the leaching toxicity of Cu and lessen the harm to the environment. Further research showed that the inhibition order of different mineral compounds on the leaching toxicity of Cu in incineration bottom ash was CaO > Fe₂O₃ > Al₂O₃ > MgO.

The leaching rate of Cr in OSA was reduced from 42.19% to 15.29%, indicating that incineration treatment can also reduce the leaching toxicity of Cr. It can be seen that the inhibition order of different mineral compounds on the leaching toxicity of Cr in incineration bottom ash was Al₂O₃ > MgO > Fe₂O₃ > CaO.

The leaching concentration of Zn in OSA was also reduced from 111.11 mg/L to 35.19 mg/L, and the leaching rate was reduced from 31.27% to 15.18%. The inhibition order of different mineral compounds on the leaching toxicity of Zn in incineration bottom ash was Al₂O₃ > MgO > Fe₂O₃ > CaO.

After incineration, the leaching concentration of Pb in OSA also decreased from 10.93 mg/L to 3.66 mg/L, and the leaching rate decreased from 18.53% to 12.34%. The inhibition order of different mineral compounds on the leaching toxicity of Pb in incineration bottom ash was Al₂O₃ > CaO > MgO > Fe₂O₃.

Compared with OS, the leaching concentration of As in OSA decreased from 11.29 mg/L to 0.53 mg/L, and the leaching rate decreased from 28.21% to 5.14%, indicating that incineration treatment can significantly reduce the leaching toxicity of As. The inhibition order of different mineral compounds on the leaching toxicity of As in incineration bottom ash was Al₂O₃ > Fe₂O₃ > CaO > MgO.

The leaching concentration of Cd in OSA decreased from 41.99 mg/L to 22.03 mg/L, indicating that incineration can reduce the leaching toxicity of Cr. At the same time, it was found that the inhibition order of different mineral compounds on the leaching toxicity of Cd in incineration bottom ash was Al₂O₃ > MgO > Fe₂O₃ > CaO.

In general, the leaching concentration of heavy metals decreased after incineration. Specifically, the leaching rate of Cu, Cr, Zn, Pb, As, and Cd in OSA were reduced by 32%, 27%, 16%, 6%, 23%, and 12%, respectively. Obviously, incineration treatment can significantly reduce the leaching toxicity of Cu and thus mitigate its environmental impact. The effect of mineral compounds on the toxic leaching of heavy metals is influenced by multiple factors, but it is primarily determined by the stability of the products formed through chemical reactions involving heavy metals during the incineration process. If the stability of these products is poor, they are easy to be leached out; otherwise, they can be stably retained in the incineration bottom ash.

It can be seen from Table 3 and Figure 2 that CaO exhibited significant inhibition of the leaching of Cu, Cr, Zn, and Pb but had a weaker inhibition effect on the leaching of Cd and As. Fe₂O₃ can inhibit the leaching of Cu, Cr, Zn, Pb, Cd, and As, especially the leaching inhibition effect of Cu and Zn. Al₂O₃ also demonstrated leaching inhibition for Cu, Cr, Zn, Pb, Cd, and As, with the most pronounced effects on Cu and Zn. MgO showed good inhibition of the leaching of Cu, Cr, Zn, Pb, and Cd but had a relatively weaker performance on As.

3.3. Effect of Mineral Compounds on Morphological Distribution of Heavy Metals

The speciation fractions of heavy metals include exchangeable fraction (F1), reducible fraction (F2), oxidizable fraction (F3), and residual fraction (F4), each exhibiting distinct characteristics.

• Exchangeable Fraction (F1): This fraction is adsorbed on solid surfaces or exists in carbonate forms. It is highly mobile, toxic, and poses significant risks to the ecological environment.
• Reducible Fraction (F2): This fraction is associated with iron and manganese oxides. It is less toxic than the exchangeable fraction but still poses notable risks.
• Oxidizable Fraction (F3): This fraction involves metal ions combined with organic matter and sulfides. It exhibits potential biological toxicity and bioavailability.
• Residual Fraction (F4): This fraction is stable under natural conditions and can remain in sediments for extended periods. It is chemically inert and has minimal biological toxicity or bioavailability.

The morphological distribution of heavy metals in different incineration bottom ashes was analyzed, and the concentrations of different heavy metals in incineration bottom ashes were obtained, as shown in Table S3. In order to verify the validity of the data, the recovery rate Rec is introduced. The sum of the four speciation fractions was compared with the total concentration of heavy metals in oily sludge or incineration bottom ash, and the recovery rate of each heavy metal was calculated, as shown in Formula (3).

$$Rec={\displaystyle \frac{F_1+F_2+F_3+F_4}{TC}}\times 100\%$$

(3)

where F₁ is concentration of heavy metals in exchangeable form (mg/L). F₂ is the concentration of heavy metals in reducible form (mg/L). F₃ is the concentration of heavy metals in oxidizable form (mg/L). F₄ is the concentration of heavy metals in residual form (mg/L).

It can be seen from Table S3 that the recovery rate of heavy metals was between 94.21% and 98.54%, indicating that the data of heavy metals obtained by BCR extraction method are effective and reliable. At the same time, the proportion of different forms of heavy metals was further analyzed, and the trends of the morphological distribution of heavy metals in different incineration bottom ashes were obtained, as shown in Figure 3.

As illustrated in Figure 3, Cu in both oily sludge and OSA mainly exists in the form of exchangeable form, accounting for 58% and 32%, respectively. This form has strong mobility and poses significant environmental risks. The addition of different mineral compounds influences the morphological distribution of Cu in various ways. In OSA-CaO, the oxidizable state and residual state of Cu accounted for a relatively high proportion, of which the oxidizable form accounted for 37% and the residual form accounted for 33%, indicating that CaO has an obvious solidification effect on Cu, which can reduce the mobility of Cu, and promoted the stable existence of Cu in the incineration bottom ash. The four forms of Cu in the OSA-Fe₂O₃ accounted for roughly the same proportion. The highest proportion of Cu in OSA-Al₂O₃ was in exchangeable form (40%), and the lowest proportion was in reducible form (11%). The exchangeable Cu accounted for the highest proportion in OSA-MgO (62%), which was higher than that of oily sludge and OSA. Overall, CaO and Fe₂O₃ had a very obvious effect on reducing the risk of Cu, while Al₂O₃ and MgO exhibited a poor effect.

Cr mainly exists in the form of exchangeable and reducible form in oily sludge, in which the exchangeable forms accounted for 40%, and the reducible form accounted for 31%. After incineration, it was found that Cr in OSA had changed from unstable form to stable form, of which the proportion of oxidizable Cr was 37% and the proportion of residual Cr was 32%. Different mineral compounds exhibited different effects on the morphological distribution of Cr. The oxidizable Cr accounted for the highest proportion of 61% in OSA-CaO, indicating that adding CaO reduced the mobility of Cr. Cr in OSA-Fe₂O₃ mainly existed in reducible form, accounting for 48%. The proportion of oxidizable Cr was relatively low, only 11%. Cr in OSA-Al₂O₃ mainly existed in the exchangeable form, accounting for 37%, while the reducible from accounted for only 14%. Cr mainly existed in residue form in OSA-MgO, accounting for 36%. The proportion of reducible form was relatively low, only 11%. It can be seen that mineral compounds used in this work showed a poor effect on reducing the risk of Cr in the incineration bottom ash.

Zn mainly existed in exchangeable and reducible form, accounting for 34% and 36%, respectively. Zn changed from unstable form to stable form after incineration, in which the residual Zn accounted for 38% in OSA. It was also found that different mineral compounds had different effects on the morphological distribution of Zn. The reducible and residual Zn accounted for 38% and 35% in OSA-CaO, respectively. It showed that CaO has a certain solidification effect on Zn and promoted the stable existence of Zn in OSA-CaO. The exchangeable Zn in OSA-Fe₂O₃ accounted for the highest proportion of 33%. While the oxidizable Zn accounted for a relatively low proportion of 17%, indicating that Fe₂O₃ has a poor curing effect on Zn. In OSA-Al₂O₃, the proportion of residual Zn was the highest, 39%, and the proportion of exchangeable was relatively low, only 5%, indicating that Al₂O₃ played an obvious role in Zn solidification. The reducible Zn accounts for the highest proportion of 36% in OSA-MgO. The proportion of the other three forms was relatively uniform, about 20%. In general, CaO and Al₂O₃ have a significant effect on reducing the risk of Zn, while Fe₂O₃ and MgO increased the risk of Zn in the bottom ash compared with OSA.

The proportions of exchangeable Pb, reducible Pb, and oxidable Pb in oily sludge were 28%, 29%, and 31%, respectively. Through incineration, Pb has obviously changed from unstable form to stable form, in which the proportion of oxidizable Pb was the highest, reaching 32%. At the same time, different mineral compounds exhibited different effects on the morphological distribution of Pb. The residual Pb accounted for the highest proportion of 35% in OSA-CaO. It showed that CaO promoted the stable existence of Pb during incineration. The exchangeable Pb in OSA-Fe₂O₃ accounted for the highest proportion, reaching 46%, while the oxidized Pb accounted for the lowest proportion of 14% in OSA-Fe₂O₃. In OSA-Al₂O₃, the exchangeable Pb accounted for the highest proportion, up to 48%, and the reducible Pb accounted for the lowest proportion, only 11%. The highest and lowest proportion of Pb were exchangeable and reducible in OSA-MgO, accounting for 48% and 15%, respectively. It can be seen that the effect of mineral compounds was poor on reducing the risk of Pb in incineration bottom ash.

The exchangeable As in OS accounts for the highest proportion, up to 35%, posing a great threat to the environment. Through incineration, the fixed form of As increased significantly, of which the proportion of oxidizable As was the highest, reaching 29%. The proportion of residual As was also high in OSA, reaching 28%, indicating that incineration improved the stability of As. At the same time, different mineral compounds showed different effects on the morphological distribution of As. In OSA-CaO, the proportion of As in oxidizable form and residue form was also high, 29% and 28%, respectively. It showed that CaO promoted the stable existence of As. The exchangeable form accounted for the highest proportion in OSA-Fe₂O₃, reaching 36%, while the reducible As accounted for 11%. The residual As in OSA-Al₂O₃ accounted for the highest proportion, up to 39%, while the reducible As accounted for 11%. The highest and lowest proportion of As were residual and reducible in OSA-MgO, accounting for 48% and 15%, respectively. It can be seen that CaO and Al₂O₃ have a significant effect on decreasing the risk of As, while Fe₂O₃ and MgO showed a poor effect.

The exchangeable Cd decreased from 36% in oily sludge to 25% in OSA after incineration, while the proportion of oxidizable Cd increased to 36% in OSA. At the same time, different mineral compounds showed different effects on the morphological distribution of Cd. The exchangeable Cd is relatively high in OSA-CaO, reaching 32%. It showed that CaO has little effect on the stabilization of Cd. The exchangeable Cd also accounted for a relatively high proportion, reaching 51%, while the reducible Cd accounted for the lowest proportion of 12% in OSA-Al₂O₃. The exchangeable Cd in OSA-MgO was as high as 57%, while the other three forms accounted for about 14%~15%. The proportion of exchangeable Cd in OSA-Fe₂O₃ was also high, reaching 48%. The proportion of reducible state was the lowest of 11%. It can be seen that mineral compounds exhibited a poor effect on reducing the risk of Cd during incineration.

Overall, incineration can effectively facilitate the transformation of heavy metals from unstable forms to more stable forms. The selected mineral compounds exhibited distinct effects on the morphological transformation of heavy metals during the incineration process. Specifically, CaO promotes the formation of oxidizable Cr and residual Cu. Fe₂O₃ enhances the formation of residual As and residual Cd. Al₂O₃ aids in the formation of oxidizable Zn and residual As. MgO supports the formation of residual Cr. The addition of these mineral compounds enhances the stability of certain heavy metals in incineration bottom ash, thereby reducing potential threats to the natural environment.

Indeed, besides focusing on the harmless disposal of solid products that has been studied in this work, the control of harmful gases during oily sludge incineration is also a crucial step. The effective management of gaseous emissions not only helps to mitigate environmental impacts but also ensures compliance with stringent regulatory standards, thereby safeguarding public health and ecological integrity.

4. Conclusions

This paper examines the impact of various mineral compounds on the migration and transformation characteristics of heavy metals. The results indicate that different minerals exert distinct effects on the behavior of heavy metals during incineration. Generally, the addition of mineral compounds during the incineration of oily sludge can enhance the retention of heavy metals in the incineration bottom ash and reduce their leaching toxicity.

CaO showed a significant adsorption effect on Cu, Cr, Pb, As and Cd. Fe₂O₃ had a great adsorption effect on Cu, As, and Cd but a weak adsorption effect on Cr, Zn, and Pb. Al₂O₃ exhibited a good adsorption effect on other heavy metals except Cr, especially on Pb, As, and other volatile heavy metals. MgO had a good adsorption effect on Cd, while it had an ordinary adsorption effect on other heavy metals.

Adding mineral compounds all showed inhibition effects on heavy metal leaching. Al₂O₃ exhibited the best performance on Cr, Zn, Pb, As, and Cd. CaO showed the best inhibition effect on Cu but a poor effect on the other heavy metals. The effect of inhibition of Al₂O₃ and MgO on the leaching behavior of different heavy metal showed no obvious regularity.

The added mineral compounds improve the stability of certain heavy metals in the incineration bottom ash to a certain extent, thus reducing threats to the natural environment. Mineral compounds showed different effects on the morphological transformation of heavy metals during incineration. CaO can promote the formation of oxidizable Cr and residual Cu. Fe₂O₃ can promote the formation of residual As and residual Cd. Al₂O₃ can promote the formation of oxidizable Zn and residual As. MgO can promote the formation of residual Cr.

Besides solid waste, the incineration of oily sludge generates a variety of gaseous pollutants, and controlling the emissions of these pollutants is crucial for achieving environmentally friendly incineration. Future work will be focused on harmful gas control during oily sludge incineration.