Application of heavy metal chelating agent SDDC in the treatment of waste incineration fly ash

1. Preface

Municipal solid waste incineration fly ash is a substance collected in the heat recovery and utilization system and flue gas purification system after incineration. The production of fly ash is related to the type of garbage, incineration conditions, incinerator type, and flue gas treatment process. The composition of municipal solid waste incineration fly ash is complex, containing high leaching concentrations of heavy metals such as Pb, Cr, Cd, Cu, Hg, and Zn, which are classified as hazardous waste and have high leaching toxicity. These heavy metals pose a serious threat to the environment. Heavy metals constantly migrate and transform in the environment, and the polluted soil and water environment cannot be eliminated through their own purification effect, and cannot be biodegraded by microorganisms. They will accumulate through the food chain. Therefore, improper fly ash treatment can lead to the loss of pollutants and heavy metals into the environment, causing serious harm to human health and the natural environment.

With the continuous improvement of national environmental protection laws and regulations, garbage incineration and power generation is the future trend in various regions. The fly ash after garbage incineration contains a lot of heavy metals and other pollution sources. If it is directly buried without solidification treatment, it will also cause damage to the environment.

 

2 Existing waste incineration fly ash treatment methods

At present, there are three main methods for treating heavy metal components in fly ash. One is to use cement solidification technology to solidify heavy metals. Heavy metals react with cement through ion exchange, passivation, adsorption, chemical adsorption, sedimentation, and hydration, ultimately forming stable compounds such as hydroxides or complexes on the surface of the hydrated silicate colloid C-H-S of cement. However, this method has the disadvantages of large capacity increase of fly ash and increased landfill volume; The second method is the melt solidification method, which is currently recognized as the safest and most stable incineration fly ash stabilization/solidification technology. This method mainly involves mixing incineration fly ash with glass powder to form pellets, and melting them at a high temperature of 1000-1400 ℃ for a period of time. However, this method has the disadvantage of high energy consumption; The third is the chemical stabilization method, in which chemical agents react with heavy metals to convert their ions into low solubility, low mobility polymer complexes or inorganic minerals. The core lies in the selection of chemical integration agents, especially the selectivity of chemical agents towards the main pollutants of heavy metal ions, as well as the stability of complex formation, especially the leaching ability of acidic solutions; With the shortage of heavy metal resources, the recovery of heavy metals from waste has received increasing attention from researchers. The use of extraction to achieve heavy metal recovery from fly ash is in line with the development direction of a resource-saving society.

At present, there are mainly two types of chelating agents used in the market for heavy metal pollution in fly ash from waste incineration plants: physical and chemical substances. Physics, commonly cured with lime or cement, but has a long curing time and is prone to secondary pollution; In the chemical category, the common substance is EDTA, which has strong metal chelation properties but has a small molecular weight. The chelated compounds formed are prone to flow into water under acidic solution leaching, causing secondary pollution. However, the reagents mainly used in the market have the following disadvantages: weak chelating ability, poor thermal stability, and complex preparation methods.The widely used organic curing stabilizers in China are mainly chelating agents, including aminothioformate (DTC), mercaptoanine salts, EDTA oligomers, and chitosan derivatives.

 

3.Advantages of using SDDC as a chelating agent for incineration fly ash

Fly ash chelating agents for garbage incineration, also known as fly ash treatment agents, fly ash solidification agents, and fly ash stabilizers; Developed based on the composition, morphological characteristics, and the presence of heavy metals in fly ash, it is suitable for the characteristics of domestic waste incineration processes. SDDC, as an effective and environmentally friendly chelating agent for solidifying heavy metals in waste incineration fly ash, can be combined with fly ash stabilization processes and systems to generate stable chelates of specific metal ions and remove them, making the treated fly ash meet the landfill requirements of the "Pollution Control Standards for Municipal Solid Waste Landfills" (GB16889-2008), with good environmental and economic benefits

3.1 Principle of Action of SDDC

The molecular structure of SDDC contains a polar group - N-C-S -. The sulfur atom in the polar group has a larger radius, carries a negative charge, and is easily polarized and deformed to generate a negative electric field. It can capture cations and tend to form bonds, resulting in insoluble dithiocarbamate (DTC) salts. Some of the insoluble DTC salts formed in this way are ionic or strongly polar bonds, such as DTC Ag, while most are valence bonds, such as DTC Cu, DTC Zn, DTC Fe, etc.

3.2 Quality indicators of SDDC 

SDDC comply with enterprise standards, with specific indicators shown in the table below:

Grade	Solid	Liquid
Appearance	White or off white crystalline	light yellow transparent liquid
Content, m/m,%	90-95	40-41
Free base, (calculated as NaOH) m/m,%	0.05-0.50	0.05-0.50
PH	9.5-12.5	9.5-12.5

3.3 Characteristics of SDDC Use

(1) Good water solubility and dispersibility, high utilization rate of chemicals, and stable distribution of fly ash treatment performance;

(2) Can be used in different urban temperature environments, can complete effective reactions within a wide pH range, and is not affected by the concentration of heavy metal ions, which can meet the use of different heavy metal contents in fly ash;

(3) Different heavy metal ions in fly ash have a good chelating effect, with a wide range of precipitated and chelated metals, especially for difficult to treat heavy metals such as lead and mercury; And the reaction time is short, with good removal effect;

(4) Appropriate adjustments can be made based on specific waste incineration processes and heavy metal content;

(5) Compared with similar products in the market, SDDC has significant advantages in heavy metal ion removal, COD removal, sludge reduction, flocculation effect, etc;

(6) No cement or less cement can be added; Combining a small amount of inorganic materials such as cement can further increase stability and strength;

(7) The processed fly ash has stable and reliable performance, a long cycle, and can meet the standards of sanitary landfill.

(8) It can effectively precipitate various heavy metal ions in wastewater, even if the treated wastewater contains complex salts of heavy metal ions such as citric acid and tartaric acid EDTA、 Cyanide NH3、 Complex copper and other complex components can also be used to treat wastewater that meets discharge standards;

(9) It has the characteristics of low processing cost, excellent processing effect, easy operation and use, environmental protection and non toxicity.

3.4 Main application areas

The chelating agent SDDC has been widely used in the stabilization of heavy metals in incineration fly ash, such as fly ash from different urban waste power plants, fly ash from waste incineration plants, and solidification of smelting slag, due to its simplicity of operation, excellent treatment effect, and low treatment cost. It has also been widely used in industries such as electroplating, electronics, and circuit boards, especially in environments with high heavy metal concentration and complex composition. Its main application areas include:

(1) Garbage power plants have unique effects in the treatment of heavy metals in fly ash from garbage incineration power plants, and are suitable for stabilizing heavy metal chelates in fly ash from garbage incineration power plants.

(2) The field of solid waste treatment includes solid waste treatment systems containing heavy metals, river sludge, urban sludge, industrial sludge containing heavy metals, stabilization of slag, and remediation of heavy metal contaminated soil.

(3) In the field of wastewater treatment, heavy metal wastewater is treated, and heavy metal ions are captured from wastewater discharged by industries such as mining, electroplating, electronics, and circuit boards.

(4) Metal ion capture of radioactive wastewater from nuclear power plant reactors, uranium thorium hydrometallurgy plants, hospitals, isotope testing reactors, and production reactors.

 

4.Introduction to the Operation Process of Using Chelating Agent SDDC to Treat Waste Incineration Fly Ash

The amount of heavy metal chelating agent used for solidification of waste incineration fly ash is closely related to the composition and heavy metal content of the fly ash. Due to the large fluctuations in the metal content of fly ash produced at different times, the heavy metal content is also closely related to the season and temperature. Therefore, multiple samples should be taken at different time periods and seasons, and a chart of excessive heavy metals should be drawn through long-term accumulation to grasp the dosage of chelating agents. The research results on the characteristics of heavy metals in fly ash at home and abroad show that heavy metals with high boiling points are prone to uniform condensation during combustion, forming the core of fly ash, while heavy metals that are volatile at high temperatures will condense on the surface of fly ash as the temperature decreases. Heavy metals in fly ash increase as the particle size of fly ash decreases.

General process for on-site handling (for reference only):

(1) After the fly ash collected by the flue gas purification system is sent to the ash silo, it is quantitatively transported to the mixer;

(2) At the same time, relative to the mass of fly ash processed each time, 25% to 30% of water (or leachate) is input, mixed with the required amount of fly ash stabilizing agent, and injected into the mixer;

(3) Fly ash, water (or leachate from garbage), and stable boiling agent of fly ash can be briefly stirred in the mixer before being discharged by opening the gate;

(4) The processed fly ash falls into the ton bag below, and after 24 hours of maintenance (subsequent reaction), it can be transported to a regular household waste landfill.

Note: 1 Satisfy the sanitary landfill standard dosage:

① No cement is added in the processing technology: the amount of chemical added is about 2% to 5% of the mass of fly ash;

② Adding cement in the processing technology: the amount of chemicals added can be correspondingly increased;

Note: 2 Recommended dosage to meet hazardous waste landfill standards: The dosage of chemicals added can be appropriately reduced by about 50% on the basis of the above item ①.

 

5. Safety protection during the use of SDDC

This product is alkaline, please pay attention to labor protection during operation, avoid contact with skin and eyes, and rinse with plenty of water after contact. Not flammable or explosive, non corrosive, transported as a general chemical, should be handled with care during loading and transportation, and should not be inverted, subjected to severe vibration, impact, or exposed to sunlight or rain. Do not add acid, alkali, salt or other substances to this product. Seal the container when not in use.

 

6.Prospects for the development of chelating agents such as SDDC in the stabilization treatment of waste incineration fly ash industry

Chelating agent stabilized fly ash is a technology with simple process, feasible technology, and low investment and operating costs.

Fly ash stabilizer meets the requirements of the "Pollution Control Standards for Landfills of Municipal Solid Waste" GB16889-2008 and can be buried in municipal solid waste landfills, avoiding expensive construction costs for safe landfills. Chelating agents can be used alone without adding cement, reducing landfill volume and alleviating landfill capacity pressure.

The equipment is easy to maintain and occupies a small area. There is no waste gas or wastewater generated during the treatment process, and there is no secondary pollution, which has long-term safety and stability.

This technology makes a fundamental and leapfrog progress in the stabilization treatment of waste incineration fly ash, which is of great significance for the healthy and sustainable development of the waste incineration industry.

 

7 Company Overview and Introduction to Heavy Metal Chelating Agent Products

7.1 Company Introduction

Qingdao Zhongke Rongda New Materials Co., Ltd. was founded in August 2012. It is a technology-based enterprise integrating scientific research, production, and trade, based on the Qingdao Rongda New Materials Research Institute. Headquartered at 8-1 Longkou Middle Road, Laixi Economic Development Zone, Qingdao.

Since its establishment, the company has successively developed a series of water treatment chemicals such as heavy metal chelating agents, organic synthesis intermediates (pharmaceuticals, pesticides, organic pigments), electronic chemicals, oilfield chemicals, and organic silicon new materials. It has published multiple academic papers in journals such as Shandong Chemical Industry and has obtained 21 national invention patents and 10 utility model patents. The enterprise has won the third prize of the "Shandong Province Invention Enterprise Award" and has been awarded honorary titles such as "Shandong Province High tech Enterprise", "Qingdao City Specialized and Innovative Demonstration Enterprise", "Qingdao City Thousand Sail Plan Enterprise", etc. The enterprise enjoys a good reputation among peers for stable and reliable product quality, high-quality services, and reasonable product prices.

The company currently has 38 employees, including 10 employees engaged in new product research and development, including 3 senior engineers and 4 engineers. The company has a research and development center, equipped with various necessary experimental instruments, and has established long-term and stable cooperative relationships with many higher education institutions such as China University of Petroleum, Shandong First Medical University, Qingdao University of Science and Technology. It regularly hires well-known domestic experts as technical consultants to guide new product development work and cultivate versatile talents in technology development.

The enterprise has established a new production base in Laiyang Economic Development Zone, focusing on the research and development of heavy metal chelating agents, intermediates, functional chemicals, and polymer processing aids as the main production direction. It will continue to develop new products, improve product quality and market share, and build the enterprise into a well-known enterprise and industry leader in Shandong Province and even the whole country.

The company currently produces SDDC (liquid, solid), Fumei Potassium (liquid), piperazine chelating agent piperazine-N, N-dithiocarbamate potassium series heavy metal chelating agent products, and is developing heavy metal capture agent sodium salt of polyethyleneimine dithiocarbamate and other products.

7.2 Introduction to SDDC

1. Chemical name: Sodium dimethyldithiocarbamate

2. English name: Sodium dimethyldithiocarbamate

3.  Molecular formula and molecular weight: Solid (CH₃)₂NCS₂Na · 2H₂O (SDD) 179

Liquid (CH₃) ₂NCS₂Na 143.21

4. Structural type：

5. CAS Number: 128-04-1

6. EINECS Number: 128-04-1

7. Physical and chemical properties: The solid appears as white or off white crystals; The liquid is a light yellow transparent liquid, soluble in water. Melting point -15 ℃, freezing point -7.6 ℃, boiling point 143 ℃. Non irritating to the skin, it is a water-soluble substance that is stable in alkaline media. It decomposes when exposed to acidic substances and high temperatures, and forms dimers when in contact with oxygen. Its quality decreases and it should be sealed and stored.

8. Usage: Heavy metal chelating agent; Terminating agents for emulsion polymerized styrene butadiene rubber and styrene butadiene latex; Industrial fungicides; Vulcanization of rubber products; Rubber accelerator TMTD; Intermediate of pesticide Fumeishuang; Metal treatment agents in industrial wastewater,; Sugar industry; Mineral processing reagents; Paper industry; Electroplating industry; Leather processing agent.

9. Quality indicator: Implement enterprise standards

Grade	Solid	Liquid
Appearance	White or off white crystalline	light yellow transparent liquid
Content, m/m,%	90-95	40-41
Free base, (calculated as NaOH) m/m,%	0.05-0.50	0.05-0.50
PH	9.5-12.5	9.5-12.5

10. Packaging and Storage:

The solid is packaged in clean, dry, and film lined laminated bags, with a net weight of 25 ± 0.1KG per package; Liquid is packaged in 200L plastic drums, with each drum weighing 200-220 kilograms, or packaged according to user requirements;

The product should be sealed and stored in a dry, cool, and ventilated warehouse, with a storage period of 1 year; After more than one year, re inspection should be conducted. If the indicators pass the re inspection, it can continue to be used.

7.3 Technical indicators of PDDC

PDDC comply with enterprise standards, with specific indicators shown in the table below：

Appearance	light yellow transparent liquid
Content, m/m，%	≥50
Free base (calculated as KOH) m/m,%	0.05-0.5
PH	9.5-12.5
Specific Gravity	1.2-1.3

7.4 Technical specifications of piperazine chelating agent piperazine-N, N-dithiocarbamate potassium

The piperazine chelating agent piperazine-N, N-dithiocarbamate potassium  complies with the enterprise standard, and the specific indicators are shown in the table below:

Appearance	light yellow transparent liquid
Content, m/m，%	≥40
Free base (calculated as KOH) m/m,%	0.05-0.5
PH	≥11
Specific Gravity	1.2-1.3