Method and device for co-processing sewage plant sludge and metallurgical dust with rotary hearth furnace

Abstract

The present invention discloses a method and device for collaboratively treating sewage plant sludge and metallurgical dust ash using a rotary hearth furnace. The rotary hearth furnace process is adopted to collaboratively treat sewage plant sludge and metallurgical dust ash and recycle the Fe and Zn elements therein, while achieving harmless treatment and resource utilization of sewage plant sludge, minimizing the volume of sludge, solving the problem of sludge odor, and reducing the cost of sludge treatment.

Description

Technical Field

[0001] The present invention belongs to the field of resource utilization of urban solid hazardous waste and metallurgical dust sludge, and relates to a method and device for collaboratively treating sewage plant sludge and metallurgical dust ash with a rotary hearth furnace. Background Art

[0002] The inorganic carbon content of sewage sludge is approximately 0.1-7.98%, the organic matter content is approximately 55-75%, and the calorific value of dried sludge is 15,048-20,482 kJ / kg. The organic matter in sewage sludge is primarily composed of organic matter and biomacromolecules. It contains a large number of persistent, bioaccumulative, toxic, and hazardous organic pollutants, such as persistent organic pollutants (POPs), perfluorinated compounds (PFCs), polycyclic aromatic hydrocarbons (PAHs), and other emerging organic compounds. These organic pollutants are carcinogenic, teratogenic, and mutagenic. Improper disposal poses ecological risks and also poses a threat to human health. Sludge is characterized by high moisture content and high viscosity. Gravity-thickened sludge has a moisture content of approximately 98%, while mechanically dewatered sludge has a moisture content of approximately 55-80%. Sludge with high moisture content is difficult to landfill and increases the landfill's leachate treatment load. Sludge inherently has a certain degree of viscosity. During the concentration and dehydration process, the addition of external reagents, such as flocculants, increases its viscosity. Consequently, dehydrated sludge exhibits a high degree of viscosity. Sludge is rich in metal ions. Heavy metals accumulate in sludge during wastewater treatment, leading to significant ecological risks and heavy metal pollution in landfill and dry thermal treatment processes. In sludge incineration ash, by mass, Fe ranges from 5.6-16.1%, CaO from 4.6-38%, Cu from 0.4-10.41%, Zn up to 2.02%, Ni from 0.1-0.6%, TP averages 2.32%, TK averages 1.42%, TN averages 3.58%, and Cl is less than 0.85%. Furthermore, sludge contains elements such as lead, cadmium, mercury, chromium, and arsenic. Consequently, sludge incineration fly ash contains a significant amount of metals, making subsequent disposal difficult and therefore classified as hazardous waste. Sludge is rich in nutrients and serves as a good nutrient matrix for bacteria and viruses. However, it also accumulates pollutants found in sewage, including viruses, microorganisms, parasites, heavy metals, and other harmful substances. The presence of microorganisms, pathogens, insect eggs, heavy metals, and organic pollutants in sludge poses certain ecological risks. Therefore, in the field of urban wastewater treatment, sludge also poses the risk of viral spread.

[0003] In view of the above situation, it is of great significance to reasonably dispose of large amounts of sludge, eliminate the ecological and health risks it brings, while also utilizing the large amount of carbon elements and its inherent viscosity in the sludge, and recycling the heavy metals in the sludge.

[0004] The rotary hearth furnace (RHF) direct reduction process for treating metallurgical dust and sludge and recycling the metal elements within it is gaining widespread adoption. Currently, most domestic companies utilize the RHF process for treating metallurgical dust and sludge. RHF direct reduction, performed at temperatures exceeding 1000°C, achieves higher recovery rates for valuable elements such as iron and zinc, lowers energy consumption, minimizes pollution, and delivers higher value-added products. Therefore, RHF direct reduction has become one of the best options for treating iron-zinc-containing dust and sludge. This process is characterized by high-temperature, rapid reduction, achieving iron metallization rates of 65-75% and dezincification rates of 85-95%.

[0005] Research has shown that furnace temperatures above 850°C ensure the complete combustion of organic matter and carbon-based elements in sludge. The resulting volatile matter is primarily composed of methane, hydrogen, and carbon monoxide. Therefore, high-temperature incineration can fully eliminate the ecological risks and health threats posed by organic pollutants. Furthermore, high-temperature incineration destroys and kills pathogens, including disease-causing bacteria and microorganisms, in sludge. Incineration also minimizes sludge volume, reducing subsequent disposal requirements.

[0006] Currently, sludge is primarily treated through landfill, high-temperature incineration, and dry thermal treatment. Landfill is relatively simple, low-cost, practical, and operational, but limited by land resources. Furthermore, sludge landfill presents several challenges, particularly the formation of leachate and gas. Leachate is a highly contaminated liquid that can contaminate groundwater if improperly sited or operated. Furthermore, the gas produced by landfills is primarily methane, which can cause explosions and combustion if appropriate measures are not taken. Due to stricter standards for sludge landfill, some European countries have banned its use in landfills or agriculture. High-temperature incineration can reduce, decontaminate, and stabilize sludge. The final product can also be used as raw material for producing expanded clay, mud bricks, building materials, and ecological sludge. However, its disadvantages include long disposal times, high secondary pollution, and high disposal costs. Furthermore, the introduction of sludge does not improve the performance of the original material. While the sludge incineration process generates some energy for secondary use, the ash and smoke containing heavy metals are difficult to dispose of and prone to secondary pollution. Currently, sludge incineration primarily utilizes fluidized bed incineration technology, which has the disadvantages of high investment and operating costs, and requires high maintenance requirements. Furthermore, residual materials, such as incineration ash, contain large amounts of heavy metals, making them hazardous wastes that require further disposal. Dry heating can effectively reduce sludge volume, make it more stable, and eliminate pathogens and odors. After drying, it can be used as a soil conditioner or energy source. However, this process is costly and does not account for the harmful effects of pathogens, microorganisms, and heavy metals in the sludge.

[0007] Currently, there are also technologies involving the resource utilization of sludge. For example, Chinese Patent Publication No. CN108070723A discloses a method for the resource utilization of fly ash from garbage incineration, municipal sludge, and steel metallurgical dust removal ash through metallurgical sintering. The effective chemical components in fly ash, municipal sludge, and steel metallurgical dust removal ash are used for metallurgical sintering to achieve resource utilization. However, this technology does not fully utilize the large amount of carbon base, calorific value, and viscosity in the sludge, and the secondary zinc oxide powder produced during the sintering process is not recycled and utilized, resulting in a waste of resources. Chinese Patent Application No. CN202110162736.9 discloses a method for treating municipal sludge, including adding a conditioning agent to the municipal sludge and dehydrating it; deep drying and pulverizing the dehydrated municipal sludge; and granulating the sludge and composite additives after deep drying and pulverization to prepare high-calorific value biofuel, which fully utilizes the large amount of calorific value in the sludge. However, this technology fails to recycle the metallized resources in the sludge, resulting in a waste of resources. Chinese patent application number CN201710477386.9 discloses a municipal sludge pyrolysis device, which aims to provide a municipal sludge pyrolysis device that improves calorific value utilization and reduces pollution; however, this technology fails to recycle the metallized resources in the sludge, resulting in a waste of resources.

[0008] In view of this, it is urgent to develop a new technology for treating sludge that can solve the problems existing in the existing technology. Summary of the Invention

[0009] In view of the above-mentioned defects in the prior art, the purpose of the present invention is to provide a method and device for the coordinated treatment of sewage plant sludge and metallurgical dust ash by a rotary hearth furnace, which adopts the rotary hearth furnace process to coordinately treat sewage plant sludge and metallurgical dust ash and recycle the Fe and Zn elements therein, while achieving harmless treatment and resource utilization of sewage plant sludge, minimizing the volume of sludge, solving the problem of sludge odor, and reducing the cost of sludge treatment.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] A first aspect of the present invention provides a method for collaboratively treating sewage sludge and metallurgical dust using a rotary hearth furnace, comprising the following steps:

[0012] S1, sewage treatment plant sludge is concentrated and dehydrated to obtain dehydrated sludge; the dehydrated sludge is fully mixed with metallurgical dust removal ash, and digested to obtain digested mixed ash;

[0013] S2, pressing the digestion mixed ash into pellets through a pelletizing process, and drying the pellets in a pellet drying device to obtain dry pellets;

[0014] S3, rotary hearth furnace reduction, sending the dried pellets into a rotary hearth furnace for reduction roasting to obtain metallized pellets, using inorganic carbon in the dried pellets and CO generated by combustion of organic matter as a reducing agent, and the zinc oxide in the dried pellets is reduced and then enters the high-temperature flue gas, and the iron oxide in the dried pellets is also reduced and then enters the metallized pellets;

[0015] S4, flue gas treatment, the high-temperature flue gas is passed through a pellet drying device and a waste heat boiler to recover waste heat, and then subjected to dust removal treatment to obtain secondary zinc oxide dust.

[0016] Preferably, in step S1:

[0017] The sludge dewatering equipment is used in the concentration and dehydration process; and / or

[0018] The water content of the dewatered sludge is 55-80 wt%; and / or

[0019] The mass ratio of the dewatered sludge to the metallurgical dust ash is 1:4 to 1:10; and / or,

[0020] The free calcium oxide content in the metallurgical dust is 4.5-6.5 wt%.

[0021] Preferably, in step S1:

[0022] During the digestion process, activated carbon is used to remove odors generated during the digestion process; and / or

[0023] During the digestion process, the digestion rate of free calcium oxide is greater than 80%.

[0024] Preferably, in step S2, the pellet drying equipment uses a portion of the high-temperature flue gas discharged from the rotary hearth furnace in step S4 as a heat source to heat the air, and the heated hot air removes moisture and volatile gases in the pellets.

[0025] Preferably, in step S2:

[0026] The temperature of the hot air used in the drying process is 450-500°C; and / or

[0027] The moisture content of the dried pellets is less than 2 wt % and the drop strength of 1 m is ≥ 5 times;

[0028] The dried pellets have an iron content of 45-65%, a zinc content of 2-5wt%, a potassium content of 0.5-2%, a sodium content of 0.5-1.5%, a CaO content of 5-8%, a SiO2 content of 2.5-5%, an inorganic carbon content of 1.5%-2%, and a carbon-based content of 8-14%, calculated in percentage by mass.

[0029] Preferably, in step S3, during the reduction process in the rotary hearth furnace, the dried pellets sequentially pass through the feeding zone, preheating zone, reduction zone and discharge zone of the rotary hearth furnace; the temperature of the preheating zone is 1150-1200°C, the temperature of the reduction zone is 1230-1280°C, and the discharge zone is provided with a water cooling device, so that the temperature of the metallized pellets obtained after reduction drops to below 1000°C after passing through the discharge zone.

[0030] Preferably, in step S3, the iron metallization rate of the metallized pellets is 62-75%, and the residual zinc content is less than 0.8 wt%.

[0031] Preferably, in step S4, the zinc content in the secondary zinc oxide dust is ≥ 60 wt%.

[0032] A second aspect of the present invention provides an apparatus for executing the method for co-processing sewage plant sludge and metallurgical dust ash using a rotary hearth furnace as described in the first aspect of the present invention, comprising a sludge dewatering device, a powerful mixer, a digestion unit, a briquetting machine, a pellet drying device, a rotary hearth furnace, a waste heat boiler, and a powder bag filter.

[0033] The sludge dewatering equipment is used to concentrate and dehydrate the sludge from the sludge plant, and the sludge dewatering equipment transports the concentrated and dehydrated sludge to the powerful mixer through a sludge pump;

[0034] The powerful mixer is used to mix the dewatered sludge and the metallurgical dust removal ash uniformly;

[0035] The digestion unit digests the material mixed evenly by the powerful mixer, and the digestion unit transports the digested mixed ash to the batching bin of the ball press through a belt;

[0036] The pellet press, pellet drying equipment and rotary hearth furnace are connected in sequence by belts;

[0037] The rotary hearth furnace is used to reduce and roast the dried pellets after drying in the pellet drying equipment. The rotary hearth furnace is provided with a feeding area, a preheating area, a reduction area and a discharging area in sequence on the rotating furnace bottom; the preheating area is provided with a flue gas outlet, which is connected to the flue gas inlet of the pellet drying equipment and the waste heat boiler respectively through a flue gas pipeline; the discharging area is provided with a water cooling device;

[0038] The waste heat boiler is used to recover heat from the high-temperature flue gas from the rotary hearth furnace, and the flue gas outlet of the waste heat boiler is connected to the powder bag filter through a pipeline;

[0039] The powder bag dust collector is used to remove dust from the flue gas coming out of the waste heat boiler and the pellet drying equipment, and to collect secondary zinc oxide dust.

[0040] Preferably, the sludge dewatering equipment is selected from a bag filter press or a plate and frame filter press; and / or

[0041] The pellet drying equipment is a convection pellet drying equipment; and / or

[0042] The pellet drying equipment and the flue gas duct of the waste heat boiler are both provided with ash cleaning and collecting devices; and / or

[0043] The powder bag type dust collector is provided with a zinc oxide collecting device; and / or

[0044] The device also includes a desulfurization and denitrification device, which is connected to the flue gas pipeline of the powder bag type dust collector through a pipeline.

[0045] The method and device for collaboratively treating sewage sludge and metallurgical dust with a rotary hearth furnace provided by the present invention also have the following beneficial effects:

[0046] 1. The present invention uses a rotary hearth furnace to collaboratively treat sewage sludge and metallurgical dust ash. The rotary hearth furnace process is used to collaboratively treat metallurgical dust ash and sewage sludge and recycle and utilize the Fe and Zn elements therein. At the same time, the organic matter in the sludge is used as fuel, and the inorganic carbon and volatile matter from the combustion of organic matter are used as reducing agents. The viscosity of the sludge itself replaces the binder required by the existing process. The high-temperature incineration at about 1200°C eliminates the ecological and health risks posed by pathogens, pathogens and microorganisms in the sludge. , avoiding the production of dioxins and other carcinogens caused by sludge incineration; Among them, sewage plant sludge is fully mixed with metallurgical dust ash, and the high moisture content of the sludge is used to digest the free CaO in the metallurgical dust ash. After pelletizing and drying, it is then incinerated in a rotary hearth furnace at high temperature to reduce the environment, effectively recovering the Fe and Zn elements in the sludge and metallurgical dust ash, while achieving harmless treatment and resource utilization of the sludge, minimizing the volume of the sludge, solving the problem of sludge odor, and reducing the cost of sludge treatment. It is the most thorough sludge disposal method;

[0047] 2. The method and apparatus of the present invention for collaboratively treating sewage plant sludge and metallurgical dust ash using a rotary hearth furnace. The moisture content of the concentrated and dehydrated sludge is 55% to 80%. The dehydrated sludge and metallurgical dust ash are fully mixed in a certain proportion. The high moisture content of the sludge is utilized to fully decompose the free CaO in the metallurgical dust ash. The decomposition rate of the free CaO can reach over 80%, thereby reducing the risk of pellet explosion and strength loss caused by the presence of free CaO in the metallurgical dust ash, reducing industrial water usage, and conserving water resources.

[0048] 3. The present invention uses sewage plant sludge as a raw material for the production process of the rotary hearth furnace, fully utilizing the calorific value of the sludge and using the organic matter in it as fuel for complete combustion, thereby reducing the use of external energy. The inorganic carbon and volatile CO generated by the combustion of organic matter serve as reducing agents in the rotary hearth furnace production, reducing Fe and Zn in the sludge and metallurgical dust, thereby reducing production and operating costs and achieving energy conservation, emission reduction, and low-carbon and environmental protection effects.

[0049] 4. The present invention combines the stickiness of sewage sludge with metallurgical dust removal ash, such as electric furnace ash, blast furnace secondary ash, and LT ash, in a certain proportion and digests it to replace the binder required in the existing process. No additional binder is needed to produce carbon-containing pellets, thereby reducing the pelletizing cost and operating cost of the existing rotary hearth furnace process.

[0050] 5. This invention utilizes a high-temperature incineration environment of approximately 1200°C in a rotary hearth furnace to fully crack and burn organic pollutants in sludge, reducing the production of dioxin precursors and avoiding the generation of dioxins and other carcinogens caused by incineration. High-temperature incineration kills pathogenic bacteria, microorganisms, pathogens, insect eggs and other organisms in the sludge, eliminating the ecological risks and health threats caused by improper sludge disposal.

[0051] 6. The present invention utilizes a high-temperature reduction process in a rotary hearth furnace to effectively recover and utilize the metallic elements Fe and Zn in sludge. Heavy metals such as Cr, Hg, As, and Cd are present in the metallized pellets, while Pb, Cd, and the like are collected in the secondary zinc oxide dust. This solves the ecological risks and resource utilization difficulties currently caused by improper disposal of heavy metals in sludge. The present invention effectively reduces and recovers Fe and Zn in metallurgical dust removal ash. The produced metallized pellets can be used as raw materials for ironmaking or steelmaking, and the secondary zinc oxide dust can be directly sold as a raw material for zinc smelting, thereby achieving good economic and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0053] Figure 1 This is a schematic flow chart of a method for collaboratively treating sewage sludge and metallurgical dust using a rotary hearth furnace according to the present invention;

[0054] Figure 2 A schematic structural diagram of an apparatus for the method of the present invention for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace;

[0055] Figure 3 This is the drying principle of the convection pellet drying equipment of the present invention. DETAILED DESCRIPTION

[0056] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below with reference to embodiments.

[0057] Combine Figure 1 As shown, the present invention provides a method for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace, comprising the following steps:

[0058] S1, the sewage treatment plant sludge is dehydrated by the sludge dehydration equipment to obtain dehydrated sludge; the dehydrated sludge is fully mixed with the metallurgical dust removal ash and digested to obtain digested mixed ash;

[0059] Specifically, sewage plant sludge is the sediment left during the sewage purification process. It is concentrated and dehydrated in sludge dewatering equipment to produce dewatered sludge with a moisture content of 55-80 wt%. The inorganic carbon content of sewage plant sludge is approximately 0.1-7.98%, and the organic matter content is approximately 55-75%, calculated as a percentage of dry matter by weight. Incineration ash, obtained from the incineration of sewage plant sludge, contains 5.6-16.1% Fe, 4.6-38% CaO, 0.4-10.41% Cu, up to 2.02% Zn, 0.1-0.6% Ni, an average TP of 2.32%, an average TK of 1.42%, an average TN of 3.58%, and less than 0.85% Cl, calculated by mass fraction. Furthermore, sewage sludge contains small amounts of heavy metals such as lead, cadmium, mercury, chromium, and arsenic.

[0060] Free calcium oxide accounts for 4.5-6.5wt% in metallurgical dust removal ash; metallurgical dust removal ash is a mixture of blast furnace secondary ash, electric furnace ash, and / or LT ash; blast furnace secondary ash is dry dust collection ash produced during the blast furnace ironmaking process, and the chlorine content of the blast furnace secondary ash is 2-5.55%, the potassium content is 0.5-2%, the sodium content is 0.5-1.5%, the zinc content is 1.5-2%, the iron content is 36-45%, and the C content is 15-25%, calculated in mass percentage; electric furnace ash is dry dust removal ash collected and processed during electric furnace steelmaking; LT ash is dry dust removal ash collected during converter steelmaking. LT ash and electric furnace ash have an iron content of 40-50%, a zinc content of 6-13%, and a CaO content of 8-12.5%, calculated in mass percentage;

[0061] The dewatered sludge is transported to a powerful mixer under the action of a sludge pump. Under the action of the powerful mixer, the dewatered sludge is fully mixed and digested with metallurgical dust ash in a certain proportion by utilizing the high water content of the dewatered sludge, thereby reducing the use of industrial water and saving water resources. After the dewatered sludge and metallurgical dust ash are digested, digestion mixed ash is obtained, in which the dewatered sludge and metallurgical dust ash are mixed in a mass ratio of 1:4 to 1:10.

[0062] In the above process, the sludge pump can be selected to be suitable for sludge with a moisture content of 55% to 80%. During the digestion process, the free calcium oxide in the metallurgical dust reacts with the water in the dehydrated sludge to produce calcium hydroxide. The specific reaction is as follows:

[0063] CaO+H2O=Ca(OH)2

[0064] The above reaction can reduce the risk of pellet explosion and strength reduction caused by the presence of free CaO in metallurgical dust removal ash. Activated carbon is used in the digestion process to remove odors that may be generated during the digestion process. The digestion mixed ash after digestion is put into the batching bin for standby use.

[0065] S2, pressing the digested mixed ash into pellets through a pelletizing process, and drying the pellets in a pellet drying device to obtain dry pellets;

[0066] Specifically, the digested mixed ash obtained after the treatment in step S1 is conveyed by a belt to a pelletizing machine, and is pressed into 25x25x15mm oval pellets through a pelletizing process. The pellets are conveyed by a belt to a pellet drying device (such as the principle Figure 3 The pellets are dried using a convection-type pellet drying device (shown in the figure). The pellet drying device uses a portion of the high-temperature flue gas discharged from the rotary hearth furnace in step S4 as a heat source. The high-temperature flue gas is mixed with air in a mixing duct and then heated. The heated air contacts the pellets, removing moisture from the pellets and volatile gases generated by the pellets. The exhaust gas from the pellet drying device then enters the powder bag filter for treatment along with the flue gas treated by the waste heat boiler.

[0067] The moisture content of the dried pellets is less than 2wt%, and the drop strength of 1m is ≥5 times; the iron content of the dried pellets is 45-65%, the zinc content is 2-5wt%, the potassium content is 0.5-2%, the sodium content is 0.5-1.5%, the CaO content is 5-8%, the SiO2 content is 2.5-5%, the inorganic carbon content is 1.5%-2%, and the carbon-based content is 8-14%, calculated in mass percentage.

[0068] The temperature of the hot air used in the above-mentioned drying process is 450-500°C, wherein the hot air used in the drying process uses the high-temperature flue gas from the rotary hearth furnace as a heat source. Since the temperature of the high-temperature flue gas is relatively high, mixed air is required. The temperature of the high-temperature flue gas after mixing with the air is 700-750°C.

[0069] S3, rotary hearth furnace reduction, the dried pellets are sent to the rotary hearth furnace for reduction roasting to obtain metallized pellets. The inorganic carbon in the dried pellets and the CO generated by the combustion of organic matter serve as a reducing agent. The zinc oxide in the dried pellets is reduced and then enters the high-temperature flue gas. The iron oxide in the dried pellets is also reduced and then enters the metallized pellets.

[0070] Specifically, the dried pellets after the treatment in step S2 are conveyed by a belt into a rotary hearth furnace for reduction roasting to obtain metallized pellets. After entering the rotary hearth furnace, the dried pellets sequentially pass through the feeding zone, preheating zone, reduction zone and discharge zone of the rotary hearth furnace. The temperature of the preheating zone is set to 1150-1200°C, the temperature of the reduction zone is set to 1230-1280°C, and the discharge zone is provided with a water cooling device. The metallized pellets obtained after reduction are discharged from the rotary hearth furnace after the temperature drops below 1000°C after passing through the discharge zone.

[0071] During the reduction roasting process, the inorganic carbon and organic matter in the dry pellets are burned to generate CO that participates in the reduction reaction as a reducing agent; the organic matter in the dry pellets is fully burned in a high-temperature incineration environment to release a large amount of heat to maintain the high-temperature environment required by the rotary hearth furnace, thereby reducing the use of external energy. During the reduction roasting process, pathogenic bacteria, microorganisms, pathogens, insect eggs and other organisms in the sludge can be completely killed, eliminating the ecological risks and health threats brought by the sludge, eliminating the need for secondary treatment of the pellets, and avoiding the production of dioxin precursors and thus avoiding the production of toxic and harmful substances such as dioxins as incineration by-products, thereby reducing the difficulty of subsequent flue gas treatment. In addition, acidic gases such as SO2, HF, and HCl generated during the reduction roasting process may react with Ca(OH)2 generated during the digestion process as follows, appropriately reducing the production of acidic gases in the flue gas:

[0072] SO2+Ca(OH)2=CaSO3+H20

[0073] Part of SO2 undergoes the following reaction: SO2+1 / 202+Ca(OH)2=CaS04+H20,

[0074] Reactions of other acidic substances (such as SO3, HF, HCl):

[0075] 2HCl+Ca(OH)=CaCl2+H20;

[0076] 2HF+Ca(OH)2=CaF2+2H2O

[0077] SO3+Ca(OH)2=CaS04+H20

[0078] During the reduction roasting process, the zinc oxide in the dry pellets is reduced and enters the high-temperature flue gas, and the iron oxide in the dry pellets is reduced and enters the metallized pellets. The specific reduction reactions involved are as follows:

[0079] ZnO(s)+C(s)=Zn(g)+CO(g)

[0080] ZnO(s)+CO(g)=Zn(g)+CO2(g)

[0081] ZnO(s)+Fe(s)=Zn(g)+FeO

[0082] 3Fe2O3(s)+C(s)=2Fe3O4(s)+CO(g)

[0083] Fe3O4(s)+C(s)=3FeO(s)+CO(g)

[0084] FeO(s)+C(s)=Fe(s)+CO(g)

[0085] 3Fe2O3(s)+CO(g)=2Fe3O4(s)+CO2(g)

[0086] Fe3O4(s)+CO(g)=3FeO(s)+CO2(g)

[0087] FeO(s)+CO(g)=Fe(s)+CO2(g)

[0088] C(s)+CO2(g)=2CO(g)

[0089] PbO(l)+C(s)=Pb(l)+CO(g)

[0090] PbO(l)+CO(g)=Pb(l)+CO2(g)

[0091] The metallized pellets obtained in the above process have an iron metallization rate of 62-75%, a residual zinc content of less than 0.8wt%, and elements such as Pb, Cr, Hg, As, and Cd in sewage plant sludge are enriched in the pellets with mass fractions of less than 1.2%. The zinc removal rate in the pellets is 85-95%. The metallized pellets can be used as high-quality raw materials for ironmaking or steelmaking.

[0092] S4, flue gas treatment, the high-temperature flue gas passes through the pellet drying equipment and the waste heat boiler to recover the waste heat, and then undergoes dust removal treatment to obtain secondary zinc oxide dust.

[0093] Specifically, a large amount of high-temperature flue gas is discharged during the reduction roasting process in step S3. The flue gas contains SO2 and NOx, which are recovered and reused through the waste heat boiler and the pellet drying equipment to reduce emissions. Part of the high-temperature flue gas from the flue gas outlet of the rotary hearth furnace enters the pellet drying equipment as a heat source, and part enters the waste heat boiler to recover waste heat. After that, the low-temperature flue gas from the waste heat boiler is reduced to about 200°C; the flue gas from the pellet drying equipment and the exhaust gas from the pellet drying equipment (if the flue / exhaust gas temperature is high, it can be reduced to about 180°C by mixing cold air with the air mixing tube) and the low-temperature flue gas from the waste heat boiler are all sent to the powder bag dust collector for dust removal. The dust-removed gas is treated by the desulfurization and denitrification equipment and is discharged through the blast furnace chimney after meeting the standards.

[0094] After the treatment in step S3, the zinc oxide in the dried pellets is reduced to elemental zinc. The elemental zinc is volatilized and then collected through the pellet drying equipment / waste heat boiler and powder bag filter. The following reactions are involved in the Zn powder collection process:

[0095] 2Zn(g)+O2(g)=2ZnO(s)

[0096] The zinc content in the secondary zinc oxide dust is 65-75%, the chlorine content is less than 2%, and the potassium and sodium content is less than 0.5%. Calculated by mass percentage, the Pb and Cd elements contained in the sludge are also collected in the secondary zinc oxide dust, with the mass fractions of both being less than 1%, indicating good quality. The secondary zinc oxide dust prepared as described above can be used as a high-quality raw material for zinc smelting.

[0097] In the method of the rotary hearth furnace of the present invention for collaboratively treating sewage plant sludge and metallurgical dust ash, the viscosity of the sewage plant sludge itself and the free calcium oxide in the metallurgical dust ash are effectively eliminated, the elimination rate of the free calcium oxide is greater than 80%, the return rate in the pelletizing process is less than 10%, and the pulverization rate of the metallized pellets after reduction is less than 16%.

[0098] Combine Figure 2 As shown, the present invention also provides a device for executing the above-mentioned method of co-processing sewage plant sludge and metallurgical dust ash in a rotary hearth furnace, comprising a sludge dewatering device 1, a strong mixer 2, a digestion unit 3, a pelletizing machine 4, a pellet drying device 5, a rotary hearth furnace 6, a waste heat boiler 7, and a powder bag dust collector 8.

[0099] Combine Figure 2 As shown, the sludge dewatering equipment 1 concentrates and dehydrates the sewage plant sludge (for example, dehydrating sludge with a moisture content of 99% to dehydrated sludge with a moisture content of 50% to 80%), and then transports the concentrated and dehydrated dehydrated sludge to the high-pressure mixer 2 through a sludge pump to be evenly mixed with the metallurgical dust removal ash; in a specific embodiment, the sludge dewatering equipment 1 is selected from a bag filter press or a plate and frame filter press.

[0100] Combine Figure 2 As shown, the digestion unit 3 digests the material evenly mixed by the powerful mixer, and the digestion unit 3 transports the digested mixed ash to the batching bin of the ball press 4 through a belt.

[0101] Combine Figure 2 As shown, the pellet press 4, the pellet drying equipment 5, and the rotary hearth furnace 6 are sequentially connected by belts.

[0102] Combine Figure 2 As shown, the smoke outlet and exhaust gas outlet of the pellet drying equipment 5 are connected to the smoke of the powder bag dust collector 8 through a pipeline. In a specific embodiment, the pellet drying equipment 5 adopts a convection pellet drying equipment, and its working principle is as follows: Figure 3 As shown, the high-temperature flue gas serves as a heat source to heat the air, and the heated hot air moves relative to the pellets. During this process, the hot air takes away the moisture in the pellets and the volatile gases generated by the pellets.

[0103] Combine Figure 2 As shown, the rotary hearth furnace 6 is used to reduce and roast the dried pellets after drying in the pellet drying equipment 5. The rotating furnace bottom of the rotary hearth furnace 6 is sequentially provided with a feeding area, a preheating area, a reduction area and a discharge area; the preheating area is provided with a flue gas outlet, which is connected to the flue gas inlets of the pellet drying equipment 5 and the waste heat boiler 7 through flue gas pipes respectively; the discharge area is provided with a water cooling device to reduce the temperature of the metallized pellets to below 1000°C.

[0104] Combine Figure 2 As shown, in order to control the temperature of the hot air, a mixing air tube is provided on the flue gas duct between the pellet drying equipment 5 and the rotary hearth furnace 6. If the high-temperature flue gas temperature is too high, the high-temperature flue gas is mixed with the mixed air to heat the air.

[0105] Combine Figure 2 As shown, the waste heat boiler 7 is used to recover the heat in the high temperature flue gas from the rotary hearth furnace 6. The flue gas outlet of the waste heat boiler 7 is connected to the powder bag type dust collector 8 through a pipeline. In a specific embodiment, the waste heat boiler 7 should adopt a suitable x Equipment that produces corrosive gases.

[0106] In a specific embodiment, the flue gas ducts of the pellet drying equipment 5 and the waste heat boiler 7 are both provided with dust cleaning and collecting devices to collect dust while ensuring effective heat exchange between the pipe wall and the flue gas. The collected dust is sold as finished zinc oxide product.

[0107] Combine Figure 2As shown, the powder bag filter 8 is used to remove dust from the flue gas from the waste heat boiler 7 and the pellet drying equipment 5, and collects the secondary zinc oxide dust. In a further embodiment, the powder bag filter 8 is equipped with a zinc oxide capture device to prevent condensation in the flue gas, which could cause the ash bin to become compacted or sticky. The collected dust is then sold as finished secondary zinc oxide.

[0108] Combine Figure 2 As shown, the rotary hearth furnace of the present invention can be used to collaboratively treat sewage sludge and metallurgical dust, and the device also includes a desulfurization and denitrification device 9 (such as an integrated desulfurization and denitrification device). The desulfurization and denitrification device 9 is connected to the flue gas pipe of the powder bag filter 8 through a pipeline; the desulfurization and denitrification device 9 needs to ensure that the subsequent flue gas contains SO2 and NO x Able to meet emission standards.

[0109] The following further describes a method and apparatus for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace according to the present invention, with reference to specific examples.

[0110] Example 1

[0111] In this embodiment, sludge from a municipal sewage treatment plant is concentrated and dehydrated by a plate and frame filter press. The water content of the dehydrated sludge after concentration and dehydration is 65%.

[0112] Dewatered sludge and metallurgical dust removal ash are mixed in a high-pressure mixer at a mass ratio of 1:6 and conveyed by belt conveyor to the digestion unit for digestion. The digested mixed ash enters the batching bin and is then conveyed by belt conveyor to the briquetting machine for pressing into 25x25x15mm oval pellets. The pellets are then conveyed by belt conveyor to a convection pellet drying equipment for drying. The dried pellets with a moisture content of less than 2% are directly fed into a rotary hearth furnace for reduction.

[0113] The drop strength of the dried pellets at 1m is ≥6 times; the iron content of the dried pellets is 56.42%, the zinc content is 3.82%, the potassium content is 0.92%, the sodium content is 0.85%, the CaO content is 5.45%, the SiO2 content is 2.82%, the inorganic carbon content is about 1.85%, and the carbon-based content is 12.32%, calculated by mass fraction.

[0114] The temperature of the preheating zone of the rotary hearth furnace is set to 1180±10℃, and the temperature of the reduction zone is set to 1250±20℃. The reduced pellets are discharged from the rotary hearth furnace after the temperature of the discharging zone drops to below 1000℃ to obtain metallized pellets.

[0115] The high-temperature flue gas from the rotary hearth furnace is processed through a pellet drying unit and a waste heat boiler for waste heat recovery. After dust removal, it is treated in desulfurization and denitrification equipment to meet standards before being discharged through the blast furnace chimney. The dust removal process collects secondary zinc oxide dust, which contains 63.9% zinc, 2.2% chlorine, less than 1% potassium and sodium, and less than 0.5% of elements such as lead and cadmium, calculated by mass percentage.

[0116] In the above process, the return rate of the pelletizing process was 8%, and the powderization rate of the metallized pellets after reduction was 15.6%. The residence time of the dried pellets in the rotary hearth furnace was 24 minutes;

[0117] The total iron content of the metallized pellets is 63.92%, the metallization rate is 69.2%, the residual zinc content in the pellets is 0.48%, the zinc removal rate is 90.7%, and it contains very small amounts of Pb, Cr, Hg, As, Cd and other elements, all less than 0.8%. The metallized pellets are used as raw materials for ironmaking; the by-product secondary zinc oxide dust is a high-quality raw material for zinc smelting.

[0118] Example 2

[0119] In this embodiment, sludge from a municipal sewage treatment plant is concentrated and dehydrated by a plate and frame filter press. The water content of the dehydrated sludge after concentration and dehydration is 70%.

[0120] Dewatered sludge and metallurgical dust removal ash are mixed in a high-pressure mixer at a mass ratio of 1:6.5 and conveyed by belt conveyor to the digestion unit for digestion. The digested mixed ash enters the batching bin and is then conveyed by belt conveyor to the briquetting machine for pressing into 25x25x15mm oval pellets. The pellets are then conveyed by belt conveyor to a convection pellet drying equipment for drying. The dried pellets with a moisture content of less than 2% are directly fed into a rotary hearth furnace for reduction.

[0121] The drop strength of the dried pellets at 1m is ≥5.8 times; the iron content of the dried pellets is 63.28%, the zinc content is 3.14%, the potassium content is 0.78%, the sodium content is 0.81%, the CaO content is 5.14%, the SiO2 content is 3.22%, the inorganic carbon content is about 1.79%, and the carbon-based content is 13.16%, calculated by mass fraction.

[0122] The temperature of the preheating zone of the rotary hearth furnace is set to 1180±10℃, and the temperature of the reduction zone is set to 1250±20℃. The reduced pellets are discharged from the rotary hearth furnace after the temperature of the discharging zone drops to below 1000℃ to obtain metallized pellets.

[0123] The high-temperature flue gas from the rotary hearth furnace is processed through a pellet drying unit and a waste heat boiler for waste heat recovery. After dust removal, it is treated in desulfurization and denitrification equipment to meet standards before being discharged through the blast furnace chimney. The dust removal process collects secondary zinc oxide dust, which contains 62.5% zinc, 2.2% chlorine, less than 1% potassium and sodium, and less than 0.8% of elements such as lead and cadmium, calculated by mass percentage.

[0124] In the above process, the return rate of the pelletizing process was 7.5%, and the powderization rate of the metallized pellets after reduction was 14.8%. The residence time of the dried pellets in the rotary hearth furnace was 24 minutes;

[0125] The total iron content of the metallized pellets is 68.6%, the metallization rate is 73%, the residual zinc content in the pellets is 0.58%, the zinc removal rate is 92%, and it contains very small amounts of Pb, Cr, Hg, As, Cd and other elements, all less than 1.2%. The metallized pellets are used as raw materials for ironmaking; the by-product secondary zinc oxide dust is a high-quality raw material for zinc smelting.

[0126] Example 3

[0127] In this embodiment, sludge from a municipal sewage treatment plant is concentrated and dehydrated by a plate and frame filter press. The water content of the dehydrated sludge after concentration and dehydration is 68%.

[0128] Dewatered sludge and metallurgical dust removal ash are mixed in a high-pressure mixer at a mass ratio of 1:6.5 and conveyed by belt conveyor to the digestion unit for digestion. The digested mixed ash enters the batching bin and is then conveyed by belt conveyor to the briquetting machine for pressing into 25x25x15mm oval pellets. The pellets are then conveyed by belt conveyor to a convection pellet drying equipment for drying. The dried pellets with a moisture content of less than 2% are directly fed into a rotary hearth furnace for reduction.

[0129] The drop strength of the dried pellets at 1m is ≥6.2 times; the iron content of the dried pellets is 52.94%, the zinc content is 2.89%, the potassium content is 0.96%, the sodium content is 0.93%, the CaO content is 5.68%, the SiO2 content is 3.47%, the inorganic carbon content is about 1.68%, and the carbon-based content is 14.16%, calculated by mass fraction.

[0130] The temperature of the preheating zone of the rotary hearth furnace is set to 1180±10℃, and the temperature of the reduction zone is set to 1250±20℃. The reduced pellets are discharged from the rotary hearth furnace after the temperature of the discharging zone drops to below 1000℃ to obtain metallized pellets.

[0131] The high-temperature flue gas from the rotary hearth furnace is processed through a pellet drying unit and a waste heat boiler for waste heat recovery. After dust removal, it is treated in desulfurization and denitrification equipment to meet standards before being discharged through the blast furnace chimney. The dust removal process collects secondary zinc oxide dust, which contains 60.2% zinc, 2.6% chlorine, less than 1% potassium and sodium, and less than 0.8% of elements such as lead and cadmium, calculated by mass percentage.

[0132] In the above process, the return rate of the pelletizing process was 7.4%, and the powderization rate of the metallized pellets after reduction was 14.2%. The residence time of the dried pellets in the rotary hearth furnace was 24 minutes;

[0133] The total iron content of the metallized pellets is 62.5%, the metallization rate is 65%, the residual zinc content in the pellets is 0.48%, the zinc removal rate is 92.5%, and it contains very small amounts of Pb, Cr, Hg, As, Cd and other elements, all less than 1%. The metallized pellets are used as raw materials for ironmaking; the by-product secondary zinc oxide dust is a high-quality raw material for zinc smelting.

[0134] Those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present invention and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A method for co-processing sewage sludge and metallurgical dust using a rotary hearth furnace, characterized in that: The following steps are involved: S1, sewage treatment plant sludge is concentrated and dehydrated to obtain dehydrated sludge; the dehydrated sludge is fully mixed with metallurgical dust removal ash, and digested to obtain digested mixed ash; The sludge dewatering equipment is used in the concentration and dehydration process; The water content of the dewatered sludge is 55-80wt%; The mass ratio of the dewatered sludge to the metallurgical dust ash is 1:4 to 1:10; The free calcium oxide content in the metallurgical dust is 4.5 to 6.5 wt%; S2, pressing the digestion mixed ash into pellets through a pelletizing process, and drying the pellets in a pellet drying device to obtain dry pellets; The temperature of the hot air used in the drying process is 450-500°C; The moisture content of the dried pellets is less than 2 wt % and the drop strength of 1 m is ≥ 5 times; The dried pellets have an iron content of 45-65%, a zinc content of 2-5wt%, a potassium content of 0.5-2%, a sodium content of 0.5-1.5%, a CaO content of 5-8%, a SiO2 content of 2.5-5%, an inorganic carbon content of 1.5%-2%, and a carbon-based content of 8-14%, calculated in percentage by mass; S3, rotary hearth furnace reduction, sending the dried pellets into a rotary hearth furnace for reduction roasting to obtain metallized pellets, using inorganic carbon in the dried pellets and CO generated by combustion of organic matter as a reducing agent, and the zinc oxide in the dried pellets is reduced and then enters the high-temperature flue gas, and the iron oxide in the dried pellets is also reduced and then enters the metallized pellets; During the reduction process in the rotary hearth furnace, the dried pellets sequentially pass through the feed zone, preheating zone, reduction zone, and discharge zone of the rotary hearth furnace; the temperature of the preheating zone is 1150-1200°C, the temperature of the reduction zone is 1230-1280°C, and the discharge zone is equipped with a water cooling device. The temperature of the metallized pellets obtained after reduction drops to below 1000°C after passing through the discharge zone; The iron metallization rate of the metallized pellets is 62-75%, and the residual zinc content is less than 0.8 wt%; S4, flue gas treatment, the high-temperature flue gas is passed through a pellet drying device and a waste heat boiler to recover waste heat, and then subjected to dust removal treatment to obtain secondary zinc oxide dust.

2. The method for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace according to claim 1, characterized in that: In the step S1: During the digestion process, activated carbon is used to remove odors generated during the digestion process; and / or During the digestion process, the digestion rate of free calcium oxide is greater than 80%.

3. The method for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace according to claim 1, characterized in that: In step S2, the pellet drying equipment uses a portion of the high-temperature flue gas discharged from the rotary hearth furnace in step S4 as a heat source to heat the air, and the heated hot air removes moisture and volatile gases in the pellets.

4. The method for co-processing sewage sludge and metallurgical dust with a rotary hearth furnace according to claim 1, characterized in that: In the step S4, the zinc content in the secondary zinc oxide dust is ≥ 60 wt%.

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