CN114394800A

CN114394800A - Method for resource utilization of sludge biochar

Info

Publication number: CN114394800A
Application number: CN202210059954.4A
Authority: CN
Inventors: 余广炜; 王玉
Original assignee: Institute of Urban Environment of CAS
Current assignee: Institute of Urban Environment of CAS
Priority date: 2022-01-19
Filing date: 2022-01-19
Publication date: 2022-04-26
Anticipated expiration: 2042-01-19
Also published as: CN114394800B

Abstract

The invention discloses a method for recycling sludge biochar. Comprises finely grinding and magnetically separating the biochar prepared from the sludge to obtain a ferrous magnetic material and biochar micro powder without iron; wherein the biochar micro powder and cement and early strength agent are premixed in a dry mode to obtain a premix; then adding river sand and mixing to obtain a secondary mixture; uniformly dispersing the water reducing agent and water in a stirrer, and adding the secondary mixture to obtain a third mixture; feeding into a mould for pressure forming; placing the mold in a steam curing box, cooling to room temperature after steam curing, and removing the mold after the blank body is solidified; curing at normal temperature to obtain the concrete solid brick, building block or heat-insulating material. The method solidifies harmful heavy metals in the sludge, avoids secondary pollution to the environment, directly prepares the sludge into the concrete solid brick, realizes resource utilization and large-scale treatment of the sludge, and is particularly suitable for treating the biochar obtained by pyrolyzing the sludge containing the heavy metals. Simple process, low cost and good economic and environmental benefits.

Description

Method for resource utilization of sludge biochar

Technical Field

The invention relates to the field of comprehensive utilization of solid waste resources, in particular to a method for resource utilization of sludge biochar.

Background

Although the sludge contains organic matters with potential utilization value and nutrient elements such as N, P, K, the salt, pathogenic microorganisms, organic high polymers and particularly heavy metals in the sludge directly enter the environment to cause pollution to the atmosphere, water and soil. Researches show that the larger the city is, the higher the concentration of heavy metals in the sludge is. The heavy metal in the sludge in China has obvious regional characteristics. For example, northern coal heating is common, and As and Hg levels are therefore higher; the south nonferrous metal mining area is centralized in distribution and high in human activity development degree, so that the contents of Cr, Ni, Cu, Zn, Cd and Pb are high. At present, the problem of high heavy metal content in sludge in China is mainly in great relation with the constraint that no relevant standard exists in the wastewater treatment process, and the heavy metal pollution problem limits the treatment and resource utilization of the sludge. If the heavy metals in the sludge are not treated, the heavy metals in the sludge can be converted into metal organic compounds with stronger toxicity under the action of microorganisms, and the metal organic compounds enter a human body through a biological chain, so that the protein is inactivated and deformed, normal physiological metabolic activity is damaged, and the health of the human body is harmed.

The sludge pyrolysis technology is a process of thermally decomposing sludge under the anoxic or anaerobic condition, and the target product is biochar. This technique has the following advantages: (1) by adopting a thermochemical treatment technology, the treatment speed is high, the period is short, and the treatment capacity is large. (2) The reduction is thorough, and the volume and the total amount of the sludge are reduced by more than 90% at one time. (3) The high temperature in the treatment process can realize 100 percent killing of various harmful pathogenic bacteria. (4) The pyrolysis can completely decompose and harmlessly treat residual antibiotics which cannot be completely removed by the composting technology. (5) The emission of CO2, NOx and other gases in the incineration process is greatly reduced, no fly ash pollution is caused, and the pollution control link is simple. (6) The produced tar and combustible gas can be used as supplementary fuel to reduce energy supply. Researches on the migration and transformation rules of heavy metals in the sludge pyrolysis process are carried out in detail by researchers, and the results show that most of Cr, Ni, Cu, Zn and Pb still exist in the biochar under the pyrolysis condition, and the bioavailability and the ecological toxicity of the heavy metals are reduced. Due to the influence of the total amount of heavy metals, the large-scale utilization of the sludge pyrolytic biochar to soil is still limited. Therefore, in order to realize the wide application of the sludge pyrolysis technology, a sludge biochar resource utilization scheme with the total heavy metal content exceeding the standard is inevitably developed.

The main component of the sludge biochar is an inorganic substance, and the biochar is used as a brick making raw material, so that the cost required by brick making can be reduced, the problem of large-scale disposal of the biochar can be solved, and the sludge biochar has a good application prospect. The invention discloses a preparation method of a sludge biochar water permeable brick with high adsorption performance, dewatered sludge is leached to remove part of heavy metals, plate-and-frame filter pressing is carried out, then fly ash or blast furnace slag is added to be uniformly mixed, after granulation and drying, sludge biochar is obtained through high-temperature cracking, after crushed stone aggregate is mixed with treated waste fabrics, the stirred concrete and the sludge biochar are filled into a mold, pressurized, formed, demoulded and maintained together, and the sludge biochar water permeable brick is obtained, and can prevent heavy metal nitrogen, phosphorus, organic pollutants and the like in surface runoff from migrating to underground water and prevent and control underground water pollution. The invention discloses a preparation method of a sludge biochar water permeable brick for water quality purification, which comprises the steps of leaching dewatered sludge to remove part of heavy metals, adding fly ash or blast furnace slag for uniform mixing, carrying out pyrolysis to obtain sludge biochar, mixing the sludge biochar with waste fabrics, adding water for stirring, adding cement and a water reducing agent for stirring, adding water for stirring, molding, forming, demolding and maintaining, and obtaining the sludge biochar water permeable brick capable of purifying water quality. According to the method, the sludge is leached to remove part of heavy metals, then is subjected to plate-frame filter pressing, then is added with fly ash or blast furnace slag, is mixed, granulated and dried, and is subjected to pyrolysis for 3-4 hours to obtain the sludge biochar, and the preparation process of the sludge biochar is complex and the applicability is poor; and in addition, waste fabrics are added in the brick making process, so that the whole process has high cost and limited application. Because the existing sludge pyrolytic biochar process adopts sludge biochar which is obtained by directly entering a pyrolysis furnace for pyrolysis for 0.5-1 hour after sludge is dehydrated and dried in the large-scale production process, the development of the technology for preparing building bricks from the sludge biochar is beneficial to the development of resource utilization ways of the sludge biochar, and the method has good environmental, social and economic benefits.

Disclosure of Invention

The invention aims to provide a method for solidifying harmful heavy metals in sludge, which does not cause secondary pollution to the environment, is directly prepared into a concrete solid brick, reduces the using amount of other components in the solid brick, realizes sludge resource utilization and large-scale treatment, has simple process and low product cost, and has good economic benefit and environmental benefit.

In order to realize the purpose, the fine grinding: finely grinding and magnetically separating the biochar prepared from the sludge to obtain a ferrous magnetic material and biochar micro powder without iron;

preparing a primary mixture: carrying out dry premixing on the biochar micro powder and cement and an early strength agent to obtain a premix;

preparing a secondary mixture: adding river sand into the obtained premix, and carrying out secondary mixing to obtain a secondary mixture;

preparing a third mixture: uniformly dispersing the water reducing agent and water in a stirrer, and adding the prepared secondary mixture to obtain a third mixture;

molding: feeding the prepared third mixture into a mold for pressure forming;

low-temperature steam curing: placing the mold in a steam curing box, cooling to room temperature after steam curing, and removing the mold after the blank body is solidified;

and (3) normal temperature maintenance: and (3) placing the blank after the mould is removed in a normal-temperature curing box, standing and curing for 8-20 days to obtain the concrete solid brick, the building block or the heat-insulating material.

Further, the sludge is sludge containing heavy metals; preferably, the sludge contains excessive heavy metals.

Further, the biochar is a product obtained by pyrolyzing sludge at 400-600 ℃, and the pyrolysis time is 0.5-2 hours;

further, the fine grinding is such that its particle size range is <100 mesh; the fine grinding mode is ball milling or air flow milling.

Further, in the step of preparing the primary mixture, the cement is ordinary portland cement, sulphoaluminate cement or aluminoferrite cement;

the early strength agent is one or more of sodium silicate, sodium sulfate and calcium chloride;

optionally, the mass content of the non-iron-containing biochar micro powder is 20-40% of the total mass of the premix dry basis; the cement adding proportion is 58-78% of the total dry-basis mass of the premix, and the early strength agent adding proportion is 1-2% of the total mass of the premix.

Further, in the step of preparing the secondary mixture, the weight ratio of the premix to the river sand is 1: 1-1: 3.

further, in the step of preparing the third mixture, the water-cement ratio of the water added into the third mixture is 0.3-0.4, the stirring time is 3-10min, and the rotating speed of the stirrer is 800 r/min;

the water reducing agent is one or a mixture of a naphthalene water reducing agent and a polycarboxylic acid water reducing agent; optionally, the addition amount of the water reducing agent is 0.1-0.2% of the total mass of the third mixture.

Further, in the low-temperature steam curing step, the steam curing temperature is 40-55 ℃, the humidity is more than or equal to 65%, and the steam curing is carried out for 8-15 hours;

optionally, after the low-temperature steam curing is finished, cooling to room temperature at a cooling rate of less than or equal to 15 ℃/h.

Further, in the normal-temperature curing step, the temperature of standing curing is 20-27 ℃, and the humidity is more than or equal to 65%.

In the preparation process, the biochar micro powder is finely ground to ensure that the particle size range is less than 100 meshes, so that conditions can be provided for separating and recovering the iron-containing magnetic components, and the micro powder can exert the synergistic hydration performance with cement to the maximum extent. The mass content of the biochar micro powder is 20-40% of the total dry basis mass of the premix, the addition amount is too small to realize large-scale utilization of the sludge biochar, and the performance of the product is affected if the addition amount exceeds 40% of the total dry basis mass of the premix.

Has the advantages that: (1) compared with the prior art, the biochar generated by independent pyrolysis of sludge can be directly used, and heavy metals do not need to be removed by leaching; (2) the biochar micro powder and the cement are cooperatively hydrated to eliminate the negative influence on the quality of the solid brick; the additive is added to adjust and enhance the adhesive force between materials, and the combined action of the biochar micro powder and hydration products of cement is excited to solidify trace harmful heavy metals, so that the biochar products do not generate secondary pollution to the environment. (3) The biochar is added to prepare the concrete solid bricks, building blocks or heat-insulating materials, so that the consumption of raw materials of building material products is reduced, the resource utilization and large-scale treatment of the sludge biochar can be realized, the process is simple, the product cost is low, and good economic benefits and environmental benefits are achieved.

Drawings

FIG. 1 is a process flow diagram of a method for utilizing sludge biochar according to the present invention.

FIG. 2 is a density chart of solid bricks with different addition ratios of biochar micropowder.

FIG. 3 is a water absorption diagram of solid bricks with different addition ratios of biochar micropowder.

FIG. 4 is a graph of breaking strength of solid bricks with different addition ratios of biochar micropowder.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. The examples do not specify particular techniques or conditions, and are performed according to the techniques or conditions described in the literature in the art or according to the product specifications. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available. "%" means weight percent unless otherwise specified.

Example 1:

the biochar adopted in the test is prepared from the biochar formed by pyrolyzing sludge of a certain sewage treatment plant in the city of mansion at 600 ℃, and is used as a brick making material after being crushed and sieved; standard 42.5 grade portland cement is used; normal fine-grained river sand; the water for the ingredients is common tap water.

Fine grinding: finely grinding the biochar to be less than 100 meshes, and carrying out magnetic separation to obtain a ferrous magnetic material and biochar micro powder without iron; wherein the iron-containing magnetic material is used as a smelting raw material or a chemical raw material.

Preparing a primary mixture: carrying out dry premixing on the non-iron-containing biochar micro powder, cement and an early strength agent to obtain a premix; the early strength agent is calcium chloride, and the addition proportion of the early strength agent is 1 percent.

Preparing a secondary mixture: adding river sand into the obtained premix, and carrying out secondary mixing to obtain a secondary mixture; the proportion is adjusted to ensure that the adding proportion of the sludge biochar micro powder is respectively as follows: 0%, 10%, 20%, 30%, 40%, 50%, 60% (all by weight, the same applies hereinafter).

Preparing a third mixture: uniformly dispersing a water reducing agent and water in a stirrer, and adding the prepared secondary mixture to obtain a third mixture, wherein the water-cement ratio of the third mixture is 0.32; the water reducing agent is a polycarboxylic acid water reducing agent, and the adding proportion is 0.1%.

Molding: feeding the prepared third mixture into a mold for pressure forming; the dimensions of the die are 192mm by 92mm by 42.4 mm.

Low-temperature steam curing: placing the mold in a steam curing box, performing steam curing at 40-55 ℃ and humidity of more than or equal to 65% for 8-15 hours, cooling to room temperature at a cooling rate of less than or equal to 15 ℃/h, solidifying the blank body, and then removing the mold;

and (3) normal temperature maintenance: and (4) placing the blank with the mold removed in a normal-temperature curing box for standing and curing for 14 days to obtain the biochar concrete solid brick. The temperature of standing and maintaining is 20-27 ℃, and the humidity is more than or equal to 65%.

And (3) determining the flexural strength, the water absorption and the water content of the biochar concrete solid brick products after curing according to the basic requirements and the operation methods in GB/T21144-2007 concrete solid bricks and GB/T4111-2013 concrete block and brick test methods. The results are shown in FIGS. 2-4.

The density of the concrete solid brick has a direct relation with the used material, and the densityThe smaller the amount of the light weight material added, the larger the density, the smaller the amount of the light weight material added. Meanwhile, the density also affects the water absorption rate of the water-absorbing material. As can be seen from fig. 2, the density of the concrete solid brick is gradually reduced with the increase of the addition amount of the biochar micro powder, because the mass density of the biochar micro powder is very small, and when the addition ratio of the biochar micro powder is increased, the density of the concrete solid brick is smaller. Meanwhile, according to GB/T21144-2007 Standard of concrete solid bricks, when the proportion of the biochar micro powder is 0%, 10% and 20%, the density of the concrete solid bricks is 1874.5kg/m³、1828.8kg/m³And 1746.0kg/m³And all reach density level B; when the addition proportion of the biochar micro powder in the concrete solid brick is 30 percent, 40 percent, 50 percent and 60 percent, the density is 1673.5kg/m respectively³、1601.0kg/m³、1528.5kg/m³、1457.1kg/m³A density level C is reached.

The maximum water absorption of the concrete solid block is plotted as shown in fig. 3. The water absorption of the prepared concrete solid brick is gradually increased along with the increase of the addition amount of the biochar micro powder, which is related to the strong water absorption of the biochar, and the larger the addition amount of the biochar micro powder is, the larger the water absorption of the concrete solid brick is. The maximum water absorption rate has a crucial influence on the durability and the mechanical property of the solid brick, and directly influences whether the product can be used or not. When the biochar micro powder is not added, the water absorption of the concrete solid brick is 10.33 percent, reaches A grade, and meets the maximum water absorption requirement of the density grade; when the addition ratio is 10% and 20%, the maximum water absorption is 11.24% and 13.294%, and the maximum water absorption requirement for achieving the density grade is B grade. The maximum water absorption rate of the concrete solid bricks with other addition proportions is grade C.

The flexural strength of the solid brick added with the biochar micro powder in different proportions is shown in figure 4. As can be seen from fig. 4, the flexural strength of the concrete solid brick gradually decreases as the addition ratio of the biochar increases. When no biochar micro powder is added into the concrete, the breaking strength of the prepared solid brick is 6.9MPa, and the national breaking strength standard of the breaking strength C6.0 is reached. When the addition ratio is 10% and 20%, the flexural strength is 4.5MPa and 3.1MPa respectively, which respectively reach the national flexural strength standard of C4.0 and C3.0, and when the addition ratio is more than 30%, the flexural strength is reduced to below 3.0MPa, which is basically not met. Therefore, in the process of preparing the concrete solid brick by using the biochar micro powder, the adding proportion of the sludge biochar micro powder is less than or equal to 20 percent.

The heavy metal content in the biochar micro powder is respectively Cr: 2819 mg/kg; mn: 2180 mg/kg; ni: 471.4 mg/kg; cu: 2371 mg/kg; zn: 943 mg/kg; as: 9.145 mg/kg; cd: 0.510 mg/kg; pb: 21.26 mg/kg. The content and leachability of heavy metals in the concrete solid bricks obtained by adding the biochar micro powder in different proportions are shown in the following table 1.

TABLE 1 heavy metal content and leachability in biochar micropowder and concrete solid brick

As can be seen from the above table, although the content of heavy metal in the sludge biochar micropowder is high, the leaching of heavy metal TCLP in the concrete solid brick prepared from the sludge biochar micropowder is lower than the relevant standard limit value, which indicates that the addition of the sludge biochar for preparing the concrete solid brick can not only realize the resource utilization of the sludge biochar, but also further realize the solidification stability of heavy metal in the sludge biochar, and the application ecological risk is small, which has important significance for the resource utilization of heavy metal sludge.

Example 2: analysis of existence form of heavy metal in biochar micropowder and solid brick

The existence of heavy metals is closely related to their harmfulness. At present, the BCR continuous extraction method is a main test method for the heavy metal form in the biochar and products thereof. In total, the F1 state is a weak acid extraction state, mainly refers to the adsorption on the particle surface or exists in a carbonate combined form, and is easily influenced by the types and adsorption and desorption of ions in the water body; the F2 state is a reducible state, mainly refers to a state combined with the ferro manganese oxide, and is less stable in the presence of oxygen deficiency or oxygen exclusion; the F3 state is an oxidizable state, mainly refers to a heavy metal state combined with organic matters, sulfides and the like, and can be degraded under specific conditions so as to be biologically utilized; the F4 state is a residue state, and mainly refers to a heavy metal state bonded to a silicate mineral, a crystalline iron magnesium oxide, or the like, and its existing form is stable and difficult to activate or utilize. And (3) pretreating the heavy metal form in the sample by adopting a BCR continuous extraction method, and detecting the leaching liquor and the digestion liquor by using ICP-MS. The specific procedure is shown in table 2.

TABLE 2 pretreatment procedure table of continuous extraction method for heavy metal BCR in biochar micropowder and solid brick

The heavy metal Potential ecological Risk Index (RI) is widely used for safety assessment of heavy metals in SS and sludge biochar, and is calculated as follows:

C_f＝W_s/ W_n (1)

E_r＝T_f·C_f (2)

RI＝ΣE_r (3)

in the formula: w_sThe content of heavy metal (F1+ F2+ F3) in the sample; w_nContent in F4 state; t is_fIs heavy metal biotoxicity response factor, and the response factor of each heavy metal is Cd (30)>Pb(5)＝Ni(5)>Cr(2)>Zn(1)＝Mn(1)；E_rIs a single potential ecological risk coefficient; RI is the heavy metal potential ecological risk index. The indexes of potential ecological risks of heavy metals are shown in table 3.

TABLE 3 evaluation index table for potential ecological risks

The morphology distribution of heavy metal BCR in the biochar micro powder raw material and the concrete solid brick prepared by adding 20% of the biochar micro powder is shown in tables 4 and 5.

TABLE 4 morphology distribution table (/%) of heavy metal BCR in the charcoal micropowder

Heavy metals	F1	F2	F3	F4
					Cr	0.06	0.13	10.88	88.93
Mn	3.09	20.05	20.45	56.41
					Ni	2.30	2.37	52.38	42.95
Cu	6.00	0.00	38.28	55.73
					Zn	30.36	8.59	43.38	17.66
As	1.15	3.38	1.93	93.54
					Cd	23.94	11.97	25.50	38.59
Pb	0.11	0.85	0.99	98.05

Table 5 distribution table of heavy metal BCR form in concrete solid brick with 20% charcoal micro powder added (/%)

Heavy metals	F1	F2	F3	F4
					Cr	0.46	0.35	0.15	99.04
Mn	7.50	32.65	20.43	39.42
					Ni	1.95	4.23	6.60	87.23
Cu	10.34	13.30	37.47	38.89
					Zn	22.79	24.95	13.06	39.19
As	0.83	0.59	0.39	98.20
					Cd	21.68	24.40	12.33	41.60
Pb	0.98	0.93	2.01	96.08

And (3) evaluating the potential ecological risks of the heavy metals by combining the heavy metal forms in the tables 4 and 5, as shown in the table 6.

Table 6 biochar micropowder raw material and heavy metal risk assessment table for concrete solid brick added with 20% biochar micropowder

The results show that the heavy metal potential ecological risk index RI of the solid brick added with the biochar is reduced to 54.35 from 74.65, and further prove that the solid brick prepared from the sludge biochar can realize stable solidification of heavy metal, the product application ecological risk is small, and the method has good practical application value.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.

Claims

1. A method for recycling sludge is characterized by comprising the following steps,

fine grinding: finely grinding and magnetically separating the biochar prepared from the sludge to obtain a ferrous magnetic material and biochar micro powder without iron;

molding: feeding the prepared third mixture into a mold for pressure forming;

and (3) normal temperature maintenance: and (4) placing the blank after the mould is removed in a normal-temperature curing box, standing and curing for 8-20 days to obtain the concrete solid brick.

2. The method for resource utilization of sludge according to claim 1, wherein the sludge is sludge containing heavy metals; preferably, the sludge contains excessive heavy metals.

3. The method for recycling sludge as claimed in claim 1, wherein the biochar is a product obtained by pyrolyzing sludge at 400-600 ℃, and the pyrolysis time is 0.5-2 hours.

4. The method for resource utilization of sludge according to claim 1, wherein the fine grinding is performed so that the particle size range is <100 mesh; the fine grinding mode is ball milling or air flow milling.

5. The method for recycling sludge according to claim 1, wherein in the primary mixture preparation step, the cement is ordinary portland cement, sulphoaluminate cement or aluminoferrite cement;

optionally, the mass content of the added biochar micro powder is 20-40% of the total mass of the premix dry base; the cement adding proportion is 58-78% of the total dry-basis mass of the premix, and the early strength agent adding proportion is 1-2% of the total mass of the premix.

6. The method for resource utilization of sludge according to claim 1, wherein in the step of preparing the secondary mixture, the weight ratio of the premix to the river sand is 1: 1-1: 3.

7. the method for recycling sludge as claimed in claim 1, wherein in the step of preparing the third mixture, the water-cement ratio of the water added into the third mixture is 0.3-0.4, the stirring time is 3-10min, and the rotation speed of the stirrer is 300-;

8. The method for resource utilization of sludge according to claim 1, wherein in the low-temperature steam-curing step, the steam-curing temperature is 40-55 ℃, the humidity is more than or equal to 65%, and the steam-curing is carried out for 8-15 hours;

9. The method for resource utilization of sludge according to claim 1, wherein in the normal temperature curing step, the temperature of standing curing is 20 ℃ to 27 ℃, and the humidity is not less than 65%.