CN113398979A

CN113398979A - Catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastic and preparation method thereof

Info

Publication number: CN113398979A
Application number: CN202110625527.3A
Authority: CN
Inventors: 王婷; 杨朝合; 张金庆; 谭映临; 李正; 张新功
Original assignee: Qingdao Hui Cheng Environmental Technology Co ltd; China University of Petroleum East China
Current assignee: Qingdao Hui Cheng Environmental Technology Co ltd; China University of Petroleum East China
Priority date: 2021-06-04
Filing date: 2021-06-04
Publication date: 2021-09-17
Anticipated expiration: 2041-06-04
Also published as: CN113398979B

Abstract

A catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastics comprises a silicon dioxide substrate and a molecular sieve, wherein the mass of the silicon dioxide substrate is 20-50 wt% of the mass of the catalyst, and the pore diameter on the silicon dioxide substrate is 20-60 nm; the molecular sieve comprises one or more of Y-type, beta molecular sieve, USY molecular sieve, SBA-15 and MCM-41. By the method, the total oil yield of gasoline and diesel oil can reach more than 90 percent. The NaY mother liquor is recycled in the preparation process of the catalyst, so that on one hand, a path can be provided for treating the NaY mother liquor, and on the other hand, the cost of the catalyst can be reduced.

Description

Catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastic and preparation method thereof

Technical Field

The invention relates to a catalyst in the field of chemical engineering, in particular to a catalyst used in the field of waste plastic recovery and a preparation method thereof.

Background

The waste plastics recycling mainly comprises four methods of landfill, incineration, physical screening, separation and recycling and oiling technology, the first three methods not only can cause secondary pollution to the ecological environment, but also can harm human health, the oiling technology can crack fuel oil, namely gasoline and diesel oil, the cracked product has high use value, and thus the urgent need of exhaustion of petroleum resources is relieved, natural resources are supplemented, and energy is regenerated. Meanwhile, the requirements of circular economy and environment-friendly economy are met, and huge economic and environmental benefits are brought. Because polypropylene is widely used for preparing drinking cups, straws, packing bags, woven bags and the like, the consumption is large, the oiling of waste polypropylene plastic is very important and meaningful work, and the key of the oiling technology is that the quality of a catalyst directly influences the yield of catalytic cracking. In the prior art, the catalyst specially used for preparing the oil by catalytic cracking of the waste polypropylene plastic is less.

Chinese patent application publication No. CN101168678 discloses a catalyst for preparing fuel oil by cracking polyethylene and polypropylene waste plastics, which is prepared by phosphorus modification of KDC-1 type molecular sieve and kaolin, and the yield of gasoline and diesel oil is about 80%.

Chinese patent application publication No. CN101284235 discloses a catalyst for preparing fuel oil by catalytic cracking mixed waste plastics and a preparation method thereof, wherein the first stage catalyst comprises 2.0 wt% -30.0 wt% of metal oxide and 70.0 wt% -98.0 wt% of clay or montmorillonite, the second stage catalyst comprises iron oxide, molybdenum oxide, zinc oxide, cerium oxide, lanthanum oxide, nickel oxide or copper oxide and ZSM-5, MCM-22, USY, REY, Beta or MOR molecular sieves, the first stage catalyst is subjected to secondary catalytic cracking, isomerization and aromatization modification reactions on cracked gas, the fraction ratio of cracked gasoline and diesel oil is increased, and the oil yield of qualified fuel oil is more than 70%. The industry is constantly seeking techniques to increase the oil yield of waste plastic recovery.

Disclosure of Invention

An object of the present invention is to provide a catalyst for catalytic cracking of polypropylene, which can improve the oil yield of waste polypropylene from cracking into fuel oil.

The second purpose of the invention is to provide a preparation method of the catalyst for catalytic cracking of the waste polypropylene.

The third purpose of the invention is to provide a method for catalytic cracking of waste polypropylene, which can improve the oil yield of fuel oil at a relatively low cracking temperature.

In order to realize the aim of the invention, the catalyst for polypropylene catalytic cracking comprises a silicon dioxide substrate and a molecular sieve, wherein the mass of the silicon dioxide substrate is 20 wt% -50 wt% of the mass of the catalyst, and the pore diameter on the silicon dioxide substrate is distributed between 20nm and 60 nm; the molecular sieve comprises one or more of Y-type, beta molecular sieve, USY molecular sieve, SBA-15 and MCM-41.

Through the organic combination of the silicon dioxide matrix and the molecular sieve, the waste polypropylene macromolecules can enter a matrix pore channel to carry out a cracking reaction, the diffusion mass transfer of macromolecule raw materials and intermediate products is promoted, the deep reaction on the outer surface and an orifice is avoided, a large amount of coke is generated and the orifice is blocked, and more intermediate products are subjected to the cracking reaction on the molecular sieve to prepare the fuel oil.

A preparation method of a catalyst for polypropylene catalytic cracking comprises the following steps:

adding NaY mother liquor into the slurry of the silica sol and mixing to obtain slurry A, wherein Na is contained in the slurry A₂O/SiO₂The mass ratio of (A) to (B) is 1: 1-2.5: 1;

adding a molecular sieve into the slurry A, and mixing to obtain slurry B;

and granulating the slurry B, and roasting to obtain the catalyst.

The prepared catalyst is applied to the preparation of fuel oil by polypropylene catalytic cracking, wherein the reaction temperature is 300-350 ℃.

By the catalytic cracking method, the total oil yield of gasoline and diesel oil can reach over 90 percent.

Drawings

FIG. 1 is a distribution diagram of the pore diameters of micropores of the catalyst obtained in example 1

FIG. 2 is a mesoporous distribution diagram of the catalyst obtained in example 1

FIG. 3 shows NH of the substrate obtained in example 1₃-TPD spectrum

Detailed Description

The catalyst for catalytic cracking of waste polypropylene and the preparation method thereof according to the present invention will be described in further detail below. And do not limit the scope of the present application, which is defined by the claims. Certain disclosed specific details provide a thorough understanding of various disclosed embodiments. One skilled in the relevant art will recognize, however, that embodiments may be practiced without one or more of the specific details, with other materials, etc.

Unless the context requires otherwise, in the description and claims, the terms "comprise," comprises, "and" comprising "are to be construed in an open-ended, inclusive sense, i.e., as" including, but not limited to.

Reference in the specification to "an embodiment," "another embodiment," or "certain embodiments," etc., means that a particular described feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, "an embodiment," "another embodiment," or "certain embodiments" do not necessarily all refer to the same embodiments. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, the features disclosed are merely generic examples of equivalent or similar features.

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out according to conventional conditions or according to conditions recommended by the manufacturers. All percentages, ratios, proportions, or parts are by weight unless otherwise specified.

In the present invention, the concentration unit "M" of the solution means mol/L.

A particular type of molecular sieve herein has properties common to that type of molecular sieve. Such as:

y-type molecular sieve (prepared by dividing according to Si/Al ratio) and having general formula of MO-₂O₃﹒SiO₂﹒yH₂O, M represents K, Na, Ca, etc., SiO₂/Al₂O₃The molar ratio is more than 3.0, and the molecular sieve is a Y-type molecular sieve.

The beta molecular sieve is a high-silicon zeolite with three-dimensional twelve-element ring pore structure, and has the structural characteristics of double six-element ring unit crystal cavity structure of two four-element rings and four five-element rings.

SBA-15 is a mesoporous molecular sieve with a two-dimensional hexagonal through-hole structure.

MCM-41 is a long-range ordered mesoporous material with uniform pore size.

In the cracking process of the waste polypropylene, the waste polypropylene is firstly contacted with a surface active center of a catalyst to perform surface catalytic reaction to generate smaller molecular fragments, and the small molecular fragments enter a catalyst pore channel to perform secondary catalytic cracking to generate compounds with smaller molecular weight, so that whether the small molecular fragments can smoothly enter the catalyst pore channel to contact the catalytic center depends on the size of the catalyst pore diameter. By adjusting the pore structure and acid property of the catalyst substrate and the synergistic effect of the catalyst substrate and the molecular sieve, the cracking process of the waste polypropylene is optimized, and the yield of gasoline and diesel oil is improved.

A catalyst for preparing fuel oil by polypropylene catalytic cracking comprises a silicon dioxide substrate and a molecular sieve, wherein the mass of the silicon dioxide substrate is 20-50 wt% of the mass of the catalyst, and the pore diameter on the silicon dioxide substrate is 20-60 nm; the molecular sieve comprises one or more of Y-type, beta molecular sieve, USY molecular sieve, SBA-15 and MCM-41.

The mass of the molecular sieve is 20 wt% -50 wt% of the mass of the catalyst.

Preferably, the acid content of the silica matrix is 0.3 to 0.8 mmol/g.

The acid amount of the silica matrix refers to the amount of the substance of hydrogen protons per unit weight of the matrix. By using NH₃The TPD method analyzes the acid properties of the matrix.

The catalyst for preparing fuel oil by catalytic cracking of waste polypropylene has a proper matrix pore structure, initial cracking macromolecules of the waste polypropylene after reaction on the surface of the catalyst can enter a matrix pore channel as soon as possible to perform cracking reaction, deep reaction on the outer surface and the pore opening is avoided, coke is generated and the pore opening is blocked, the proper matrix pore structure can promote diffusion mass transfer of the initial cracking macromolecules and intermediate products, and more raw materials are provided for the cracking reaction on a subsequent molecular sieve. Since polypropylene has methyl side chains and is readily available as carbonium ions, it does not require too many acid sites for the cleavage reaction. More preferably, the number of acid centers in the matrix is such that the acidity requirement for initial cleavage of the polypropylene is met, and the use of such a catalyst can substantially reduce the activation energy of the reaction, resulting in a reduction in the cleavage temperature of about 75 ℃.

In certain embodiments, the pore size distribution of the catalyst for the catalytic cracking of polypropylene is such that there are two types of pores within 0.5 to 5nm and 20 to 60 nm.

In some embodiments, the silica matrix has a pore volume of 0.8 to 1.2cm³(ii) in terms of/g. Further, the specific surface area of the silicon dioxide substrate is 300-500 m²/g。

The pore diameter of the molecular sieve is distributed in the range of 0.5-5 nm.

In some embodiments, the catalyst for preparing fuel oil by catalytic cracking of polypropylene further comprises a binder, wherein the binder comprises silica, and the mass of the binder is 10 wt% to 30 wt% of the mass of the catalyst.

In certain preferred embodiments, the Y-type molecular sieve is loaded with a rare earth element. The performance of the catalytic cracking waste polypropylene fuel oil is superior to that of a molecular sieve which does not load rare earth elements.

The catalyst for polypropylene catalytic cracking with the characteristics can be prepared by various methods. In the present application, the following method is used, among others.

On the other hand, the preparation method of the catalyst for polypropylene catalytic cracking comprises the following steps:

adding NaY mother liquor into the slurry of the acidic silica sol and mixing to obtain slurry A, wherein Na in the slurry A₂O/SiO₂The ratio of the amount of the substance is 1: 1-2.5: 1, and H in the acidic silica sol⁺/SiO₂The mass ratio of the substances is 0.1: 1-1.0;

adding a molecular sieve into the slurry A, and mixing to obtain slurry B;

and granulating the slurry B, and roasting to obtain the catalyst.

The main components of the NaY mother liquor contain sodium element and silicon element, wherein the sodium element is Na₂The content is 15 g/L-30 g/L calculated by O; silicon element is SiO₂The content is 30g/L to 60 g/L.

In some embodiments, the slurry of silica sol is mixed with NaY mother liquor at a temperature of 30 to 60 ℃. Preferably, the temperature is 35-50 ℃.

The mixing time of the slurry of the silica sol and the NaY mother liquor is controlled to be 20 min-60 min.

In the slurry A, Na₂O/SiO₂In the proportional relationship of the amounts of substances, Na₂O represents the amount of NaY mother liquor, SiO₂Denotes SiO in silica sol₂The amount of (c).

By controlling the reaction conditions, the size of the silica gel particles generated by the reaction can be controlled, and the pore structure size of the formed silica matrix can be further controlled.

In certain embodiments, the acidic silica sol is SiO₂Meter, SiO₂25-50 wt% of acidic silica sol.

An appropriate amount of acid can be added into the acidic silica sol to adjust the ratio of hydrogen ions to silicon dioxide in the acidic silica sol, namely H⁺/SiO₂The mass ratio of the substances is 0.1: 1-1.0. The added acid includes, but is not limited to, one of hydrochloric acid, sulfuric acid, boric acid, or phosphoric acid.

In certain embodiments, in the acidic silica sol, H⁺/SiO₂The amount ratio of the substances is 0.2-0.5.

In some instancesIn the embodiment, the adding amount of the molecular sieve is the molecular sieve and SiO in the slurry A₂The mass ratio of (A) to (B) is 1: 4-1: 1.

In the actual treatment process, the preparation conditions of the substrate and the proportion and the type of the molecular sieve are properly adjusted according to the specific requirements of the yield of fuel oil (gasoline and diesel oil) and the diesel-gasoline ratio.

In certain embodiments, Na₂O/SiO₂The ratio of the amount of the substances is 1: 1-2.5: 1. Under the condition, the mesoporous structure of the silicon dioxide matrix in the obtained catalyst is more uniform and more.

In the application, the silicon dioxide matrix and the molecular sieve are bonded together through the silicon-based binder, and compared with an aluminum-based binder or a silicon-aluminum binder, the catalyst prepared by using the silicon-based binder is used in the reaction of preparing fuel by catalytic cracking of waste polypropylene, so that the coke yield is low.

In the application, the roasting temperature is controlled to be 450-600 ℃. Preferably, the roasting temperature is controlled to be 500-600 ℃.

In certain embodiments, the calcined catalyst is washed with water and dried. The drying temperature is controlled to be about 100 ℃, and the moisture can be evaporated to dryness.

The catalyst provided by the application can be used for catalytically cracking waste polypropylene plastics or waste modified polypropylene plastics. Common polypropylene waste plastics types, such as drinking cups, straws, woven bags, and the like.

The polypropylene catalytic cracking catalyst selects the silicon dioxide substrate with large particle size, can effectively promote the diffusion mass transfer of polypropylene macromolecular raw materials and intermediate products, has high utilization rate of acid sites, improves the yield of gasoline and diesel oil in a cracked oil product, and simultaneously has better coke selectivity and hydrothermal stability and can be repeatedly used. The catalyst can reduce the polypropylene cracking temperature by about 75 ℃, and the gasoline and diesel oil yield is improved to more than 90%.

The catalyst preparation method can solve the recycling problem of the NaY mother liquor, the production amount of the NaY mother liquor is far larger than the recycling amount at present, and the NaY mother liquor is recycled into the catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastics, so that on one hand, a path can be provided for treating the NaY mother liquor, and on the other hand, the cost of the catalyst can be reduced.

The catalyst of the present invention and its catalytic effect are further illustrated below with reference to specific examples.

Example 1:

(1) at room temperature, to silica Sol (SiO)₂Acid silica sol with a content of 30%) is added with hydrochloric acid to obtain a slurry so that H is⁺/SiO₂The mass ratio of (A) is 0.2, and the mixture is fully stirred for 10 min;

(2) adding NaY mother liquor (SiO) into the slurry in the step (1)₂The concentration is 50g/L, Na₂O concentration of 25g/L) so that Na in NaY mother liquor₂O and SiO in the slurry in the step (1)₂The mass ratio of the substances is 1, the temperature is controlled to be 30 ℃, and the reaction is carried out for 20min to obtain slurry A;

(3) adding REUSY molecular sieve (representing USY molecular sieve containing rare earth elements, wherein La and La elements are added, the content is 2 wt% of the molecular sieve) into the slurry A in the step (2), and stirring for 1h at room temperature to obtain slurry B, wherein the molecular sieve and all Si elements in the slurry A are SiO₂The mass ratio is 1: 1.86.

(4) And (4) carrying out spray granulation on the slurry B obtained in the step (3), roasting at 500 ℃, washing with water, and drying at 100 ℃, and recording as the fuel oil catalyst Cat-1 prepared by catalytic cracking of the waste polypropylene. In Cat-1, the matrix silica content was 44 wt% and the molecular sieve content was 35 wt%

The pore size distribution of micropores of the catalyst Cat-1 is shown in figure 1, the micropores are provided by a molecular sieve, the pore size is about 0.8nm, the pore size distribution of mesopores is shown in figure 2, the mesopores are provided by a substrate, and the pore size is about 30 nm. In FIGS. 1-2, the ordinate "dV/dlogW" is the differential value of pore volume versus pore diameter. The acid amount distribution of the matrix is shown in figure 3, most of the acid amount distribution is weak acid sites, and a small number of strong acid sites exist, so that the requirement of plastic pre-cracking on the acid amount can be met.

Example 2:

(1) at room temperature, to silica Sol (SiO)₂Acid silica sol with a content of 30%) is added with hydrochloric acid to obtain a slurry so that H is⁺/SiO₂The mass ratio of (A) to (B) is 0Fully stirring for 10 min;

(2) adding NaY mother liquor (SiO) into the slurry in the step (1)₂The concentration is 45g/L, Na₂O concentration of 22g/L) so that Na in NaY mother liquor₂O and SiO in the slurry in the step (1)₂The mass ratio of the substances is 1.5, the temperature is controlled to be 35 ℃, and the reaction is carried out for 30min to obtain slurry A;

(3) adding USY molecular sieve into the slurry A in the step (2), and stirring for 1h at room temperature to obtain slurry B, wherein the molecular sieve and all Si elements in the slurry A are in SiO₂The mass ratio is 1: 1.86. (ii) a

(4) And (4) carrying out spray granulation on the slurry B obtained in the step (3), roasting at 500 ℃, washing with water, and drying at 100 ℃, and recording as the fuel oil catalyst Cat-2 prepared by catalytic cracking of waste polypropylene. In Cat-2, the content of matrix silica was 49 wt%; the content of molecular sieve was 35 wt%.

Example 3:

(1) at room temperature, to silica Sol (SiO)₂Acid silica sol with a content of 30%) is added with hydrochloric acid to obtain a slurry so that H is⁺/SiO₂The mass ratio of (A) is 0.5, and the mixture is fully stirred for 10 min;

(2) adding NaY mother liquor (SiO) into the slurry in the step (1)₂The concentration is 55g/L, Na₂O concentration of 30g/L) so that Na in NaY mother liquor₂O and SiO in the slurry in the step (1)₂The mass ratio of the substances is 1.75, the temperature is controlled to be 40 ℃, and the reaction is carried out for 40min to obtain slurry A;

(3) adding an MCM-41 molecular sieve into the slurry A in the step (2), and stirring for 1h at room temperature to obtain slurry B, wherein the molecular sieve and all Si elements in the slurry A are in SiO₂The mass ratio is 1: 1.86;

(4) and (4) carrying out spray granulation on the slurry B obtained in the step (3), roasting at 550 ℃, washing with water, drying at 100 ℃, and recording as the fuel oil catalyst Cat-3 prepared by catalytic cracking of waste polypropylene. In Cat-3, the content of matrix silica was 50 wt%; the content of molecular sieve was 35 wt%.

Example 4:

(1) at room temperature, to silica Sol (SiO)₂Acid silica sol with a content of 30%) is added with hydrochloric acid to obtain a slurry so that H is⁺/SiO₂The mass ratio of (A) is 0.65, and fully stirring is carried out for 10 min;

(2) adding NaY mother liquor (SiO) into the slurry in the step (1)₂The concentration is 55g/L, Na₂O concentration of 30g/L) so that Na in NaY mother liquor₂O and SiO in the slurry in the step (1)₂The mass ratio of the substances is 2.1, the temperature is controlled to be 45 ℃, and the reaction is carried out for 45min to obtain slurry A;

(3) adding SBA-15 molecular sieve into the slurry A in the step (2), and stirring for 1h at room temperature to obtain slurry B, wherein the molecular sieve and all Si elements in the slurry A are in SiO₂The mass ratio is 1: 1.86;

(4) and (4) performing spray granulation on the slurry B obtained in the step (3), roasting at 550 ℃, washing with water, drying at 100 ℃, and recording as the fuel oil catalyst Cat-4 prepared by catalytic cracking of waste polypropylene. In Cat-4, the content of matrix silica was 52 wt%; the content of molecular sieve was 35 wt%.

The pore diameter structures of the catalysts Cat-2, Cat-3 and Cat-4 obtained in the embodiment 2-4 are respectively distributed with two types of pores within 0.5-5 nm and 20-60 nm.

Comparative example 1:

(1) uniformly mixing 100g of kaolin and 40g of KDC-1 type molecular sieve, and drying in a drying oven at 120 ℃ for 2 hours;

(2) adding 400g of 10% phosphoric acid solution into the mixture, stirring, and standing for 2 hours;

(3) placing the mixture after standing in a drying oven to dry for 4h at 120 ℃;

(4) roasting at 500 deg.c for 4 hr;

(5) the catalyst was removed and pressed to crush and sieved to obtain 20-40 mesh particles, which were designated as catalyst Cat-5.

Evaluation of catalyst: the evaluation of the catalyst is carried out on a fixed fluidized bed, 100-mesh polypropylene particles are selected as raw materials, the feeding amount is 500g, the reaction temperature is 300 ℃, and the space velocity is 10h^-1The evaluation results are shown in the following table, wherein the dose ratio is 0.1.

Catalyst and process for preparing same	Cat-1	Cat-2	Cat-3	Cat-4	Cat-5
						Gasoline yield/wt%	48	46	45	45	43
Diesel oil yield/wt%	42	44	46	48	39
						Fuel oil yield per wt%	90	90	91	93	82

Claims

1. A catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastics comprises a silicon dioxide substrate and a molecular sieve, wherein the mass of the silicon dioxide substrate is 20-50 wt% of the mass of the catalyst, and the pore diameter on the silicon dioxide substrate is 20-60 nm; the molecular sieve comprises one or more of Y-type molecular sieve, beta molecular sieve, USY molecular sieve, SBA-15 and MCM-41;

preferably, the mass of the molecular sieve is 20 wt% to 50 wt% of the mass of the catalyst.

2. The catalyst according to claim 1, wherein the acid amount of the silica substrate is 0.3 to 0.8 mmol/g.

3. The catalyst according to claim 1 or 2, wherein the pore size distribution of the catalyst is such that there are two types of pores within 0.5 to 5nm and 20 to 60 nm;

preferably, the pore volume of the silicon dioxide matrix is 0.8-1.2 cm³/g。

4. A catalyst as claimed in any one of claims 1 to 3, further comprising a binder, the binder comprising silica, the mass of the binder being from 10% to 30% by weight of the mass of the catalyst.

5. A preparation method of a catalyst for preparing fuel oil by catalytic cracking of waste polypropylene plastics comprises the following steps:

adding NaY mother liquor into the slurry of the acidic silica sol and mixing to obtain slurry A, wherein Na in the slurry A₂O/SiO₂The mass ratio of (A) to (B) is 1: 1-2.5: 1;

adding a molecular sieve into the slurry A, and mixing to obtain slurry B;

granulating the slurry B, and roasting to obtain a catalyst;

preferably, H in the acidic silica sol⁺/SiO₂The amount ratio of the substances (A) is 0.1: 1-1: 1.

6. The catalyst according to claim 5, wherein the slurry of silica sol is mixed with NaY mother liquor at a temperature of 30-60 ℃;

preferably, the temperature is 35-50 ℃.

7. The catalyst according to claim 5 or 6, wherein the silica sol is SiO₂Meter, SiO₂25-50 wt% of acidic silica sol.

8. Catalyst according to any of claims 5 to 8, characterized in that the slurry of silica sol contains H⁺/SiO₂Between 0.2 and 0.5.

9. The catalyst of any of claims 5 to 8 wherein the molecular sieve is added in an amount such that the molecular sieve is in slurry A with SiO₂The mass ratio of (A) to (B) is 1: 4-1: 1.

10. The method according to any one of claims 5 to 9, wherein the preparation of the acidic silica sol comprises: adding an acidic solution to the silica sol to adjust H in the silica sol⁺/SiO₂The mass ratio of the substances is 0.1: 1-1.0;

preferably, the acidic solution comprises one of hydrochloric acid, sulfuric acid, boric acid or phosphoric acid.