US20090320680A1

US20090320680A1 - Methods of Manufacturing Bentonite Polution Control Sorbents

Info

Publication number: US20090320680A1
Application number: US12/534,509
Authority: US
Inventors: Xiaolin D. Yang; Pascaline H. Tran; Lawrence Shore; Barry K. Speronello; Ronald T. Mentz
Original assignee: BASF Catalysts LLC
Current assignee: BASF Catalysts LLC
Priority date: 2005-11-30
Filing date: 2009-08-03
Publication date: 2009-12-31
Also published as: US20070119300A1; US7578869B2

Abstract

Methods of manufacturing bentonite sorbents for removal of pollutants including mercury from gas streams, such as a flue gas stream from coal-fired utility plants are disclosed. The methods include mixing bentonite sorbent particles with a sulfide salt and a metal salt to form a metal sulfide on the outer surface of the bentonite sorbent particles.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 11/291,091, filed on Nov. 30, 2005, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the invention relate methods for the manufacture of bentonite pollution control sorbents.

BACKGROUND ART

Emission of pollutants, for example, mercury, from sources such as coal-fired and oil-fired boilers has become a major environmental concern. Mercury (Hg) is a potent neurotoxin that can affect human health at very low concentrations. The largest source of mercury emission in the United States is coal-fired electric power plants. Coal-fired power plants account for between one-third and one-half of total mercury emissions in the United States. Mercury is found predominantly in the vapor-phase in coal-fired boiler flue gas. Mercury can also be bound to fly ash in the flue gas.
On Dec. 15, 2003, the Environmental Protection Agency (EPA) proposed standards for emissions of mercury from coal-fired electric power plants, under the authority of Sections 111 and 112 of the Clean Air Act. In their first phase, the standards could require a 29% reduction in emissions by 2008 or 2010, depending on the regulatory option chosen by the government. In addition to EPA's regulatory effort, in the United States Congress, numerous bills recently have been introduced to regulate these emissions. These regulatory and legislative initiatives to reduce mercury emissions indicate a need for improvements in mercury emission technology.
There are three basic forms of Hg in the flue gas from a coal-fired electric utility boiler: elemental Hg (referred to herein by the symbol Hg⁰); compounds of oxidized Hg (referred to herein by the symbol Hg²⁺); and particle-bound mercury. Oxidized mercury compounds in the flue gas from a coal-fired electric utility boiler may include mercury chloride (HgCl₂), mercury oxide (HgO), and mercury sulfate (HgSO₄). Oxidized mercury compounds are sometimes referred to collectively as ionic mercury. This is because, while oxidized mercury compounds may not exist as mercuric ions in the boiler flue gas, these compounds are measured as ionic mercury by the speciation test method used to measure oxidized Hg. The term speciation is used to denote the relative amounts of these three forms of Hg in the flue gas of the boiler. High temperatures generated by combustion in a coal boiler furnace vaporize Hg in the coal. The resulting gaseous Hg⁰exiting the furnace combustion zone can undergo subsequent oxidation in the flue gas by several mechanisms. The predominant oxidized Hg species in boiler flue gases is believed to be HgCl₂. Other possible oxidized species may include HgO, HgSO₄, and mercuric nitrate monohydrate (Hg(NO₃)₂.H₂O).
Gaseous Hg (both Hg⁰and Hg²⁺) can be adsorbed by the solid particles in boiler flue gas. Adsorption refers to the phenomenon where a vapor molecule in a gas stream contacts the surface of a solid particle and is held there by attractive forces between the vapor molecule and the solid. Solid particles are present in all coal-fired electric utility boiler flue gas as a result of the ash that is generated during combustion of the coal. Ash that exits the furnace with the flue gas is called fly ash. Other types of solid particles, called sorbents, may be introduced into the flue gas stream (e.g., lime, powdered activated carbon) for pollutant emission control. Both types of particles may adsorb gaseous Hg in the boiler flue gas.
Sorbents used to capture mercury and other pollutants in flue gas are characterized by their physical and chemical properties. The most common physical characterization is surface area. The interior of certain sorbent particles are highly porous. The surface area of sorbents may be determined using the Brunauer, Emmett, and Teller (BET) method of N₂adsorption. Surface areas of currently used sorbents range from 5 m²/g for Ca-based sorbents to over 2000 m²/g for highly porous activated carbons. EPA Report, Control of Mercury Emissions From Coal-Fired Electric Utility Boilers, Interim Report, EPA-600/R-01-109, April 2002. For most sorbents, mercury capture often increases with increasing surface area of the sorbent.
Mercury and other pollutants can be captured and removed from a flue gas stream by injection of a sorbent into the exhaust stream with subsequent collection in a particulate matter control device such as an electrostatic precipitator or a fabric filter. Adsorptive capture of Hg from flue gas is a complex process that involves many variables. These variables include the temperature and composition of the flue gas, the concentration of Hg in the exhaust stream, and the physical and chemical characteristics of the sorbent. Of the known Hg sorbents, activated carbon and calcium-based sorbents have been the most actively studied.
Currently, the most commonly used method for mercury emission reduction is the injection of powdered activated carbon into the flue stream of coal-fired and oil-fired plants. Currently, there is no available control method that efficiently collects all mercury species present in the flue gas stream. Coal-fired combustion flue gas streams are of particular concern because their composition includes trace amounts of acid gases, including SO₂and SO₃, NO and NO₂, and HCl. These acid gases have been shown to degrade the performance of activated carbon. Though powdered activated carbon is effective to capture oxidized mercury species such as Hg⁺², powdered activated carbon (PAC) is not as effective for elemental mercury which constitutes a major Hg species in flue gas, especially for subbituminous coals and lignite. There have been efforts to enhance the Hg⁰trapping efficiency of PAC by incorporating bromine species. This, however, not only introduces significantly higher cost, but a disadvantage to this approach is that bromine itself is a potential environmental hazard. Furthermore, the presence of PAC hinders the use of the fly ash for cement industry and other applications due to its color and other properties.
As noted above, alternatives to PAC sorbents have been utilized to reduce mercury emissions from coal-fired boilers. Examples of sorbents that have been used for mercury removal include those disclosed in United States Patent Application Publication No. 2003/0103882 and in U.S. Pat. No. 6,719,828. In United States Patent Application Publication No. 2003/0103882, calcium carbonate and kaolin from paper mill waste sludge were calcined and used for Hg removal at high temperatures above 170° C., preferably 500° C. U.S. Pat. No. 6,719,828 teaches the preparation of layered sorbents such as clays with metal sulfide between the clay layers and methods for their preparation. The method used to prepare the layered sorbents is based on an ion exchange process, which limits the selection of substrates to only those having high ion exchange capacity. In addition, ion exchange is time-consuming and involves several wet process steps, which significantly impairs the reproducibility, performance, scalability, equipment requirements, and cost of the sorbent. For example, a sorbent made in accordance with the teachings of U.S. Pat. No. 6,719,828 involves swelling a clay in an acidified solution, introducing a metal salt solution to exchange metal ions between the layers of the clay, filtering the ion exchanged clay, re-dispersing the clay in solution, sulfidation of the clay by adding another sulfide solution, and finally the product is filtered and dried. Another shortcoming of the process disclosed in U.S. Pat. No. 6,719,828 is that the by-products of the ion exchange process, i.e., the waste solutions of metal ions and hydrogen sulfide generated from the acidic solution, are an environmental liability.
There is an ongoing need to provide improved pollution control sorbents and methods of their manufacture. It would be desirable to provide sorbents containing metal sulfides on the sorbent substrate that can be manufactured easily and inexpensively. In this regard, simple and environmentally friendly methods that effectively disperse metal sulfide on readily available substrates, which do not require the numerous steps involved in an ion exchange process are needed.

SUMMARY

Aspects of the invention include methods of manufacturing bentonite sorbents for removal of pollutants such as heavy metals from gas streams. The sorbents are useful for, but not limited to, the removal of mercury from flue gas streams generated by the combustion of coal.
In a first aspect, a method of making sorbent particles for the removal of mercury from a gaseous stream is provided. The method comprises mixing a metal salt with bentonite particles by grinding or milling; mixing a sulfide salt with the bentonite particles and the metal salt; and drying the mixture. In certain embodiments, the method may further include reducing the particle size of the particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is graph showing mercury removal versus copper loading on a copper weight percent basis; and

FIG. 2 is a graph showing the effect of the addition of chloride salts on mercury removal.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.
One aspect of the present invention relates to methods of manufacturing bentonite sorbents. Bentonite is an aluminum phyllosilicate clay consisting mostly of montmorillonite, (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂.nH₂O, which may also be referred to as Fuller's earth or montmorillonite. Applicants have determined that an ion exchange process such as the type disclosed in U.S. Pat. No. 6,719,828, is not necessary for mercury capture by the bentonite sorbent material.
According to one or more embodiments, incipient wetness or solid-state reactive grinding processes are used to disperse metal sulfide salts on the surface of bentonite sorbent particles. The incipient wetness or solid-state reactive grinding methods disclosed herein do not require excessive liquids associated with wet processes, thus eliminating problems associated with wet processes. These problems may include disposal of waste of metal ions solution or hydrogen sulfide. In addition, certain embodiments of the present invention provide accurate control of the amount of metal sulfide on the surface of the sorbent. Further, the processes according to certain embodiments are much faster and significantly reduce the equipment and resources required for the large-scale production. Moreover, the highly dispersed metal sulfide on the surface of the sorbent provides better contact between Hg and metal sulfide when used for mercury removal, as evidenced by the higher and faster mercury capture than those obtained by the ion exchange process when measured by an in-flight test with simulated flue gases.
Another embodiment of the invention pertains to the addition of at least one halogen-containing salt to the sorbent. The presence of chloride significantly promotes the Hg-capture, possibly due to the formation of mercury chloride complexes.
According to one embodiment of the invention, the steps for making surface-dispersed metal sulfide include: mixing a metal salt with a bentonite substrate particle by grinding or milling; adding and mixing, preferably by grinding, a sulfide salt with the metal salt and substrate particle mixture; and drying the mixture. In certain embodiments, the resultant material is optionally milled to the desired particle size. For low metal sulfide loading, the sulfide can be added by incipient wetness as described below. For high metal sulfide loading, the sulfide can be added by solid-state grinding.
While embodiments of the present invention should not be limited by a particular theory of operation or scientific principle, it is believed that the metal salt (e.g., CuCl₂) reacts with sulfide (e.g., Na₂S) in situ to form a metal sulfide (for example, CuS) on the surface of the substrate, particles (for example, kaolin clay). The formation of the metal sulfide can occur either by contact via incipient wetness of the reaction or said solid-state reactive grinding. Since most metal salts and sulfides are crystal hydrates, for high metal sulfide loading, the water released from the chemicals during mixing is sufficient to moisten the mixture. As an example, the following reaction,
CuCl₂.2H₂O+Na₂S.9H₂O→CuS+2NaCl+11H₂O,
demonstrates that no additional water is required to disperse CuS in the mixture.
The metal salts used according to method embodiments include any metal salt that can release a metal ion with any oxidation states when the salt contacts a sulfide salt and thereafter forms water insoluble metal sulfide on the surface of a substrate. The metal includes alkaline earth metals and the metals that have an atomic number of between 21 and 30, between 38 and 50, and between 56 and 79 and combinations thereof. Examples of metals include copper, titanium, tin, iron, and manganese. A presently preferred metal ion is Cu⁺²and presently preferred salts are nitrate, chloride, sulfate, and acetate and combinations thereof. The loading level of metal is between about 0 and 100 weight percent, preferably between about 1 and 50 weight percent, and most preferably between about 1 and 20 weight percent.
Any sulfide precursor that forms the S⁻²anion can be used in accordance with embodiments of the invention. This includes, but is not limited to, Na₂S and (NH₄)₂S. In a specific embodiment, the sulfide precursor is Na₂S. Sulfide loading level can be stoichiometric (1:1 atomic ratio) or different than that of the metal ion.
Dispersion of the metal sulfide can be accomplished by any method as long as the metal sulfide is well dispersed on the surface of the substrate. Such methods include, but are not limited to, incipient wetness, solid-state mixing, spray-drying, sprinkling of solution on the solid, precipitation, co-precipitation, etc. Detailed embodiments use solid-state reactive grinding for high metal sulfide loading, incipient wetness for low metal sulfide loading, or a combination of grinding and incipient wetness. The order of adding the metal salts and sulfide salts can be altered, e.g., the sulfide salt can be added to the substrate first followed by addition of the metal salt. The metal sulfide can be added to the substrate one salt at a time (e.g., add CuCl₂first, followed by adding Na₂S), two salts at the same time (e.g., co-precipitation), or directly mixing a fine metal sulfide powder with the substrate.
Additional steps according to embodiments of the invention may include drying and milling the sorbent. Drying may be accomplished by any means such as static, spray-drying, microwave drying, or on a moving belt at a temperature in the range of about 25° and 200° C. for 0 to 15 hours. In detailed embodiments, drying occurs within the temperature range of about 60° and 150° C. In further detailed embodiments, drying occurs in the range of about 90° and 140° C. The sorbent can optionally be milled to an average particle size below about 80 μm, preferably below about 40 μm.
Without intending to limit the invention in any manner, the present invention will be more fully described by the following examples.

EXAMPLES

Several samples were prepared in accordance with the methods for manufacturing sorbent substrates described above. Table 1 lists the sample number, the sulfide salt formed on the surface of the sorbent (D), the sorbent substrate (A), the metal salt (B), and the precursor sulfide salt (C). The last column of Table 1 indicates the order of mixing of each of the ingredients. For example, A-B-C indicates that the metal salt (B) was added to the substrate (A) first, and then precursor sulfide salt (C) was added to the mixture.
For example, sample ECS22, 1.18 g of CuCl₂.2H₂O was mixed with 10.0 g of bentonite by a thorough solid-state grinding. Then 1.67 g of Na₂S.9H₂O was dissolved in de-ionized water and added to the solid mixture by what is termed herein as an incipient wetness process, in which the solution was added drop-wise to the solid mixture which was stirred rigorously. The resultant moistened solid was wet enough to completely disperse CuS on the bentonite, but dry enough so that the paste did not flow. The moistened paste was then dried at 105° C. in air overnight and milled to a particle size of D₉₀<10 μm.
For sample ECS24, 2.44 g of CuCl₂.2H₂O was mixed with 10.0 g of bentonite by a thorough solid-state grinding Then, 2.91 g of Na₂S.9H₂O powder was added in by another thorough solid-state grinding. The moistened paste was then dried at 105° C. in air overnight and milled to a particle size of D₉₀<10 μm.
The remaining samples were prepared in a very similar way as the above two samples. The source and purity of the raw chemicals are listed in Table 2, and the main characteristic properties of the sorbent substrates are listed in Table 3.

TABLE 1

Summary of the Hg-removal sorbent
composition and preparation methods

Example	Metal Sulfide (D)	Substrate (A)	Metal Salt (B)	Precursor Sulfide (C)	Prep

ECS01	CuS (10% Cu basis)	10 g Na-bentonite	2.95 g CuCl₂•2H₂O	4.16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS02	CuS (10% Cu basis)	10 g Na-bentonite	4.32 g CuSO₄•5H₂O	4.16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS03	CuS (20% Cu basis)	10 g Na-bentonite	5.9 g CuCl₂•2H₂O	8.33 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS04	Fe₂S₃(6% Fe basis)	10 g Na-bentonite	3.12 g FeCl₃•6H₂O	16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS06	CuS (20% Cu basis) +	10 g Na-bentonite	5.9 g CuCl₂•2H₂O	2.70 g Na₂S•9H₂O	A-B-C
	CuCl_2,		solid-state grinding	solid-state grinding
ECS07	CuS (10% Cu basis) +	10 g Na bentonite	2.95 g CuCl₂•2H₂O	0.56 g sulfur powder	A-B-C
	sulfur powder		solid state grinding
			heated to 150° C. for 0.5 hr
ECS08	MnS (4% Mn basis)	10 g Na-bentonite	1.71 g MnCl₂•4H₂O	4.16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS09	S (10% S basis)	10 g Na-bentonite	0	1.11 g sulfur powder	A-B
ECS10	none	10 g Na-bentonite	0	0	A
ECS11	MnS₂(3% Mn basis)	10 g Na-bentonite	1.37 g KMnO₄	8.33 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS12	KMnO₄(4% Mn basis)	10 g Na-bentonite	1.37 g KMnO₄	solid-state grinding	A-B
			solid-state grinding
ECS13	AgNO₃(1% Ag basis)	10 g Na-bentonite	0.159 g AgNO ₃	0	A-B
ECS15	CuS (20% Cu basis)	10 g Na-bentonite	1.48 g CuCl₂•2H₂O	2.08 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS20	CuS (15% Cu basis)	10 g bentonite	4.43 g CuCl₂•2H₂O	6.25 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS21	CuS (10% Cu basis)	20 g bentonite	0.90 g CuCl₂•2H₂O	8.33 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
				(food processor blending)
ECS22	CuS (4% Cu basis)	10 g bentonite	1.18 g CuCl₂•2H₂O	1.67 g Na₂S•9H₂O	A-B-C
			solid-state grinding	incipient wetness
ECS23	CuS (4% Cu basis)	10 g bentonite	1.72 g CuSO₄•5H₂O	1.67 g Na₂S•9H₂O	A-B-C
			solid-state grinding	incipient wetness
ECS24	CuS (7% Cu basis)	10 g bentonite	2.44 g CuCl₂•2H₂O	2.91 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS25	CuS (7% Cu basis)	10 g bentonite	3.01 g CuSO₄•5H₂O	2.91 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS26	CuS (10% Cu basis)	20 g bentonite	5.9 g CuCl₂•2H₂O	8.33 g Na₂S•9H₂O	A-B-C
			solid-state grinding	blending by blender
ECS27	CuS (10% Cu basis) +	10 g bentonite	4.32 g CuSO₄•5H₂O +	4.161 g Na₂S•9H₂O	A-B-C
	MgCl₂		1.00 g MgCl₂	solid-state grinding
			solid-state grinding
ECS28	CuS (10% Cu basis) +	10 g bentonite	4.32 g CuSO₄•5H₂O +	4.16 g Na₂S•9H₂O	A-B-C
	MgCl₂		2.00 g MgCl₂	solid-state grinding
			solid-state grinding
ECS29	CuS (10% Cu basis) +	10 g bentonite +	4.32 g CuSO₄•5H₂O +	4.16 g Na₂S•9H₂O	A-B-C
	MgCl₂	5.00 g MgCl₂	solid-state grinding	solid-state grinding
ECS30	CuS (10% Cu basis)	10 g bentonite	2.16 g CuSO₄•5H₂O +	4.16 g Na₂S•9H₂O	A-B-C
			1.48 g CCl₂•2H₂O	solid-state grinding
			solid-state grinding
ECS31	CuS (10% Cu basis)	10 g bentonite	3.24 g CuSO₄•5H₂O +	0.74 g Na₂S•9H₂O	A-B-C
			1.48 g CCl₂•2H₂O	solid-state grinding
			solid-state grinding
ECS32	CuS (10% Cu basis)	10 g bentonite	3.45 g Cu(acetate)₂•H₂O	16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS33	CuS (7% Cu basis)	10 g bentonite	2.42 g Cu(acetate)₂•H₂O	2.91 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS34	CuS (4% Cu basis)	10 g bentonite	1.38 g Cu(acetate)₂•H₂O	1.66 g Na₂S•9H₂O	A-B-C
			solid-state grinding	incipient wetness
ECS35	CuS (100% Cu basis)	10 g bentonite	4.02 g Cu(NO₃)₂• 5/2H₂O	16 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS36	CuS (7% Cu basis)	10 g bentonite	2.82 g Cu(NO₃)₂• 5/2H₂O	2.91 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS37	CuS (4% Cu basis)	10 g bentonite	1.61 g Cu(NO₃)₂• 5/2H₂O	1.66 g Na₂S•9H₂O	A-B-C
			solid-state grinding	incipient wetness
ECS38	CuS (10% Cu basis)	5 g bentonite	0.83 g CuS	—	A-B
			solid-state mixing
ECS39	CuS (20% Cu basis)	5 g bentonite	1.66 g CuS	—	A-B
			solid-state mixing
ECS40	CuS (1% Cu basis)	10 g Bentonite	0.295 g CuCl₂•2H₂O	0.416 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS41	CuS (2% Cu basis)	10 g Bentonite	0.59 CuCl₂•2H₂O	0.832 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS42	CuS (3% Cu basis)	10 g Bentonite	0.885 g CuCl₂•2H₂O	1.248 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS43	CuS (1% Cu basis)	10 g Bentonite	0.433 g CuSO₄•5H₂O	0.416 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS44	CuS (2% Cu basis)	10 g Bentonite	0.866 g CuSO₄•5H₂O	0.833 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS45	CuS (3% Cu basis)	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-C-B
			incipient wetness	solid-state grinding
ECS46	CuS (4% Cu basis) +	10 g Bentonite	1.732 g CuSO₄•5H₂O	1.664 g Na₂S•9H₂O	A-B-C
	MgCl₂		0.100 g MgCl₂	incipient wetness
			solid-state grinding
ECS47	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	MgCl₂		0.100 g MgCl₂	incipient wetness
			solid-state grinding
ECS48	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	MgCl₂		0.200 g MgCl₂	incipient wetness
			solid-state grinding
ECS49	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	MgCl₂		0.500 g MgCl₂	incipient wetness
			solid-state grinding
ECS50	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	NaCl		0.100 g NaCl₂	incipient wetness
			solid-state grinding
ECS51	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	NaCl		0.200 g NaCl₂	incipient wetness
			solid-state grinding
ECS52	CuS (3% Cu basis) +	10 g Bentonite	1.299 g CuSO₄•5H₂O	1.248 g Na₂S•9H₂O	A-B-C
	NaCl		0.500 g NaCl₂	incipient wetness
			solid-state grinding
ECS57	Fe₂S₃(10.0% S basis)	10 g Na-bentonite	6.24 g FeCl₃•6H₂O	8.33 g Na₂S•9H₂O	A-B-C
			solid-state grinding	solid-state grinding
ECS58	CuS (5.0% S basis)	10 g bentonite	4.33 g CuSO₄•5H₂O	4.16 g Na₂S•9H₂O	A-B-C
			co-precipitation	co-precipitation

TABLE 2

Source and purity of the raw chemicals

Raw Chemical	Supplier or Source	Purity

CuCl₂•2H₂O	Alfa-Aesar	99+%
CuSO₄•5H₂O	Alfa-Aesar	98.0-102.0%
Cu(Acetate)₂•H₂O	Aldrich	98+%
Cu(NO₃)₂•5H₂O	Aldrich	98%
Na₂S•9H₂O	Alfa-Aesar	98.0-103.0%
FeCl₃•6H₂O	Aldrich	98+%
MnCl₂•4H₂O	Aldrich	98+%
MgCl₂	Alfa-Aesar	99%
KMnO₄	Alfa-Aesar	99.0%

TABLE 3

Source Treatment and Propeties of Bentonite Substrate

	Supplier or Source	Black Hill Bento
	Pre-treatment	Milled to a particle size of D₉₀<10 μm
	N₂Surface Area	35.4 m²/g
	N₂Pore Volume	0.11 cc/g
	XRD Crystallinity	Crystalline
	Structure	Layered - Interlayer accessible
	Pore Diameter	12.2 nm

Main Chemical Composition (%)

SiO₂	Al₂O₃	Fe₂O₃	K₂O	MgO	Na₂O	TiO₂	CaO

67.50	20.20	3.20	0.51	2.06	2.64	0.23	2.42

The formation of metal sulfide (CuS) on the substrate particles is evidenced by the darkening of the paste and the heat release due to the following process:
CuCl₂.2H₂O+Na₂S.9H₂O→CuS+2NaCl+11H₂O
Spectroscopic data also supports this observation of CuS formation on the substrate. Infrared (IR) spectra of bentonite-supported chemical CuCl₂, Na₂S, CuS, NaCl, and three sorbents that were prepared from the solid-state grinding of CuCl₂.2H₂O, Na₂S.9H₂O and bentonite, as described above, confirmed that the three sorbents (2.5, 5.0 and 10.0% S basis) contain predominantly CuS and NaCl and very little CuCl₂and Na₂S precursors. X-ray powder diffraction provided similar evidence of the formation of CuS. X-ray elemental microprobe also showed that the space distribution of Cu on the surface of substrate particles is identical to that of sulfur, indicating again the formation of CuS on the substrate surface.

Mercury Removal Evaluation

The Hg-removal performance of the sorbents described above was evaluated by an in-flight test, which is a commonly used fast screening method used to rank sorbents. The measurement includes the total mercury removal from simulated flue gas (Hg]_{injection start}−[Hg]_{injection stop})/[Hg]_{injection start}*100%) and the kinetics, or rate, of the Hg-capture (−d(Hg %)/dt). Both parameters are important since the former measures the total Hg-capture while the latter is directly related to the strength of adsorption sites on the sorbent material. Thus, good sorption may be characterized by both high and fast Hg removal at fixed sorbent injection rate.
Mercury concentration at the outlet of the sorbent injection chamber was measured using an Ohio Lumex cold vapor atomic absorption instrument. The simulated flue gas consisted of 1600 ppm SO₂, 400 ppm NOx, 12% CO₂, 6% oxygen, 2% water, and balanced by nitrogen. The flow rate was 944 sccm, sorption pressure 12 psia, and sorption temperature 140.5° C.
Table 4 summarizes the in-flight test results of metal sulfide/substrate sorbents from Table 1. All of the sorb ent samples were sieved through a 325 mesh sieve prior to the injection. In-flight test results were compared to samples made in accordance with the ion exchange methods disclosed in U.S. Pat. No. 6,719,828, which demonstrated a total Hg removal between 70 to 90% (injection rate 6-10 lb/MMacf).
The data in Table 4 shows that the CuS/bentonite samples prepared by incipient wetness or solid-state grinding has the same or better Hg-removal than sorbents made by ion exchange methods.

TABLE 4

Selected In-Flight Test Results

				Rate
		Injection	Hg	d
Sample		Rate	removal	(Hg %)/
#	Sorbent*	(lb/MMacf)	(%)	dt

ECS-01	CuS/bentonite	8.4	93	0.133
	10% Cu basis
ECS-02	CuS/bentonite	8.4	93	0.133
	10% Cu basis
	CuSO₄precursor
ECS-03	Cu/bentonite	9.9	89	0.264
	20% Cu basis
ECS-04	Fe₂S₃/bentonite	7.4	17	0.044
	10% S basis
ECS-06	(CuS + CuCl₂)/bentonite	8.2	62	0.067
	20% Cu basis; Cu:S = 2:1
ECS-07	(CuS + S)/bentonite	9.0	90	0.406
	10% Cu basis; Cu:S = 1:2
ECS-08	MnS/bentonite	5.9	73	0.030
	5% S basis
ECS-09	Sulfur/bentonite	5.5	32	2.5
	10% S basis
ECS-10	Bentonite, As-is	5.0	27	0.021
ECS-11	MnS₂/bentonite	8.6	47	0.086
	10% S basis
ECS-12	KMnO₄/bentonite	9.3	9	2.8
	6% Mn basis
ECS-15	CuS/bentonite	11.6	90	68.3
	20% Cu basis
	dried at 150° C.
ECS-21	CuS/bentonite	8.2	92	29.1
	10% Cu Basis
ECS-22	CuS/bentonite	6.2	93	43.0
	4% Cu basis
ECS-23	CuS/bentonite	7.2	76	24.6
	4% Cu basis, CuSO₄
ECS-24	CuS/bentonite	8.8	89	22.3
	7% Cu basis
ECS-25	CuS/bentonite	7.3	62	22.0
	7% Cu basis, CuSO₄
ECS-26	CuS/bentonite	5.7	94	30.8
	10% Cu basis
	(blender scale-up)
ECS-27	CuS/bentonite	7.7	94	47.4
	10% Cu + 10 g MgCl₂
ECS-28	CuS/bentonite	10.3	94	20.5
	10% Cu + 2.0 g MgCl₂
ECS-29	CuS/bentonite	7.8	36	6.2
	10% Cu + 5.0 g MgCl₂
ECS-30	CuS/bentonite	7.5	93	47.4
	10% Cu, CuSO₄/CuCl₂= 1.5
ECS-31	CuS/bentonite	7.0	91	39.5
	10% Cu, CuSO₄/CuCl₂= 2.2
ECS-32	CuS/bentonite	9.9	31	28.5
	10% Cu, Cu acetate
ECS-33	CuS/bentonite	7.8	59	24.6
	7% Cu, Cu acetate
ECS-34	CuS/bentonite	11.5	23	18.9
	4% Cu, Cu acetate
ECS-35	CuS/bentonite	4.5	58	24.0
	10% Cu, Cu nitrate
ECS-36	CuS/bentonite	6.3	54	14.6
	7% Cu, Cu nitrate
ECS-37	CuS/bentonite	5.3	55	19.6
	4% Cu, Cu nitrate
ECS-38	CuS/bentonite	3.2	19	10.0
	10% Cu, CuS
ECS-39	CuS/bentonite	3.6	32	14.1
	2% Cu, CuS
ECS-40	CuS/bentonite	6.1	93	31.9
	1% Cu, incipient wet
ECS-41	CuS/bentonite	6.0	93	31.0
	2% Cu, incipient wet
ECS-42	CuS/bentonite	7.0	98	28.0
	3% Cu, incipient wet
ECS-43	CuS/bentonite	5.9	36	12.3
	1% Cu, CuSO₄, incipient wet
ECS-44	CuS/bentonite	6.9	31	12.0
	2% Cu, CuSO₄, incipient wet
ECS-45	CuS/bentonite	75.	40	118.4
	3% Cu, CuSO₄, incipient wet
ECS-46	CuS/bentonite	7.3	87	27.9
	4% Cu, CuSO₄+ 0.1 gMgCl₂
ECS-47	CuS/bentonite	7.7	83	31.1
	3% Cu, CuSO₄+ 0.1 gMgCl₂
ECS-48	CuS/bentonite	7.5	80	33.5
	3% Cu, CuSO₄+ 0.2 gMgCl₂
ECS-49	CuS/bentonite	6.2	81	27.1
	3% Cu, CuSO₄+ 0.5 gMgCl₂
ECS-50	CuS/bentonite	6.4	87	30.5
	3% Cu, CuSO₄+ 0.1 gNaCl
ECS-51	CuS/bentonite	6.7	83	27.4
	3% Cu, CuSO₄+ 0.2 gNaCl
ECS-52	CuS/bentonite	5.6	89	18.1

(*unless stated otherwise, CuCl₂•2H₂O and Na₂S•9H₂O precursors were used)

FIGS. 1 and 2 further demonstrate the optimized copper loading and importance of the presence of chloride anion in the sorbent. As shown in FIG. 1, at the Cu loading of 4 wt %, the Hg-removal seems to have reached the maximum level. Samples were prepared by the incipient wetness and solid state grinding techniques described using the bentonite substrate and the precursors noted in Table 1. The copper loading level was varied between 0 and 10% and the Hg removal was measured as described above. Different metal precursors exhibited different Hg-removal efficiency. For the results shown in FIG. 1, only the sorbents made using a CuCl₂precursor provided Hg-removal of above 80%. The highest Hg removal was 98% at 3% Cu loading level, which is not shown in the Figures.
Adding a small amount of chloride, such as NaCl or MgCl₂, to copper sulfate significantly increases its Hg removal, shown in FIG. 2 where all sorbents have 3% Cu loading level on bentonite substrate loaded with copper sulfate precursors as described above and prepared by incipient wetness. FIG. 2 demonstrates that adding a small amount of chloride in the sorbents effectively enhances Hg-removal. This also has practical importance. For example, the use of pure chloride salt precursor can lead to corrosion of stainless steel reaction vessels. Therefore, the addition of mixed salts not only enhances mercury removal but reduces corrosion of equipment.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. For example, while the sorbents disclosed herein are particularly useful for removal of mercury from the flue gas of coal-fired boilers, the sorbents can be used to remove heavy metals such as mercury from other gas streams, including the flue gas of municipal waste combustors, medical waste incinerators, and other Hg-emission sources. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A method of removing pollutants from a flue gas stream comprising injecting a sorbent into a coal-fired boiler flue gas stream, the sorbent comprising bentonite particles having a metal sulfide dispersed on the surface of the particles by a grinding, milling or incipient wetness process.

2. The method of claim 1, wherein the pollutants include mercury.

3. The method of claim 1, wherein the metal sulfide is the reaction product of a metal salt and a sulfide salt.

4. The method of claim 3, wherein the reaction product is an in situ reaction product.

5. The method of claim 1, wherein the bentonite particles have an average size less than about 80 μm.

6. The method of claim 3, wherein the metal salts are salts selected from the group consisting of alkaline, alkaline earth metals and metals having an atomic number in the range of 21 to 30, 38 to 50 and 56 to 79, and combinations thereof.

7. The method of claim 6, wherein the metal salt includes a metal selected from the group consisting of copper, titanium, tin, iron, manganese and mixtures thereof.

8. The method of claim 6, wherein the metal salt is selected from the group consisting of nitrate, chloride, sulfate, acetate salts and mixtures thereof.

9. The method of claim 1, wherein the loading level of the metal sulfide is in the range of about 1 to about 20 weight percent.

10. The method of claim 3, wherein the sulfide salt is a sulfide precursor that forms a S²⁻ anion.

11. The method of claim 10, wherein the sulfide salt is selected from the group consisting of Na₂S and (NH₄)₂S.

12. The method of claim 1, wherein the metal sulfide is selected from the group consisting of copper sulfides, tin sulfides, manganese sulfides, titanium sulfides and iron sulfides.

13. A method of removing pollutants from a flue gas stream comprising:

preparing sorbent particles by mixing a solid metal salt with bentonite particles, adding a sulfide salt into the mixture using a grinding process, milling process or an incipient wetness process so that the metal salt and sulfide salt react to form a metal sulfide on the surface of the bentonite particles, and drying the mixture; and

injecting the sorbent particles into a coal-fired boiler flue gas stream.

14. The method of claim 13, wherein the method does not utilize ion exchange to form the metal sulfide on the surface of the particles.

15. The method of claim 13, wherein the metal sulfide forms in situ on the particles.

16. The method of claim 13, wherein the metal salts are salts selected from the group consisting of alkaline, alkaline earth metals and metals having an atomic number in the range of 21 to 30, 38 to 50 and 56 to 79, and combinations thereof.

17. The method of claim 13, wherein the sulfide salt is a sulfide precursor that forms a S²⁻ anion.

18. The method of claim 13, further comprising reducing the average particle of the particles to less than about 80 μm.