US3094846A - Treatement of earth strata containing acid forming chemicals - Google Patents

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June 25, 1963 TREATMENT OF EARTH STRATA CONTAINING ACID FORMING CHEMICALS Filed Feb. 27, 1961 C. E. PEELER, JR

3 Sheets-Sheet l l4 v V 1: l6 l6 l6 l6 1 I6 ACID BEARING SHALE )SILICA GEL V V V V V V 2s 2s 26 2e INVENTOR GLETUS E. PEELER, JR.

ATTORNEY 3 Sheets-Sheet 2 I INVENTOR GLETUS E. PEELER, JR.

ATTORNEY C. E. PEELER, JR

I o o 0 o o 0 0 0 0 0 0 0 0 0 0 O 0 WMM fF M June 25, 1963 TREATMENT OF EARTH STRATA CONTAINING ACID FORMING CHEMICALS Filed Feb. 27, 1961 June 25, 1963 Q PEELER, JR 3,094,846

TREATMENT OF EARTH STRATA CONTAINING ACID FORMING CHEMICALS Filed Feb. 27, 1961 3 Sheets-Sheet 5 I I v I INVENTOR CLETUS E. PEELER, JR.

ATTORNEY 3,094,846 TREATMENT OF EARTH STRATA (IONTAINKNG ACE) FORMING CHEMICALS Cletus E. Peeler, Jr., Painesville, Ohio, assignor to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware Filed Feb. 27, 1.961, Ser. No. 91,712 14 Claims. (til. 61-36) This invention relates to a new and improved method for in situ treatment of soil contaminated with acids and/ or acid-forming chemicals to minimize and/ or prevent leaching therefrom of objectionable chemicals with subsequent acid pollution of ground and surface waters.

. One of the most objectionable and heretofore difficult to control sources of large scale water pollution has been the leaching out of sulfuric acid or sulfuric acid containing chemicals from coal mines or the like in areas where air comes in contact with sulfide-containing minerals, notably pyrites (generally FeS or the like, marcasite associated with coal. In the United States the so-called acid mine drainage is responsible for considerable stream pollution and up to the present time there has been no generally applicable feasible method of combating this large scale drainage problem.

Part of the difticulty has been because acid mine drainage is a natural phenomenon not subject to uniform controls possible with many industrial wastes. Also, it is generally difficult to seal abandoned mines where much of the problem originates. In addition, because of differing geologic conditions, acid occurrence varies widely in regional and local areas even, for example, in coal pro-. ducing areas in. adjacent mines. Further, the weight of acid discharged mine water often varies from hour to hour and from day today.

' Generally the approach up to this time has involved one of several approaches, -i.e., to prevent Water entering the zones of acid-forming chemicals, to promote drainage of abandoned mines, and trying to segregate the sulfuric acid type materials. Unfortunately, for the reasons indicated, none of these approaches has been entirely satisfactory and the problem remains.

, Acid waters from mining, notably coal mining, are generally derived from several different sources: abandoned mines which are often difficult to treat, active underground mines, strip mines, or the like.

Chemically speaking, the difficulty arises in forming the so-called acid water because of iron sulfide, e.g.,' pyrite is oxidized by atmospheric oxygen, especially in the case of an abandoned mine where air entry is facilitated by the mine shaft or header and tunnels, to generate sulfuric acid in the soil which acid is carried out of the mine in the natural route by percolating or seeping ground water.

The chemical reaction involved is illustrated by the following equation:

2FeS +7O +2I-l O 2FeSO +2H SO Mine sealing programs (to prevent access of air) have been tried to minimize the drainage problem. However, this has not been entirely satisfactory because in many instances the oxygen contained in the sealed mine differs only slightly from that before sealing. Further, the non uniform nature of the earth itself often permits access of oxygen for continued acid formation. Accordingly, it will be appreciated that the problem of stream pollution via acid mine water with the associated ice lat area, in the United States pollution of the waters of the Ohio River Valley via mine acid contamination in the States of Illinois, Indiana, Kentucky, New York, Ohio, Pennsylvania, Virginia and West Virginia is a problem of great concern.

Generally the present invention comprises contacting the acid-forming chemicals, notably iron pyrites, in the soil, i.e., termed herein as in situ treatment, with an aqueous alkali metal silicate-containing solution to form by reaction with the sulfuric acid a silica gel dispersed throughout the soil formation. In practice the seeping or percolating ground water carrying the dissolved alkali metal silicate in following its natural course through the soil formations will as it normally contacts the sulfuric acid-forming chemicals set up reaction of the acid with the silicate to form within the soil a silica gel which effectively destroys the sulfuric acid at its source and precludes leaching out of the sulfuric acid or associated acid values. Continuous delivery of silicate solution provides a silicatesaturated soil formation whereby continuous conversion of the acid to a silica gel is established and maintained. Thus it Will be appreciated that this invention avoids the necessity of sealing a mine or the other heretofore proposed methods of combating this difiicult problem. Simply by providing in the ground water an adequate supply of an alkali metal silicate, as from a standpipe connected to one or more Well points in the area Where control is necessary, a convenient and practicable control is achieved.

Referring now to the accompanying drawing,

FIG. 1 illustrates in schematic form the treatment of soil in accordance with this invention;

FIG. 2 is an enlarged fragmentary view, partially in section, illustrating the in situ formation of a silica gel in accordance with this invention;

FIG. 3 illustrates the practice of the present invention in treating underground earth strata containing a horizontal opening such as a mine shaft;

FIG. 4 illustrates the formation of a silica gel barrier in earth strata around an open top cavity;

FlG. 5 shows another embodiment of the present invention in forming a silica gel material in and around an underground cavity.

FIG. 6 illustrates in schematic, sectional form, the formation around an open top cavity of a containing barrier formed via another embodiment of the present invention as shown more clearly by consideration together of FIGS. 6 and 7, the latter comprising a schematic plan view.

Reference is now made to the accompanying drawings which illustrate, in fragmentary and schematic form, the present invention. In FIGURE 1 there is illustrated one embodiment of the invention in treating mine overburden through which surface and/or ground water percolates downward and dissolves sulfuric acid from the acid bearing shale. As there shown, the practice of the present invention contemplates providing a source of aqueous alkali metal silicate, notably and preferably a standpipe or elevated tank, indicated generally at 10, disposed above the zone to be treated at a height suflicient to provide enough pressure head to force the silicate solution through a discharge pipe 12, lateral extending headers or pipes indicated generally at 14 and thence downward to perforated well points indicated generally at 16.

As illustrated in more detail in the fragmentary partial section view of FIGURE 2, through the holes 18 in the well point 16, the aqueous alkali metal silicate is discharged throughout a surrounding soil zone, in which zone the inevitable presence of oxygen and ground water generate with the mineral sulfides sulfuric acid. By the introduction of an alkali metal silicate in this manner, there is formed within the soil itself a silica gelwhich effectively chemically converts the sulfuric acid in situ to a a substantially innocuous alkali metal sulfate and effectively encapsulates any unreacted sulfides and/or acid within the gel, whereby further contamination by the acidcontaining water is precluded.

It will be appreciated that by providing the well points 16 at lateral and vertical intervals dictated by the geological formation encountered in the area to be treated and the extent of the treatment desired, it is possible from one convenient source to provide a continuous or, if desired, intermittent treatment of a large zone of acid-forming strata such as so-called acid bearing shale which heretofore has been exceedingly difiicult if not impossible effectively to treat.

Further, it will be appreciated that by providing a continuous supply of aqueous alkali metal silicate, in the strata itself, any earth heaving or other geological shifting of strata which might open fissures or other cavities would not circumvent or render inoperable the present invention because the water itself in following any newly opened passageways, would continue to carry the alkali met-a1 silicate solution into contact with new sites of acid formation whereby the encapsulation and conversion to silica gel of the sulfuric acid and associated materials effectively will continue.

As shown in FIGURE 3, again in schematic form with certain parts broken away and a somewhat simplified application illustrated for the purposes of clarity, a standpipe 20 provided with a discharge 22 and laterally extending headers 24 connected downwardly disposed well points indicated generally at 26 are provided. Again, as illustrated in FIGURE 2 the introduction of an alkali metal silicate under the hydrostatic pressure (or if desired at a higher pressure generated by a pump (not shown) included in the fluid flow system at any convenient point) permeates and saturates the porous earth strata containing the acid-forming chemicals whereby the inevitable reaction of oxygen, water and sulfide minerals, notably pyrites, is met with the presence of alkali metal silicate whereby, as indicated hereinbefore, the sulfuric acid is converted substantially as it is formed into a silica gel which elfectively entraps any as yet unreaoted acid or aicd-formers discharge of acid values through perco-- lating water is avoided.

As further indicated in FIGURE 3, well points 26 may be of such a length and include, if desired, lower imperferate portions 28 which can extend through a transversely extending mine shaft or other opening and yet provide in the strata beneath the transverse tunnel 'or shaft a further impregnation of the soil with the alkali metal silicate solution from openings in the lower portion of the well point. It will also be appreciated that such extended well points, .designated generally with the numeral 27, may be disposed outside of the vertical walls of the horizontal shaft and may in such instance or otherwise if desired be perforated for substantially their entire length whereby a more extended zone of alkali metal silicate impregnation can be provided.

Referring to FIGURE 3, for the purpose of illustration there is shown a discharge stream 32 of mine water which otherwise would be contaminated with sulfuric acid, typically to the extent of about 600 to 1250 ppm. (equivalent CaCO and having a pH from about 3 to 5. Via the practice of this invention as indicated the acid content of such a stream is substantially if not completely eliminated, as FIGURE 4 illustrates, again in a schematic form partially in section, how a series of well points or drilled openings can be arranged, with a suitable series of points indicated generally at 40, can be connected to a suitable header to provide a substantially uniform alkali metal silicate saturated zone surrounding an acid-containing cavity 44 whereby a surface pool may be effectively treated via this invention against undesired downward and lateral acid seepage.

FIGURE 5 illustrates, again in schematic form, how the practice of the present invention is advantageous in a schematically indicated well point 46 disposed in the earth extends into a cavity 48;- When filled with an alkali metal silicate solution, the surrounding soil is impregnated with the :alkali metal silicate whereby acid therein is converted to the silica gel as pointed out hereinbefore. FIGURES 6 and 7 illustrate in schematic form another embodiment of the practice of this invention wherein a central cavity 54, is surrounded by a series of drilled holes 50 and 52, through which an alkali metal silicate is introduced to generate via reaction with contained acid a surrounding zone of silica gel 56.

As used throughout the specification and claims, the expression alkali metal silicate is intended to include various alkali metal silicates including the silicates of sodium, potassium, cesium and rubidium. A preferred type of alkali metal silicate is sodium silicate because it is more readily available commercially and (for that reason is particularly referred to hereinafter. It is preferred to employ a sodium silicate having an SiO :Na O ratio within the range from about 2 to 4:1. The alkali metal silicate preferably is introduced into the soil in the form of an aqueous solution as is the form of many commercially available or, preferably, somewhat diluted sodium silicates. It, of course, also is contemplated that the silicate employed may comprise a mixture of commercially available aqueous sodium silicates, or other silicates, as well as in certain instances of solid alkali metal silicates either alone or in combination. in manyinstances, advantageous results are obtained using a silicate solution containing 10 to 30% by volume, e.g., (l1-12 B. 20 C.) of alkali metal silicate, preferably sodium silicate.

The following are illustrative of suitable commercially available sod1um silicates:

Percent Sp. Gravity S10z:NazO Ratio (Wgt. Ratio) Solids Baum) 7 (Average) In order that those skilled in the art may more completely understand the present invention and the preferred methods by which the same may be carried into eifect, the following specific examples are offered:

Example I To illustrate the practice of this invention there are combined 5 g. of FeS (pyrites) with 25 ml. of a 10%: aqueous solution of sodium silicate (10 parts by volume of a commercial aqueous sodium silicate having an Na O:SiO ratio of 1:322, a solids content of 38.35%, and a 4-1.5 B. value at 20 C. and a viscosity of 75 second Stormer with parts by volume of water). To this silicate solution (solely to insure the presence of sufficient oxygen to cause reaction to form the desired gel) is added three drops of a 30% hydrogen peroxide solution. There results a rigid gel which both converts the sulfuric acid to silica gel and also seals 01f, encapsulates or encloses within the gel itself any remaining as yet unreacted source of acid formation.

Example II Example III Further to illustrate the practice of the present invention there are combined at room temperature sulfuric acid and a sodium silicate having the following formula Na SiO -2.33SiO -23.36H O 'to form a rigid gel having the composition Na SO +3.3Si(OH) 17 .7H O. Thus it will be appreciated that the chemical reaction converts the sulfuric acid into sodium sulfate which, together with any unreacted sulfuric acid, is physically entrapped within the formed silica gel. Expressed more simply with a different silicate, the reaction is as follows:

By the foregoing examples it will appear and be understood that the practice of the present invention affords I a dual approach to the problem of dealing with the acid- Example IV To a sample of acid-bearing shale is added an aqueous sodium silicate. There results through chemical reaction with the FeS content of the shale, and from the encapsulation effect of the gel a complete conversion of acid into the gel material and/ or enclosure of any as yet unreacted material within the gel.

While reference has been made to iron sulfide (FeS as illustrative of the principal sulfuric acid-forming ingredient of the mineral sulfides encountered in mining operations, it will be understood, of course, that other sulfides as well are at times found in such strata, e.g., PbS, CuFeS Cu S, CuS, Cu AsS Cu SbS Cu SbS Cu AsS- ZnS, NiS(NiFe) S Sb S HgS, M08 CdS, V 8 Bi S CoAsS, or the like.

It will be appreciated that the practice of this invention solves a long-standing exceedingly difiicult problem by providing in ground water a dilute alkali metal silicate which in following its normal often indescribable path through the acid-producing earth strata carries with it the means to react when necessary to be converted into a rigid acid neutralizing and encapsulating silica gel by reaction with acid as it is formed by oxid-ative and hydrolytic reactions of the mineral sulfided in the earth.

While in most instances where formation of sulfuric acid occurs there is sufficient oxygen present, in the event that additional oxidation effect is desirable, an oxidizing agent such as hydrogen peroxide, an alkali metal peroxide, e.g., sodium peroxide, an alkali metal hypochlorite, e.g., sodium hypochlorite, an alkali metal permanganate such as potassium permanganate or the like, or an alkali metal bichromate such as sodium bichromate can be incorporated into the silicate, the proportions depending on the specific problems encountered. In general about up to about 25% by weight of oxidizing agent (based on the silicate solution used) is sufficient.

-It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. The method of treating earth strata containing naturally present acid-forming chemicals which comprises providing within the soil an alkali metal silicate convertible therein by said chemicals to a silica gel.

2. The method according to claim 1 wherein the alkali metal silicate is sodium silicate.

3. The method of treating earth strata containing sulfide minerals convertible to sulfuric acid by exposure to oxygen and water which comprises providing in such strata an alkali metal silicate whereby conversion of the sulfuric acid in situ physically and chemically into a silica gel is achieved.

4. The method according to claim 3 wherein an aqueous alkali metal silicate is continuously provided in said strata.

5. The method according to claim 3 wherein said strata are saturated with an aqueous alkali metal silicate containing solution.

6. The method according to claim 3 wherein the strata containing sulfuric acid is treated with an alkali metal silicate to prevent seepage therefrom of said water.

7. The method of treating an earth stratum in or adjacent a coal mine, said stratum containing at least one sulfide mineral convertible therein to sulfuric acid by available oxygen and water, which comprises supplying into said stratum an aqueous alkali metal silicate solution whereby a dilute :alkali metal silicate is placed in immediate contact with ground water containing sulfuric acid in proportions at least sufficient to cause formation of a silica gel by reaction with said sulfuric acid thus chemically and physically precluding further movement of said sulfuric acid in said ground water.

8. The method of preventing stream pollution which comprises contacting with an alkali metal silicate acid bearing strata through which water feeding said stream percolates.

9. The method of forming a gel which comprises reacting FeS with a dilute aqueous solution of sodium silicate containing a small amount of hydrogen peroxide.

10. The product of the method of claim 1.

11. The method according to claim 7 wherein a dilute silicate solution is continuously added to said strata.

12. The method of treating earth strata containing acid-forming chemicals which comprises providing within the earth an oxidizing agent and an alkali metal silicate convertible therein by said chemicals to a silica gel.

13. The method .of treating earth strata containing sulfide minerals convertible to sulfuric acid by exposure to oxygen and water which comprises providing in such strata an alkali metal silicate, whereby conversion of the acid formed from such minerals in situ physically and chemically until a silica gel is achieved.

14. The product of the method of claim -11.

References Cited in the file of this patent UNITED STATES PATENTS 1,384,990 Campbell July 19, 1921 Il,428,633 Hofiman Sept. 12, 1922 2,207,759 Reimers July 16, 1940 2,580,409 Cornish Jan. 1, 11952 2,647,084 McDonald July 28, 1953 FOREIGN PATENTS 404,170 Germany Oct. 13, 1924 415,326 Great Britain Aug. 23, 1934 415,565 Great Britain Aug. 30, 1934 OTHER REFERENCES Condensed Chemical Dictionary, Fifth Edition, published by Reinhold (NY), 1956. Pages 1133, 980, 988, relied on.

Claims (1)

1. THE METHOD OF TREATING EARTH STRATA CONTAINING NATURALLY PRESENT ACID-FORMING CHEMICALS WHICH COMPRISES PROVIDING WITHIN THE SOIL AN ALKALI METAL SILICATE CON-

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