US 3359037 A
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United States Patent 3,359,037 PHOSPHATE SLURRY MINING PROCESS Richard L. Every and Ralph C. Hughes, Ponca City, Okla., assignors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware No Drawing. Filed Mar. 30, 1966, Ser. No. 538,572 6 Claims. (Cl. 2994) ABSTRACT OF THE DISCLOSURE Phosphate matrix comprising an agglomerate of discrete phosphatic particles cemented together by nonphosphatic materials is mined by contacting the deposit with a mineral acid to physically disintegrate the deposit, followed by withdrawing a suspension of particles in acid.
Related application This application is a continuation-in-part of US. Ser. No. 491,898, filed Sept. 30, 1965.
Disclosure This invention relates to a method of mining phosphate rock. In one aspect, the invention relates to removal of phosphate values from beneath an overburden, without the necessity of removing the overburden, by converting the values into a slurried condition in situ. In another aspect, the invention relates to a method for breaking down the physical structure of a phosphate-bearing matrix by leaching with a dilute mineral acid.
Phosphate ore or matrix occurs in deposits located in various spots around the world, and in differing conditions. For instance, some phosphate deposits form outcrops to the surface, While more commonly they are buried beneath varying amounts of overburden which itself has little or no value. The phosphate values have, in the past, been recovered by various techniques. Most common is removal of the overburden physically by such as draglines, followed by mining the matrix. This technique requires a substantial investment in earth-moving equipment, and becomes uneconomical where the deposlt underlies a thick overburden. Another method involves vertical shaft mining, with underground recovery of the matrix by manual and machine labor. Again, substantial equipment investment is required. A third technique which has been considered is the use of impingement water, applied through nozzles in a well bore penetrating the phosphate stratum, to place the matrix in a slurry condition so it can be pumped from the formation. Such a method is described, for instance, in US. Patent 931,057 to Goldsmith, issued August 17, 1909. This method suffers from a low recovery of matrix from the formation due to limitations in the radius in which an impingement stream 15 effective, estimated in the order of only 63% of the total matrix, and from a relatively large investment for the large-diameter boreholes required for the nozzles.
It is an object of this invention to provide method for recovering phosphate matrix from a formation which is under a thick overburden in an economical manner. Another object of this invention is to provide method for recovering phosphate matrix without investment in heavy earth-moving equipment. Another object of the invention is to provide method for recovering a high percentage of the total phosphate matrix contained beneath a thick overburden.
Other aspects, objects and the several advantages of this invention will become apparent upon study of this disclosure and the appended claims.
According to this invention, it has been discovered that phosphate matrix can be physically broken down into discrete, relatively fine particles of solid values by exposing the phosphate stratum to contact with a dilute mineral acid. The resulting fine particles of phosphate values can then be recovered from the formation by slurrying in a liquid which is pumped above ground for further beneficiation. Phosphate matrix can be considered as an agglomerate of discrete phosphatic particles cemented together by nonphosphatic materials, and the present invention succeeds primarily by chemically attacking the cementing material; any chemical attack on the phosphatic particles is incidental in the process of this invention.
Exposure of the phosphatic stratum to attack by dilute acid can be effected in several manners. The presently preferred method of this invention comprises penetrating the phosphate stratum with a plurality of well bores, producing a substantially horizontal planar fracture in the pay stratum, and passing the dilute acid solution into the fracture in contact with the phosphatic stratum. After the acid solution has had suflicient time to attack the formation, a slurrying fluid such as water or additional dilute acid is passed into a least one of the well bores at suflicient velocity to form a slurry with the loosened phosphatic particles, and the slurry is recovered by way of another of the well bores. The fracture can be immediately adjacent either the lower or the upper boundary of the phosphatic zone, or can be intermediate these boundaries at some level within the zone. It is also contemplated that a plurality of fractures at different levels throughout the stratum can be produced and, if desired, held open by a suitable propping agent as known in the art. These fractures can then be produced either simultaneously or sequentially, the advantage in the latter method residing in the fact that low volumes of slurrying fluid are required to maintain slurrying velocity up until such time as the various fractures become enlarged so as to merge with each other.
In addition to the fracture method just discussed, other methods are available for subjecting the formation to acid attack. When the pay formation is of sufficient permeability, the dilute acid solution can be introduced therein under pressure so as to penetrate by pressure and capillary action. Naturally-occurring fissures in the pay formation can also be used as passageways through which to spread the acid.
Regardless of the manner in which the dilute acid is put into contact with the formation, the next step comprises maintaining the acid in contact for a time suflicient to break down the structure of a desired portion of the matrix. The exposure time required depends on several factors, but primarily on the tightness of the formation and the chemical nature of the cementing material. Other factors include the type and strength of acid used, and the formation temperature. Simple laboratory tests can be used to determine the desirable exposure time, e.g., a core sample of the matrix is subjected to the acid, and the time to produce a desired loosening is measured. Typically, this initial exposure time will vary between about a half hour and a day, and more typically from about one to about six hours.
Among the acids suitable for practice of the present invention are sulfuric, phosphoric, nitric and hydrochloric. Hydrochloricacid is presently less preferred when the phosphatic product is destined for fertilizer use, since fertilizers generally require as low a chloride ion content as possible. Nitric acid is at present one of the more expensive per unit of acid strength and, for this reason, is less preferred. A typical material with which the phosphatic grains are cemented together comprises in part dolomite and calcite in varying proportions, and it has been found that sulfuric and phosphoric acids have different effects on this material. Sulfuric acid seems to attack the calcite only, while phosphoric attacks both the dolomite and the calcite. However, the overall effect on loosening of the granules appears to be very similar in both instances. The point to be observed here is that various phosphatic deposits react differently to treatment with different acids; simple laboratory experiment serves to determine which acid is most effective on a given matrix type. This concept of differential effect of various acids on underground formations can also be useful in the acidizing treatment of e.g. oil wells.
Mixtures of these acids can also be used to advantage. One source of a mixed dilute sulfuric-phosphoric acid is spent acid from the wet process of making phosphoric acid. Acid concentration can be varied within rather broad limits, although the acid strength should not be so high as to allow appreciable reaction with the phosphatic portion of the matrix while it is still underground. Acid (3011-. centrations as low as e.g. 0.1 weight percent sulfuric and even lower are useful in breaking up the stratum, and concentrations of 10 weight percent and more are also contemplated. A preferred range of acid concentration is between about 0.1 and about 5 weight percent.
Various additives can be used in the dilute acid. For example corrosion inhibitors can be used to reduce attack of the acid solution upon equipment used to introduce the solution to and recover the slurry from the formation. Foam inhibitors or foaming agents can also be used; the choice of which of these to use, if either, will depend upon the nature of the phosphate deposit, e.g., its organic content, and upon how much beneficiation of the matrix is desired in situ. This latter factor arises from the observation that treatment of phosphate values in accordance with the method of this invention results in considerable foam formation, particularly with higher acid concentrations. One well-known method of beneficiating phosphatic ores is froth flotation, whereby non-value materials such as silica are separated from phosphatic values by becoming either the sink or the float portion of a flotation process and, as stated, such a beneficiation can be at least partially effected within the formation by practice of this invention. Further, it is noted that there is an evolution of gases during treatment of the matrix with dilute acid in accordance with this invention. This fact aids in recovering the phosphate particles slurry from the recovery well because of the gas-lift effect provided.
The present invention is also advantageous in furnishing a phosphatic product from the recovery well which is readily amenable to further processing. The surface of the particles is to an extent pre-digested or at least conditioned for the subsequent treatment with acid which typically initiates the processing of phosphate rock.
It has further been discovered that the gas, primarly carbon dioxide, evolved by this process can be utilized to control the shape of the cavern being mined, i.e., to control the direction of attack of the mining solution on the formation walls. This effect is related to the solubility of carbon dioxide in the particular mining solution being used. When sufficient pressure is applied to the mining solution, during its attack on the formation, to prevent evolution of gas bubbles by maintaining the gas in solution, the acid mining solution exhibits a substantially higher rate of attack on the phosphatic formation than when the pressure is sufficiently low to allow gas bubble formation. This effect is utilized in the following manner when, for example, it is desired to selectively mine certain ore zones. Application of a high pressure, i.e., sufficient to prevent gas bubble evolution, results in the formation being attacked in a generally upward direction, while a pressure low enough to permit gas evolution results in the cavern spreading more in a lateral direction. Control over cavern size and shape can also be exercised by the proper placement of acid injection and slurry recovery points. Thus, by gradually raising these points, the less dense injection acid flows over the top of the heavier slurry and attacks the roof matrix. As
the roof is attacked, it falls down into the lower slurry and forces the acid even higher, resulting in a gradual increase in the cavern height. Thus, by proper control of acid concentration, injection and removal point locations, and applied pressure, the cavern size and shape can be controlled and selective mining of various zones is achieved.
The method of the present invention can be effected in either a batch or a continuous manner, although the latter is presently preferred. When accomplished batchwise, the formation is exposed to the dilute acid solution for the desired contact time, and then the mass of acid and loosened particles are displaced with additional dilute acid or another displacing fluid such as water, at a velocity sufficient to produce a slurry of the loosened particles. The stratum is then again filled with fresh dilute acid, and the exposure and flushing procedures are repeated. When effected continuously, the dilute acid is placed into contact with the formataion for the desired initial exposure time, and then additional dilute acid is continuously passed into the formation at a rate sufficient to maintain the desired slurry consistency. Upon obtaining the product slurry above ground, a simple settling operation suffices to settle out the product phosphate, and the dilute acid can be recycled to the formation after being re-fortified as necessary. In a large phosphate field, a plurality of wells can be used in a manner similar to secondary recovery of oil, e.g., in the so-called five-spot, inverted five-spot, nine-spot, etc., patterns. When production from a field ceases to be economical, the last amount of product can be recovered by passing in a chaser fluid, and then the resulting cavern can be used to dispose of the large volume of wastes typically associated with e.g. phosphoric acid manufacture, such as slimes and railings. This procedure has the additional advantage of providing permanent support for the overburden.
Further understanding of the invention will be gained by consideration of the following examples.
EXAMPLE 1 A phosphate deposit occurs which is about 45 feet thick on the average, and is underneath an average of about feet of overburden. The phosphatic deposit is of about 33% BPL and 43% insolubles. The formation is pentrated by two well bores about 75 feet apart, and a substantially horizontal planar fracture is produced between these wells at the bottom of the phosphatic stratum. Sulfuric acid of about 3 weight percent in water is pumped through one of the well bores so as to substan tially fill the fracture. After allowing a reaction time of about 2 hours, acid solution is continuously pumped into one of the well bores and a slurry of phosphatic particles is recovered from the other.
EXAMPLE 2 A vessel of about 1 foot internal diameter by one foot high was charged with tightly packed phosphate or like that described in Example 1. A passage about Ar-inch in diameter was made horizontally on a diameter of the vessel. The passage was filled with 1% sulfuric acid and allowed to sit for about one hour, after which time acid of the same strength was passed continuously through the fracture at a rate of about 30 -ml./hr. The slurry product levelled off after about 4 hours of such operation at a relatively constant 45 weight percent solids.
EXAMPLE 3 The solubility of carbon dioxide in dilute sulfuric acid has been determined as having a linear relationship between acid concentration and applied pressure. For an acid strength of about 0.4 weight percent, saturation is achieved at atmospheric pressure, while with an acid strength of 3 weight percent, saturation requires a pressure of about 138 p.s.i.g. When the process of Example 1, wherein 3 weight percent sulfuric acid is used, is operated with a pressure in the cavern of about 150 p.s.i.g., the cavern remains liquid-full in spite of the evolution of carbon dioxide, and the cavern growth progresses in an essentially upward direction. In contrast, when the same acid strength is used, but operating under a pressure of about 120 p.s.i.g., carbon dioxide is evolved in gaseous form, and cavern growth is primarily laterally.
Having thus described the invention by providing specific examples thereof, it is to be understood that no undue limitations or restrictions are to be drawn by reason thereof and that many variations and modifications are within the scope of the invention.
What is claimed is:
1. The method of removing phosphatic mineral from an underground deposit thereof, said deposit comprising an agglomerate of discrete phosphatic particles cemented together by nonphosphatic materials, which comprises passing into contact with said deposit in situ a solution of a mineral acid, maintaining said contact for a time sufiicient to effect physical disintegration of a portion of said nonphosphatic material, and withdrawing acid from said contact at a velocity sufficient to eifect suspension of solid phosphatic particles therein.
2. The method of claim 1 wherein said acid is selected from the group consisting of sulfuric, phosphoric, nitric, and hydrochloric.
3. The method of claim 2 wherein said acid has a concentration, prior to said contacting, between about 0.1 and about 5 weight percent.
4. The method of recovering phosphatic mineral from an underground deposit thereof, said deposit comprising an agglomerate of discrete phosphatic particles cemented together by nonphosphatic materials, which comprises passing a Well bore from the surface of the earth into said deposit, passing a second well bore from the surface of the earth into said deposit, exerting fluid pressure by Way of one of said well bores into said deposit so as to cause a fracture therein extending between said well bores, introducing a dilute acid solution by way of one of said well bores into said fracture, subsequently passing additional dilute acid solution by way of one of said well bores into said formation at a rate sufiicient to effect suspension of phosphatic solids therein, and recovering by way of a second of said well bores a product comprising discrete particulate phosphatic solids suspended in acid.
5. The method of claim 4 wherein said acid is selected from the group consisting of sulfuric, phosphoric, nitric and hydrochloric.
6. The method of claim 5 wherein said acid has a concentration, prior to said contacting, between about 0.1 and about 5 weight percent.
References Cited UNITED STATES PATENTS 3,070,361 12/1962 Pew W 29917 3,086,760 4/1963 Bays 299-5X 3,097,922 7/1963 Beetz 23-165 3,186,793 6/1965 Gillis et a1. 23-16S 3,278,233 10/1966 Hurd et al. 299-4 1,690,446 11/1928 Grant.
2,251,916 8/1941 Cross.
ERNEST R. PURSER, Primary Examiner.