# Patent

## Title
Method and apparatus for removing silver in the form of silver salts

## Abstract

This application is a continuation of application Ser. No. 08/312,341 filed Sep. 26, 1994, now abandoned.

## Background
This invention relates to a method of beneficiating kish graphite to high purity levels in an economic manner without undue destruction of the coarse graphite flakes.

Graphite, particularly flake graphite, finds use in many applications, including refractories, as a coating material in foundries, in brake linings, batteries, carbon brushes, pencils, propellants, as a precursor for expandable graphite, as a lubricant, and in other areas.

In refractories, flake graphite imparts high thermal conductivity and improved slag resistance. At operating temperatures, residual impurities are detrimental to the high temperature stability of the flakes. Because of this there is a tendency in refractory applications to use flake graphite of higher purity. For example, U.S. Pat. No. 4,912,068 describes the significant increase in hot strength made possible by using 99% pure flake graphite in magnesia-carbon brick. Flake graphite sold to certain battery markets requires a purity level of at least 99.9% carbon. Flake graphite sold to the expandable graphite market requires a purity of at least 96%. Large, high purity flakes command a higher price and are becoming increasingly desired. All of these tendencies are driving graphite producers to find new methods of producing high purity flake graphite. Further, it is important to note that the United States is totally dependent on foreign sources for this material. Therefore, the desire exists to find a suitable domestic source of this important raw material.

Flake graphite occurs naturally as flakes disseminated in metamorphosed siliceous or lime-rich sediments such as limestone, gneiss, and schist. In unweathered deposits it is difficult to liberate and separate the flakes from the ore without destroying their size. A number of processes are utilized to upgrade flake graphite to high purity levels, that is above 96% carbon, without significantly decreasing the size of the flakes.

In addition to naturally occurring graphite, there is a type of graphite referred to as "kish" graphite.

"Kish" is a general term for waste that forms during the iron production process and during the initial steelmaking process. It contains graphite in a flake form, iron, lime-rich slag, and other materials. Efforts have been made to recover flake graphite from kish and it is this graphite that has been now termed "kish graphite".

To date, efforts to improve the purity of kish graphite and still retain its coarse size have not been commercially satisfactory. They have generally followed the accepted procedures utilized for upgrading graphite-containing ores which require the use of multi-step chemical leaching. Such a process is described in the Bureau of Mines Preliminary Report No. 415 of January 1993 and the extension of this work by the joint effort of Inland Steel Industries and Asbury Graphite Mills, Inc. which was presented at the 86th Annual Meeting of the Air & Waste Management Association on Jun. 13-18, 1993. These techniques are able to upgrade the purity of the graphite to 95-96% carbon, but at significant cost due to the numerous leaching steps required. Further upgrading of the kish graphite to 98 or 99% purity has also been reported in the Bureau of Mines report noted above, but, again, with multiple steps using strong acids bearing the attendant cost of having all of the safety features involved for handling of the acids, recovering, and disposing of the same.

In short, it has not been possible to economically beneficiate kish graphitic to 99% graphitic carbon content.

It has been discovered that flake graphite from kish can be economically upgraded to 99%+ graphitic carbon content with an acceptable rate of recovery and minimal destruction of flake size.

Briefly, the present invention comprises the method of recovering high purity flake graphite from kish, comprising first forming a concentrate, screening said kish graphite concentrate to preferably retain about a +200 mesh fraction, light milling of said fraction, screening said milled fraction to preferably retain a +200 mesh fraction, and mixing said fraction in a dilute acid to form a weakly acidic pulp, subjecting said pulp to attrition, neutralizing said acidic pulp and recovering said kish graphite by flotation.

The invention also comprises the resultant graphite product and apparatus system for effecting such method as hereinafter more fully described.

FIGS. 1a and 1b are a flow diagram depicting the method and apparatus system of the instant invention.

It will be understood at the outset that the instant invention is directed to the beneficiation of kish graphite and the term "graphite" when used alone herein refers to the fact that the starting material is, in fact, kish graphite. Moreover, as has been noted, "coarse" flake graphite is most desired because of its higher value and as used herein that term is applied to graphite in which the bulk, at least 50%, most preferably at least about 70%, or more, by weight thereof is reported on a +100 mesh screen. The mesh sizes referred to herein are Tyler standard.

The essential steps of the instant invention in producing high purity flake graphite from kish involve treatment with a dilute acid followed by attrition milling. Other steps in the process include concentration, flotation, and the use of pebble milling. However, the ability to obtain high purity kish graphite is by combining a dilute acid solution with kish graphite and then subjecting the same to attrition. This facilitates the preparation of coarse high purity flake graphite while overcoming the disposal problems of concentrated acid.

The process and apparatus system are illustrated in the accompanying drawings in which the various pumps for moving the material are indicated by a "P" in a circle. Raw kish 10 obtained from a steel mill (which has previously been concentrated to any percent desired of kish graphite using conventional beneficiation techniques such as flotation) is added to a conventional conditioning tank 11 where it is admixed with conventional flotation aids, such as #2 fuel oil and MIBC (methyl isobutyl carbinol). The percent purity of the kish graphite at this point is not critical and the concentration of the feed is dictated by the economics of the operation and the particular source of the kish. The invention will be described in conjunction with a kish concentrate containing 35% graphitic carbon. If needed, and as shown in the drawing, the graphite concentrate 12 can be further concentrated in rougher flotation cell 13, to form a concentrate 14 containing greater than 70% graphitic carbon.

The resultant concentrate 14 as an overflow from cell 13 is then passed to a wet screen 15 of any conventional type used in ore handling to collect a +200 mesh fraction. The tails 16 from cell 13 and screen 15 are combined in tails sump 17 for further treatment as described later herein. Tails 16 consist of iron 18, which can be recovered in Circuit A for reuse and remaining tail material 19 which contains large amounts of lime.

The resulting +200 mesh graphite fraction is then subjected to light milling, such as in a conventional pebble mill 20, but under conditions such as to minimize degradation of the coarse graphite flakes. It is preferred to use small low density, non-metallic, spherical or short axis cylindrical grinding media, suitably one-half inch diameter available commercially as Cylpebs. Such milling of the +200 mesh graphite concentrate in mill 20 serves to effectively liberate the flakes from the kish so the resulting discharge from mill 20 may be easily concentrated to a level of 95% graphitic carbon. Importantly, the use of the small size grinding media minimizes the destruction of coarse flakes with a suitable milling time being five to twenty minutes.

The ground flake graphite concentrate 21 is again screened on any conventional wet screen 22 to collect a +200 mesh fraction 23. The iron 18 and remaining -200 mesh tails 19 that form the underflow from screen 22 are added to sump 17 for further treatment.

Concentrate 23 is then admixed with a dilute acid 24 in a conventional mixing tank 25. Although only one mixing tank is depicted, to ensure adequate acid treatment of all the material, two or more connected tanks can be used.

An important feature of the instant invention is that the acid need not be concentrated and, in fact, can be very dilute, at a concentration on the order of less than about 7.5%. It is preferred to use hydrochloric acid because of its ease of handling and lower cost. While not completely understood, it is believed that the weak acid functions simply to soften the cement-like bonds of the oxides attached to the graphite flakes. In order to therefore best utilize the weak acid, it was determined that this weak acid treatment phase should take place only after the concentrate had been physically cleaned to the greatest possible level. This not only acts to decrease the gangue material available to react with the acid and thereby decrease the acid requirement and/or its strength significantly, but also to enable the acid to get to the interface between the gangue and the platelets and most effectively liberate high purity graphite with minimal destruction of the kish graphite particles. The residence time of the +200 mesh fraction in the mixing tank with the acid will vary, depending upon feed grade and desired product grade.

The acidic slurry or pulp 26 is subjected to attrition, preferably with the addition of an attrition adjuvant (attritants) 28 in attrition device 27. Suitable attrition adjuvants are abrasive mineral sands, graphitic ore, or mixtures thereof. The abrasive mineral sands, such as silica sand, are preferred, particularly those having a specific gravity less than 3.0 and a mesh size between 10 and 40 mesh, most preferably one having 50% 16 mesh and 50% 24 mesh.

As used herein, the term "attrition" means a rubbing or abrasive action created by the individual graphite flakes or flakes and attritants impinging and rubbing against each other in an agitated aqueous pulp with sufficient force to physically dislodge the surface impurities. As previously noted, an attrition adjuvant 28 is preferably utilized to facilitate attrition, thereby minimizing the time required to liberate the flakes to 99+ wt. % purity with only minimal size reduction of coarse flakes.

Such attrition can be satisfactorily carried out using commercially available attrition cells such as a WEMCO attrition scrubber or a DENVER attrition scrubber. Most preferably, attrition cells having at least one set of opposing pitched impellers are used. Such machines are typically used to scrub glass-grade sand, to remove clays from iron ore, and to break up weakly bonded agglomerates. For such cleaning and deagglomeration uses the machines do not contain adjuvants.

The specific optimum conditions, such as impeller speed, pulp solids concentration, attrition time and the like, can be readily determined for any kish flake graphite concentrate and attrition machine combination by routine experimentation following the detailed description set forth herein. It will be evident that the optimum conditions are readily determined by noting the purity of the flake graphite obtained and the percentage of different mesh size flakes preserved after attrition. The point at which both are at a maximum is the optimum. The pulp solids, impeller speed, attrition time and the like can be varied until such optimum is obtained.

Attrition milling may be carried out at ambient temperature with the attrition time varying, dependent on such factors, as noted above. The acidic pulp slurry is preferably combined with the attrition adjuvant in a weight ratio of concentrate, attrition adjuvant, acidic water of about 1:2:2.5. This results in a slurry of 55% solids content. This ratio can vary to give a slurry that ranges from about 20% to 80% solids. The particular solids content is dictated by ease of liberation, the feed grade, and the desired purity level of the final product which can be determined by routine experimentation.

After the attrition has been completed, the attrited material 45 is subjected to a washing/diluting water spray 29 in any device such as a conventional screen belt separator 30 in order to remove the acid and to remove all the -200 mesh gangue material. The result is a flake graphite stream 31 containing graphite concentrate 32, attrition adjuvant 33, and +200 mesh gangue. Stream 31 is preferably subjected to two flotation cycles.

Stream 31 is first placed in a conventional conditioning tank 34 and admixed with conventional reagents used in floating ores, such as, again, #2 fuel oil and MIBC. The conditioned concentrate stream 35 is then passed through conventional flotation cells 36 and 37 where the overflow 38 containing the concentrated flake graphite is first treated on conventional filter 39 to remove as much liquid as possible and filtered concentrate 40 containing 99% graphitic carbon is then passed to conventional dryers, screens, and packaging apparatus 41, which are not shown.

Tails 42 from cell 37 are returned to mill 20 for further processing.

The acidic solution and tails 43 recovered from filter 30 are conveyed to the neutralization and waste disposal circuit B.

The drawings also illustrate circuits A, B and C that can be utilized and which add to the economies of the instant process. The first is circuit A involving treatment of the tails from the initial rougher column 13 and from wet screens 15 and 22. The combined tails are rich in iron and are fed from sump 17 to a conventional separator 50, such as a Whirlsizer, to produce a high gravity iron-rich underflow concentrate 51 that can be reused. The overflow 52, a material high in lime, is combined with filtrate 43 to neutralize the acid and further treated in Circuit B which provides water for use in the process and extracts solids which are landfilled.

Circuit B contains neutralizing tank 60, into which feeds 43, 52, and 73 are fed, and may need alkali material added thereto to guarantee a neutral effluent. The neutralized feed 61 from tank 60 is conveyed to a conventional thickener 62 which removes water 63 for return to the process water stream and produces a higher percentage solids tail 64, which is landfilled.

It will be evident that by using the weak concentration of acid, very little neutralization is required. Further, by providing for recovery of the iron-rich concentrate in the Whirlsizer, one is able to scavenge the maximum amount of iron from the process.

Auxiliary Circuit C comprises wet screen 70 which receives the adjuvant and tails 71 from flotation column 36. Screen 70 is sized to separate the attrition adjuvant (sand) 72 which is returned for use in the attrition device 27 and, as noted above, the tails 73 are combined with stream 43 from the acid filter 30 and stream 52 from Whirlsizer 50 and placed in neutralizing tank 60.

It is these unique and essential steps and apparatus combination that enable the economic recovery of the desired 98 and higher percent carbon coarse kish graphite flakes. As has been discussed above, previously it has been possible to obtain a 95% carbon graphite from kish, but this has been at significant cost in time and with the use of concentrated acids. Moreover, as noted, it was not possible to get reasonable recovery with carbon concentration above 95%.

This process removes essentially all the useful materials in the kish, maintains the coarseness of the kish graphite while permitting high purity at low cost, and minimizes the amount of kish that is disposed in landfills.

The invention will be further described in connection with the following examples which are set forth for purposes of further illustration only.

A series of four different tests were carried out on the same kish utilizing the apparatus system depicted in the drawings to determine optimum milling conditions. The kish contained 37% carbon and was concentrated to .about.70-75% carbon by rougher flotation and then fed to a pebble mill. Each of the four concentrated fractions was separately fed to the same mill. Milling time varied from 5 to 20 minutes. Each of the milled fractions was then tested to determine the graphitic carbon content and total carbon recovered. The results were as follows: Milling Time Example No. (min)     % Graphitic Carbon (Cg) ______________________________________ 1            5        93.1 2           10        95.0 3           15        94.9 4           20        95.5 ______________________________________

Most importantly, there was very little destruction of the coarse graphite flakes as the milling time increased from 5 to 20 minutes. The results were as follows: Mesh Size Percentages Mesh Size Percentages Before Milling After Milling Example No. +48    +80    +100 +200 +48  +80  +100 +200 ______________________________________ 1        34.4   33.6   7.5  24.5 33.2 31.7 7.0  21.2 2        36.2   31.3   8.8  24.1 35.2 30.1 7.9  21.9 3        33.7   33.3   7.6  25.4 32.6 32.0 7.2  23.2 4        33.2   32.8   7.7  26.2 32.1 31.5 7.3  24.5 ______________________________________

In these tests the same 37% Cg kish was concentrated by roughing to a .about.+70% Cg concentrate, but it was subjected to only mechanical (physical) enhancement.

One fraction (Example 5) was milled for 5 minutes, wet screened on a 200 mesh screen with the through's rejected, and the +200 mesh fraction, which was 89.8% graphitic carbon, was attritioned for 5 minutes at 2,700 r.p.m.'s without first being treated with dilute acid. The attritioned concentrate was then subjected to two flotation cleanings and the resultant +200 mesh material analyzed. The kish graphite recovery was .about.75% and the graphitic carbon content was 97.9% with all +200 mesh graphite at +98%.

For example 6, the same concentrate and process were used except that a 200 mesh screening was substituted for the two steps of flotation and the attrition time increased to 10 minutes. There was no improvement in carbon content over Example 5, but the coarse flake graphite recovery was decreased from 18.7% to 11.5%.

These tests show that attrition alone is inadequate to achieve +99% Cg coarse flake graphite from kish.

A test was made in accord with the present invention in which a crude kish assaying at 33.6% Cg was subjected to rougher concentration to form a 72.4% Cg concentrate.

A +200 mesh fraction was obtained by wet screening and such fraction was admixed with an aqueous solution of (5%) HCl for about 10 minutes. The acidic pulp was then attritioned for 5 minutes in an attrition device whose paddles operated at 2100 r.p.m. Sand was used as an attrition adjuvant with the concentrate/adjuvant ratio being 1:2 and the solids content 55%.

After attrition, the acid pulp was neutralized, subjected to two flotations, the flake graphite recovered and tested.

The flake graphite had a purity of 99.3% graphitic carbon, of which 75% was retained on +100 mesh. The final recovery of this test was 87% of the graphitic carbon from the feed.

Thus, the instant method and apparatus system will generate a +200 mesh kish graphite of 99+ wt. % carbon purity with 70% or more of the mass retained on +100 mesh, and an overall recovery of graphitic carbon from the feed of greater than 85%. This is a highly favorable result and of great commercial significance, particularly with respect to treating what has been heretofore considered a waste material.

It will be understood that kish from iron or steelmaking processes can vary widely, but operating within the teachings of the present invention, one skilled in the art can determine the operative and optimum conditions to recover the flake graphite from any kish by routine experimentation.

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

## Claims

1. A method of beneficiating kish to produce flake graphite having 99%+ graphitic carbon content comprising concentrating the kish to form a graphite concentrate, screening the concentrate to collect an about +200 mesh fraction, milling said fraction at an energy level and for a time sufficient to liberate the flakes to a purity of at least about 90% carbon, screening said milled fraction to collect the remaining +200 mesh fraction, mixing said remaining +200 mesh fraction with an acid having a concentration of less than about 7.5% to form an acidic pulp, subjecting said pulp to attrition with a solid attrition adjuvant at an energy level and for a time sufficient to dislodge impurities remaining on the graphite and at least 50% by weight of the flake graphite in said graphite concentrate is recovered as a +100 mesh product, neutralizing said acidic pulp, and recovering the flake graphite from the neutralized pulp.

2. The method of claim 1 wherein the acid is a mineral acid at a concentration of about 7.5% or below and at least about 70% of the flake graphite is retained on +100 mesh.

3. The method of claim 2 wherein the solids content of the acidic pulp is from 20 to 80% solids.

4. The method of claim 3 wherein the weight ratio of the milled +200 mesh fraction, attrition adjuvant, and acidic water in the acidic pulp is 1:2:2.5.

5. The method of claim 4 wherein the mineral acid is hydrochloric acid.

6. The method of claim 5 wherein the weakly acidic pulp is attrited for about 5 to 20 minutes.

7. The method of claim 6 wherein said solid attrition adjuvant is an abrasive mineral sand with a specific gravity less than 3.0.