CA1040868A - Process for recovery of valuable metals from solution using neutralization by lime or limestone - Google Patents

Abstract

Abstract of the Disclosure A solution containing a valuable metal from the group: Fe, Co, Ni, Cu, Mn, Mg, Cr, Al, Be, Cd, In, Ti and Zn, is treated with a calcium base under controlled conditions to precipitate a basic compound of the valuable metal as relatively fine particles and gypsum as relatively coarse particles. The gypsum is then separated from the compound of the valuable metal by relying on the particle size difference.

Description

lQ4Q868 PC-llO0-Can.
The present invention relates to processes for the recovery of metal values from sulfate solutions.
In a large numher of hydrometallurgical ex-traction processes, the need arises to recover metal values from sulfate solutions. A common example is the leaching of slurries of minerals with sulfuric acid, after which it is necessary to recover the metal values leached. A con-venient method involves the addition of alkali to the solutions so as to precipitate a basic compound (typically the hydroxide or carbonate) of the metal value. The neu-tralization may be done so as to precipitate more than one metal value simultaneously, or alternatively controlled neutralization mav be used to recover a metal value selectively.
Metal values which can be recovered in this way include for example: iron, cobalt, nickel, copper, manganese, magnesium, chromium, aluminum, beryllium, cadmium, indium, titanium and zinc.
Current practice for the neutralization of such sulfate solutions to precipitate the metal value entails the use of bases, typically sodium hydroxide or sodium carbonate, which themselves form soluble sulfates. In this way the precipitated metal value is not contaminated hy solid sulfates produced by neutralization of the base.
However, when the solutions from which metals have been removed is recycled in such a process, sodium sulfate builds up therein and it is necessary to bleed-off, and dispose of, the sodium sulfate periodically. -As is well known, sulfate solutions can be neutra-lized by means of a calcium base, i.e. lime or limestone,and ~ `
indeed due to the cheapness of such a base this method of :'.
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:, ': ,`.: : ' lV4~8f~8 neutralization would be highly desirable. An added attraction of such a neutralization is that it would provide a bleed which prevents sulfate build-up. However, its commercial implementation has been hampered by the problems raised by the gypsum which is precipitated together with the desired basic precipitate of the metal to be extracted. It is necessary to separate the gypsum fxom the desired basic precipitate and to do so prior to further treatment, e.g., pyrometallurgicaliy, of the basic precipitate. This is because the greater bulk of the pre-cipitate creates handling problems, the gypsum contains water which increases the fuel consumption for the pyro-metallurgical treatment, and calcium and sulfur are generally undesirable contaminants of the final product.
The cost of the additional step of ~eparating the gypsum from the desired precipitate can offset any economic ad-vantages resulting from the use of the cheaper base. For this reason the precipitation of a metal from a sulfate solution with the aid of a calcium base has in the past been advocated only for limited commercial applications.
One such application is in processes where the mixture of precipitates is subjected to a leach thereby effecting separation of the valuable metal from the gypsum. Another area of application is the scrubbing of effluents, where gypsum precipitation does not raise problems since the primary object is solution purification rather than metal recovery.
It has now been found that if a solution containing the sulfate of a metal which can be precipitated by addition of alkali is neutralized with lime or limestone under specific 104~868 conditions it is possible to precipitate gypsum of a much larger particle size than the precipitated basic compound of the metal to be extracted, e.g. the hydroxide thereof.
This enables most of the gypsum to be separated from the metal hydroxide by means of a relatively inexpensive step relying on the size difference of the particles, such as screening of the solids.
According to the invention a process for recover-ing a metal value, selected from the group consisting of iron, cobalt, nickel, copper, manganese, magnesium, chromium, aluminum,beryllium,cadmium, indium, titanium and zinc,from a sulfate solution comprises treating the solution with a calcium base, selected from the group consisting of lime and limestone, at a temperature TC, which is at least 60C, for a period of t minutes wherein: ~
t ~ 15 , where T ~ 90C. -and t ~ 15 + ~ (90-T), where 60C ~ T ~ 90C; -the solution being subjected during the process to low energy agitation as herein defined, thereby forming a first set of particles comprising a basic compound of the metal :
value and a second set of particles comprising gypsum and being much larger than the first set of particles, and ~`
separating the two sets of particles from one another by a physical process relying on the difference in particle size. . `
The term "low energy agitation" is used herein to `.:-~
denote agitation which is sufficient to ensure intimate contact and reaction between the solution and added base, but which does not impart a high shear which would cause ~ :
nucleation at a larger number of sites and fragmentation ' 104~)i!3~il~

of the growing gypsum crystals thereby preventing attain-ment of the desired large gypsum particle size. The energy of agitation depends of course on the method employed for the agitation. Thus where mechanical agitation is used, the energy is a function of the design, position and speed of rotation of the blades as well as the design and position of any baffles present in the precipitation vessel. In general, success of the process of the invention requires that the agitation used be such as to impart to the slurry an energy which does not exceed 3 (and is preferably much lower, e.g., of the order of 0.2) kilowatts per cubic meter of slurry. Air agitation is a convenient way of achieving good mixing without imparting high shear, and moreover because of its low power consumption constitutes a preferred feature of the process of the present invention.
It is essential to adhere to all the criteria of reaction temperature, reaction duration (i.e. residence time) and degree of agitation itl order to achieve the desired easily separated mixed precipitate. Thus whereas neutralization can be accomplished in 15 minutes or more at 90C, a holding time of at least one hour is required at 60C.
Where lime is used as the calcium base, it can be added as such into the precipitation vessel, e.g., as a -325 mesh, Tyler Screen Size (TSS) powder. Preferably however the base is introduced in the form of a calcium hydroxide slurry. Similarly, limestone is preferably intro-duced into the precipitation vessel in the form of a slurry of 99~ -325 mesh (TSS) particles. The choice between lime and limestone will depend upon the particular metal value 104~)8ti8 to be precipitated. Thus whereas limestone can be used effectively to precipitate copper or iron from solution, it is incapable, as is well known, of raising the pH to a sufficient extent to give adequate precipitation of nickel or cobalt, for which metals lime has to be used.
The process of the invention is preferably carried out in a continuous manner, i.e. appropriate amounts of the sulfate solution and the base slurry are fed into a precipita-tion vessel, and the resulting slurry is extracted from the vessel at such a rate as to maintain a fixed volume within the vessel. The relative proportions of solution and base slurry are controlled by measurement of the pH within the precipitation vessel.
The growth of large crystals of gypsum is aided by the presence in the solution of gypsum seed crystals.
While the initial introduction of such seeds is not essential in that the crystallization can proceed on a self-nucleating 3 basis, it has been found advantageous to introduce gypsum seed initially. Of course when the precipitation is carried `
out as a continuous process, seeds will be present in solution under steady state conditions.
Any of the various known methods for separating solids of different particle size, such as hydrocycloning, elutriation or screening, may be used to separate the precipitated metal hydroxide from the gypsum. This is because -`
the process of the invention can result in gypsum particles which are at least two or three times as large as the preci-pitated metal hydroxide particles. Thus typically metal hydroxide particles of 1-10 micron diameter can be precipi-tated along with approximately rectangular gypsum particle~ -~5~

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1()4~6~3 measuring from 30 microns in the smaller dimension to 500 microns in the larqer dimension.
In order to provide a better understanding of the invention specific examples thereof will now be described.

Nickel hydroxide was precipitated by the process of the invention from a nickel sulate solution containing 16.9 g/l of nickel. One liter of this solution was heated to 95C in a two-liter cylindrical vessel. 6 grams of coarse gypsum were added to the solution, this amount cor-responding to about 10% of the stoichiometric amount of gypsum to be formed by the neutralization. Lime, in the form of a 10% solids slurry of minus 325 mesh (TSS) particles, was introduced into the vessel so as to raise the pH to 8.0 over a 30 minute period, with a further retention time of 1 hour in a quiescent state. During the neutralization process the slurry within the vessel was agitated by means of a stirrer comprising a four blade 7.8 centimeter diameter axial impeller, the blades being 2.65 cm. wide and having a 45 pitch. By rotating this impeller at 300 rev/min, the agi-tation power consumption was found to be 0.19 kW/m3. The temperature of the slurry within the vessel was maintained constant at 95C.
The resulting slurry contained nickel hydroxide particles of between 1 and 10 micron diameter, and gypsum particles having an average size of 30 x 180 microns. This slurry was separated by batch elutriation with a saturated gypsum solution in a 2.4 cm diameter column. The overflow from the elutriation contained 99.5% of the precipitated nickel and only 25% of the precipitated gypsum.

i~)4~)8~8 To investigate the precipitation of nickel together with copper, a solution was used which contained mainly sulfates of copper and nickel together with small amounts of cobalt, magnesium, calcium and iron. The concentration of the metals in the electrolyte was as follows:
Copper : 5 g/l Nickel : 15 g/l Cobalt : 0.5 g/l Magnesium : 1 g/l Calcium : 0.4 g/l ~he initial pH of the feed solution was 3.5, and it was neutralized to a pH of 8.5 using a lime slurry in a manner identical to that described in Example 1, except that the temperature was maintained at 90C and the preci-pitation was carried out in a continuous rather than batch mode. After the precipitation, the solids obtained were :
wet-screened to divide them into a +100, a +325 and a -325 mesh (TSS) fractions. The distribution of copper, nickel, .
cobalt, magnesium and calcium between the three fractions was then determined and the results are shown in Table 1 below. :

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104~868 By way of comparison a further test was conducted using the same copper-nickel solution and experimental procedure described above except that the stirrer was rotated at 600 rev/min which corresponded to an agitation power of 0.78 kW/m3. After wet screening of the resultant slurry, the screen fractions were analyzed for copper, nickel and calcium and the results are shown in Table 2 below.

10FEaction . _ . _ ..
(TSS Wt Ass~y (wt. %) Distribution (%) mesh) (~) u Ni Ca Cu Ni ~~Ca .. . _ -100) 340.04 0.1 23.4 0.3 0.2 55.2 +325) 15 .2 0.5 23.0 0.6 0.5 23.9 ; --325 51 .2 26.4 5.9 99 99 20.9 -~..- "' It will be clear from a comparison of the results of Table 1 with the corresponding ones in Table 2, that the use of a high stirrer speed led to a significant --increase in the amount of gypsum present in both the fine and intermediate screen fractions.
The separation of the precipitates into coarse, -intermediate and fine fractions was undertaken to obtain a clear picture of the metal distribution. In practice ;
it may be sufficient to undertake a two part separation using only a 100 mesh (TSS) screen, the coarser fraction then being acid washed to redissolve the copper and nickel.

, Alternatively the product can be screened to give three fractions as described, the intermediate fraction being recycled for further growth of the gypsum.

A precipitation test was carried out using air agitation instead of the mechanical agitation described in the previous examples. For this test a solution having the same composition as described in Example 2 was used.
The apparatus used comprised a conical one-liter glass vessel, 46 cm deep. The vessel was filled with solution to a level 7 cm below its top and the solution was heated to 90C by means of a centrally located heating rod.
Gypsum seeds were added to the solution as in previously described examples, and air was fed into the solution at the bottom of the vessel at a rate of 1.4 l/min through a glass tube immersed from above. Through similar glass tubes fresh nickel sulfate solution was fed into the vessel at a point 2.5 cm above the bottom of the vessel, a 100 g/l slurry of lime was fed in at a point 10 cm above the bottom of the vessel while reacted slurry was drawn out from a point 7 cm above the bottom of the vessel. The relative flow rates were controlled to maintain a constant volume within the vessel at a pH of 8.5 and to allow a one-hour residence within the vessel.
The resultant slurry was wet-screened and the screen fractions analyzed to give the results shown in Table 3 below:

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~04~)868 Fraction _ _ _ _ (TSS Wt Ass ~ %) Distribution (%) mesh) (%) Cu Nl Ca Cu ~~ Ni Ca -100) 45 0.09 0.23 23.0 1.0 0.8 75 +325) 12 0.23 0.71 20.0 0.6 0.6 17 -325 43 10.0 30.6 2.61 98.4 98.6 8 The above results compare reasonably well with those obtained by relying on mechanical agitation of the slurry.

The effectiveness of a limestone slurry as calcium -~
base in the process of the invention was tested on a solution containing 7 g/l of copper and 15 g/l of nickel as sulfates.
The solution was neutralized with a limestone slurry to a pH of 4.3 on a continuous basis using the mechanical agita- -tion procedure described in Example 2. The temperature of the reacting slurry was maintained at 60C and because of this relatively low temperature as well as the relatively low pH a residence time of 3 hours was allowed. At the -end of the precipitation, an effective separation of the gypsum from the copper and nickel hydroxides was found to -;
be obtained when the slurry was wet screened using a 48 mesh (TSS) screen. The results are shown in Table 4 below.
TABLE 4 ~-Fraction _ (TSS Wt As say (wt ~) _ Distribution (%) _ mesh) (%) Cu Ni Ca Cu 1 Ni ~Ca +48 mesh 34 6.0 1.32 18.8 9 9 72 --48 mesh 66 30.6 6.56 3.8 91 91 28 104~ 8 The high gypsum fraction can be washed with a weakly acidic solution to recover over 90~ of the copper and nickel therein for further treatment. It should be noted that at the pH of 4.3 used, 38.2% of the total copper present, but only 3.4% of the total nickel present were precipitated.

A continuous precipitation was carried out using the procedure and apparatus described in Example 2 on a solution containing 2 g/l of each of the metals chromium, aluminum, manganese, zinc and cadmium as ~ulfates. The solution was neutralized with a lime slurry to a pH of 8.5 at 90C with a residence time of 1 hour. The analysis of three fractions obtained by wet screening of the re-sultant slurry, shown in Table 5 below, revealed a good separation of the gypsum from the fine fraction which contained a very high proportion of all the metal hydroxides.

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~ o o ~,_ + '~ + , '' ' . _ _ 1!~)4~868 Pickle liquors obtained when steel is pickled with sulfuri~ acid contain, typically, 220 g/l of iron and 300 g/l of sulfuric acid. When suitably diluted such solutions can be treated in accordance with the invention to recover the iron for recycling through the steel plant, as well as a clean gypsum product for disposal. Ferrous iron can be precipitated by using lime to raise the pH
to about 8.5. Preferably however the iron in solution is first oxidized by any known means to the ferric state, whereafter it can be precipitated with limestone at a pH of about 4.
A particular solution containing 15 g/l of sulfuric acid and 11.1 g/l of ferric iron was treated with a 100 g/l limestone slurry at pH 4Ø The procedure used was as described in Example 2 abo~e, the solution temperature being 90C and the retention time 3 hours.
As shown in Table 6 below, wet screening of the precipi-tation product yielded a fine fraction containing 96~
of the iron with only 4.1% of the gypsum. ;

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Fraction Wt. Assay ~? ~ (~) (TSS mesh) (~) Fe Ca Fe Ca +65 63 0.57 26.3 2.g 83.7 ~ ~

-65, +100 4 0.74 22.5 0.3 4.9 ~ --100, +325 7 1.4 21.0 0.8 7.3 -325 26 46.5 3.2 96.0 4.1 A comparison was made between elutriation and ,: :; . ;., , ., . , . . . : .
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1~4~868 screening as means for effective separation of the coarse gypsum from the finer metal hydroxides. For this purpose a solution containing nickel sulfate was neutralized at 60C with lime at a pH of 9.0 using the low energy mechani-cal agitation method described in Example 2, to give a 7% solids slurry of gypsum and copper and nickel hydroxides.
A 400 ml sample (A) of the slurry was subjected to elutriation at 23C using a saturated solution of calcium sulfate as the elutriant. The elutriation column comprised a vertical cylinder 80 cm high and having an internal diameter of 4.5 cm, and terminating at its lower end in a cone 28 cm deep. A vertical linear velocity of 0.17 cm/sec was used. The column underflow constituted a first fraction containing the coarse (predominantly gypsum) particles. The overflow was wet screened on a 325 mesh (TSS) screen to separate a middling fraction (containing some gypsum and some hydroxides) from the fine fraction (predominantly hydroxides). To determine the degree of separation, the distribution of calcium and ;
nickel was determined in each fraction and the results are shown in Table 7, together with the results obtained for a second sample B which was screened, without elutriation, on 100, 200 and 325 mesh (TSS) screens.
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It will be seen from the above results that elutriation, though less effective than screening, nevertheless provided good separation of the gypsum from the nickel hydroxide.
Although the invention has been particularly described with reference to preferred embodiments, it will be understood that various modifications can be made to the conditions specified in these embodiments without departing from the scope of the invention, which is defined by the appended claims.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for recovering a metal value, selected from the group consisting of iron, cobalt, nickel, copper, manganese, magnesium, chromium, aluminum, beryllium, cadmium, indium, titanium and zinc, from a sulfate solution comprises treating the solution with a calcium base, selected from the group consisting of lime and limestone, at a temperature T°C, which is at least 60°C, for a period of t minutes wherein:
t ? 15 , where T 2 90°C, and t ? 15 + ? (90-T), where 60°C ? T ? 90°C;
the solution being subjected during the process to agitation which imparts thereto an energy not greater than 3 kilowatts per cubic meter, thereby forming a first set of particles comprising a basic compound of the metal value and a second set of particles comprising gypsum and being much larger than the first set of particles, and separating the two sets of particles from one another by a physical process relying on the difference in particle size.

2. A process in accordance with claim 1 wherein said separating of the two sets of particles comprises screening the particles.

3. A process in accordance with claim 1 wherein said separating of the two sets of particles comprises elutria-ting the particles.

4. A process in accordance with claim 1 wherein said agitation comprises air agitation.