SE2150867A1 - Process step in a metal recovery system - Google Patents

Abstract

The present invention relates to a method for preparation of solutions containing nickel and other unwanted components comprising fluoride and chrome, comprising the steps of feeding the solution to a precipitating unit, adding of calcium hydroxide to the solution, adding, if necessary, of natrium hydroxide to the solution for adjusting the pH level, precipitation of fluoride that is bound to the calcium hydroxide, precipitation of chrome that is bound to the fluoride, and removal of the solution for further treatment.(Fig. 2)

Description

PROCESS STEP IN A |\/IETAL RECOVERY SYSTEM TECHNICAL AREA The present invention relates to a process step in a system for metal recovery from spent pickling solutions.

BACKGROUND OF INVENTION ln a process for recovery of spent pickling solutions, ion exchange units are often used for the separation of for example nickel or other metals wherein the metal is separated from the solution by binding to the resin of the ion exchange bed. The metal containing solution produced from the ion exchange process may be fed to further processes to recover the metal. For nickel, electrowinning may be used for instance for recovery of nickel in its metal state, but other processes may also be used, such as precipitation steps for recovery of for instance nickel as nickel hydroxide. ln some instances, the solution may contain larger amounts of fluoride, which is unwanted for some reasons. For instance, if the solution also contains chrome, the chrome will not be precipitated or be bound to the resin because of the fluoride, instead staying in the solution. This is a disadvantage for subsequent treatment such as for electrowinning, which may be seriously deteriorated due to this.

Thus, it would be advantageous if both fluoride and chrome may be removed in an appropriate way before subsequent steps of metal recovery, in particular nickel.

BRIEF DESCRIPTION OF INVENTION The aim of the present invention is to handle fluorides present in acid containing solutions that also contain metals to be recovered. This aim is obtained by a method with the features according to the independent patent claim. Preferable embodiments of the invention form the subject of the dependent patent claims.

According to a main aspect of the invention, a method is provided for preparation of solutions containing nickel and other unwanted components comprising fluoride and chrome, wherein the method may comprise the steps of feeding the solution to a precipitating unit, adding of calcium hydroxide to the solution, adding, if necessary, of natrium hydroxide to the solution for adjusting the pH level, precipitation of fluoride that is bound to the calcium hydroxide, precipitation of chrome that is bound to the fluoride, and removal of the solution for further treatment.

By adding calcium hydroxide, a good process for the removal of fluorides in the solution is provided wherein the fluoride in the solution is bound to the calcium hydroxide. By precipitating the fluoride and calcium hydroxide, the fluoride may be readily removed. At the same time, the chrome that is bound to the fluoride is also precipitated.

According to one aspect, the pH level may be adjusted to around 5 - 6 for precipitating the fluoride as calcium fluoride. l\/loreover, the further treatment may comprise filtering the solution of particles and advantageously the filtered solution may be fed back to the precipitation unit and recircled through the filter until generally free of particles. ln order to remove the fluoride, the further treatment may comprise feeding of the solution through a fluoride absorbent, wherein the fluoride absorbent may be activated aluminium oxide. The solution may be recirculated and fed back to the precipitation unit and recircled through the fluoride absorbent until generally free of fluonde. ln order to ascertain a clean solution, it may be passed through a fine meshed filter for removal of residual particles. ln order to recover nickel, the treated solution may be fed to an electrowinning unit for recovery of nickel metal. As an alternative, the treated solution may be fed to a precipitation unit and a subsequent dewatering unit for recovery of nickel hydroxide. Further, the treatment of the solution may be monitored and controlled by a controller.

These and other aspects of and advantages with the present invention will become apparent from the following detailed description and from the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS ln the following detailed description of the invention, reference will be made to the accompanying drawing, where Fig. 1 is a schematic overview of a part of a system for recovering metals from pickling liquid before the process of the invention, and Fig. 2 is a schematic overview of a process step of the invention for example comprised in the system of Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION A process step 40 according to the invention is comprised in a system for recovery of metals in spent pickling solutions, such as iron, chrome and nickel. Before this process step, the pickling solution has been treated in several steps such as for example removal of iron and chrome. For example, pickling acid from a steel production plant that has been degraded with deposits to a degree that it cannot function properly is fed to an acid recovery and acid/metal separation unit 12. Here the acid solution is recovered and is fed back to the production plant. l\/letal cations may be separated from free acids by a diffusion process utilizing a particle bed of a special ion exchange resin frequently also referred to as an acid-retardation resin, sometimes also a size exclusion resin can be used. The metal cations will pass the resin bed while a large amount of the free acid is eluated from the resin by the use of water. The metal rich solution separated from the acid solution is then fed to a precipitation unit 18. Typically, the metal rich solution may contain 3-5 grams Ni/I and the solution may have a pH value in the order of 1-3. lt is of course favourable if the Ni-content is even higher.

For removing certain metal oxides and in this particular case iron and chrome, the pH level of the solution is adjusted with an appropriate additive. One preferred additive is NaOH that does not contaminate metal oxides that are not removed in this stage.

The additive may be added gradually and during stirring of the solution/additive. lt is important that the stirring is thorough so that the additive, e.g. NaOH, is dissolved in the solution. lf the additive is not dissolved properly, but there instead is a concentration of additives locally, that local concentration will cause local increase of pH levels which in turn may cause unwanted precipitation of material that should be extracted at a later stage and it might be difficult and/or time consuming to dissolve material that has been precipitated for treatment at later stages. An important factor for the efficiency of the process is temperature control. For the present embodiment described above, a temperature range between O - 65°C is desirable. The aim of the optimisation generally is to have particle sizes of the precipitation that are easy to filtrate in later stages.

For iron in particular to precipitate the pH level should be in the region of 4,5 - 5,5. lt is also important that the level is not higher because other metal oxides may precipitate at this stage, which is unwanted. The pH level is preferably monitored and controlled by a controller 22. ln this regard, the controller is preferably working dynamically controlling the pH level in the solution of the precipitation unit. lf the controller detects that the pH level is outside the preferred range, control signals are sent to the previous step to adjust the acid solution for the next batch. The controller could utilize a number of different techniques for controlling the process in the precipitation unit. For instance, pH level, conductivity, pe-level, spectrophotometric measurements, density, just to mention some.

Regarding the precipitation step described above, it is of course possible to actively and selectively chose different points along a titration curve and to pick certain pH levels depending on the resulting precipitation of metal oxides. As an example, the pH level could be chosen so that only Fe is precipitated, while Cr and Ni remains in the solution, or the pH level could be chosen such that both Fe and Cr are precipitated while Ni remains in the solution, and finally the pH level could be chosen such that both Fe, Cr, and Ni are precipitated. There could also be several different subsequent precipitations. As an alternative, instead of pH levels along a titration curve, a redox measurement could be chosen where the redox potential (ORP, pe) is utilized for the monitoring of the separation of metal oxides along the titration curve of OH- dosed versus ORP. Another alternative is to use any ion-selective measurement system that follows the specific concentrations of the Fe, Cr, Ni and other metals ions, specifically. This can be done on one or multiple ions.

The slurry with precipitated metal oxides in the precipitation unit 18 may now be fed from the precipitation unit to a dewatering unit 28. ln the dewatering unit 28, solids are separated from the liquid in the slurry, forming a cake on the filter surface of a filter component if filters are used as dewatering units. The aim should here to have as dry as possible cake of metal oxides and possibly hydroxides if hydroxides are used as pH affecting matter. The cake is removed from the dewatering unit 28, for instance by scraping off a filter surface. The recovered cake can then for example be re-used in the metal production plant.

The cake on the filter may be washed with cleaning liquids such as water by pressing water through the cake, for instance in order to wash away flourides that may be present in the cake. Such fluorides are the residues of the hydrofluoric acid previously mentioned often being part of the pickling acid. The removal of flourides is important if the resulting metal oxides and hydroxides are to be returned to the metal production plant. The fluorides are sometimes unwanted in the smelt due to metallurgical reasons and may be seen as an impurity. lt should be noted that in some steel making furnaces a small amount of fluoride containing inorganic chemical species are added as a flux component, but that this is controlled. ln this present system this fluoride component could be optimised to assist the steel maker in controlled addition of such fluoride bearing material The liquid solution from the dewatering unit is then fed to a second separation unit 34, e.g. an ion exchange unit. ln this regard, there might be a filter 35 between the dewatering unit 28 and the second separation unit 34. This may be done to ensure that no particles that might have slipped through the dewatering unit can enter the second separation unit and in particular prevent damage of a bed if an ion exchange unit is used. The process of the ion exchange will make nickel in the solution to be separated from the rest of the solution because the nickel will bind to the resin of the ion exchange bed. The process may be recirculating the solution through the bed until the bed is saturated or the solution is free of nickel. lf the solution is free from any metals, it can be sent to a deposit facility for storage and after treatment from a first outlet 38. Should the solution still contain metal, which can occur if the bed is saturated before all metal has been removed from the solution, the solution is kept in the second separation unit. When the bed has been saturated, it has to be regenerated. For this process, the solution needs to undergo an elution process with for example sulphuric acid, H2SO4. The acid solution is added via an inlet 36 and the elution process may be recirculating through the bed until the acid is saturated or there is no more Ni released from the bed, i.e. that the concentration of the acid is constant. lf the acid is saturated, then it is sent to the subsequent step.

Another variant is to perform separate precipitations of iron and chrome, for instance if it is desired to have them separate. ln that scenario, a first precipitation in step 18 may be performed at a pH level, e.g. 3 - 3.5, in which only iron is precipitated while the rest of the metals are still in the solution. The dewatering step 28 will then remove iron oxides. The solution is then fed to a subsequent precipitation step 70 having a pH level, e.g. 5 - 5.5, in which the chrome is precipitated, but not the nickel. A subsequent dewatering step 72 will then remove the chrome oxide from the solution.

The solution that is incoming to the process step 40 of the invention from for example step 34 is preferably rich with nickel in the form of nickel sulphate. However, the solution also contains some chrome and fluoride F', which is unwanted in following nickel recovery steps. ln order to remove the fluoride, the solution is fed to a precipitation unit 42 and calcium hydroxide Ca(OH)2 is added to the precipitation unit 42. The Ca(OH)2 binds to the fluoride as calcium fluoride CaFz and will be precipitated, sinking to the bottom of the precipitation unit 42. ln order to have an optimized precipitation, the pH in the precipitation unit may be adjusted to around 5-6 if not already within this range, preferably with natrium hydroxide NaOH. This is because Ca(OH)2 will bind to the fluoride but may not precipitate if the pH is too low. The pH range is also positive for the precipitation of possible residue of chrome Cr since Ca(OH)2 and Cr are coupled, the fluoride holds Cr in solution, so if Ca(OH)2 is precipitated, so is the remaining Cr.

The solution is then pumped with a pump 44 from the precipitation unit through a particle removing unit 46, such as for example a filter. lt is of course to be understood that other means of particle removal may be used; filtration, centrifugation, segmentation, floatation, just to mention a few. The treated solution is then fed back to the precipitation unit 42 and this process is continued until the solution is free of particles. The solution treated thus however still contains fluoride because the precipitation with Ca(OH)2 is not capable of removing all fluoride at this pH level. The next treatment step is then to feed the solution to a fluoride absorbent unit 48. One such type of fluoride absorbent unit 48 comprises a bed with activated aluminium oxide AlzOs. The AlzOe, will absorb the fluoride in the solution passing the bed. Preferably the pH level during this process is around 5.5. The solution will be returned to the precipitation unit 42 and recirculated through the fluoride absorbent 48 until all fluoride has been removed.

The process of removing fluoride is preferably monitored by a suitable controller 50 having an automation technology, for example electro chemistry, capable of measuring hydron Ht, fluoride F' and redox. lt is also possible that the controller can measure UV, IR, pH and temperature. The controller is in that regard connected to for instance the supply of NaOH and to a suitable heating or cooling means. ln this regard, the process may have a dynamic temperature control depending on the actual process steps. For instance, the fluoride absorbing with the activated aluminium bed may be performed at lower temperatures than when the treated solution is to be fed to for instance an electrowinning unit.

The bed of the fluoride absorbent 48 will eventually be saturated and will not be capable of absorb more fluoride. Since the bed functions as an ion exchange unit, it may be regenerated, for instance by adding NaOH that is already available in the process, which dissolves the fluoride. Further regeneration measures may be to add sulphuric acid H2SO4. The H2SO4 may also clean the bed from precipitated Ni. ln this regard, it is to be understood that other types of fluoride absorbents may be utilized like resin type ion exchange units or other ion exchange materials.

The solution treated by this process now contains more or less only Ni and will be fed to other units for further treatment. ln order to ascertain that no particles or undissolved substances remain in the solution, a fine meshed filter 52 may be arranged in the outlet of the process. Fïegarding further treatment, the solution may for example be fed to an electrowinning unit 54 for the recovery of Ni as metal. As an alternative, instead of recovering the nickel as a metal, the solution may be fed to a process step for recovery of nickel hydroxide Ni(OH)2, which is an important component for rechargeable batteries, for example for the electric car industry. The solution from the previous step is then instead led to a precipitation unit 56.

The precipitation unit 56 may be provided with means for adjusting the pH level, to a level where the nickel will be precipitated, preferably with NaOH to a level of between 8 - 8.5. The precipitated Ni solution is then fed to a dewatering unit 58. ln the dewatering unit 58 a cake with solids is formed, containing Ni(OH)2 with high purity, up to 90%. The obtained Ni(OH)2 may be treated further with different techniques for further drying and for increasing the purity further. For instance, colling crystallisation may be used for further purification.

This unit will work through either or a combination of precipitation of F' by forming a solid, for example in reaction with added Ca ions or selective ion exchange of the F' onto a size exclusion or an anion resin. To optimise the process an ion selective F' electrode may be used to continuously measure the F' level entering such a system.

According to another aspect of the system and method described, it might be advantageous to speed up the precipitation in some of the steps. One problem in this situation is that there might occur unwanted precipitation of nickel when for instance a precipitation of iron is wanted, but the nickel should be kept in the solution. One feasible solution is then to add carbonates which has been found to hold nickel as nickel-carbonate up to a pH of 6, at least for a time period. One aspect that then needs to be handled is that carbonates create carbon dioxide when added to acid solutions, which is unwanted from a production view-point as large amounts may be produced, which have to be vented away or handled in some other way. ln this regard, the acid solution entering a precipitation unit may in a first step be added with NaOH in order to increase the pH value of the solution, reducing the acidity. As a next step, carbonates are added, which also further increases the pH value, whereby less carbon dioxide is produced while still maintaining the nickel in solution. Different carbonates may be used for this. For instance, sodium bicarbonate can be used, which from a production view-point is advantageous, being of low cost. As an alternative sodium carbonate or C02 may be used. lt is to be understood that the embodiment described above and shown in the drawings is to be regarded only as a non-limiting example of the invention and that it may be modified in many ways within the scope of the patent claims.

Claims (11)

1.Method for preparation of solutions containing nickel and other unwanted components comprising fluoride and chrome, comprising the steps of: - feeding the solution to a precipitating unit; - adding of calcium hydroxide to the solution; - adding, if necessary, of natrium hydroxide to the solution for adjusting the pH level; - precipitation of fluoride that is bound to the calcium hydroxide; - precipitation of chrome that is bound to the fluoride, and - removal of the solution for further treatment.

2.Method according to claim 1, wherein the pH level is adjusted to around 5 - 6 for precipitating the fluoride as calcium fluoride.

3.Method according to claim 1 or 2, wherein the further treatment comprises filtering the solution of particles.

4.Method according to claim 3, wherein the filtered solution is fed back to the precipitation unit and recircled through the filter until generally free of particles.

5.Method according to any of the claims 1 - 4, wherein the further treatment comprises feeding of the solution through a fluoride absorbent.

6.Method according to claim 5, wherein the fluoride absorbent is activated aluminium oxide.

7.Method according to claims 5 or 6, wherein the solution is fed back to the precipitation unit and recircled through the fluoride absorbent until generally free of fluoride.

8.Method according to any of the preceding claims, wherein the treated solution is passed through a fine meshed filter for removal of residual particles.

9.Method according to any of the claims 1 - 8, wherein the treated solution is fed to an electrowinning unit for recovery of nickel metal.

10. Method according to any of the claims 1 - 8, wherein the treated solution is fed to a precipitation unit and a subsequent dewatering unit for recovery of nickel hydroxide.

11. Method according to any of the preceding claims, wherein the treatment of the solution is monitored and controlled by a controller.