WO2005097680A1

WO2005097680A1 - Process for the regeneration and recovery of ammonia

Info

Publication number: WO2005097680A1
Application number: PCT/AU2005/000494
Authority: WO
Inventors: Gary Donald Johnson
Original assignee: Western Minerals Technology Pty Ltd
Priority date: 2004-04-07
Filing date: 2005-04-06
Publication date: 2005-10-20

Abstract

A process for the recovery of ammonia from an ammonium sulphate solution, the process comprising the method steps of: (i) combining ammonium sulphate solution (10) and quicklime (12) in a milling means (14) to provide a reaction slurry; (ii) running the milling means (14) whereby the milling action thereof acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means (14) so as to liberate ammonia; and (iii) transferring the reaction slurry containing liberated ammonia from the milling means (14) to an independent stripping means (16) for ammonia recovery.

Description

"Process for the Regeneration and Recovery of Ammonia"

Field of the Invention

The present invention relates to a process for the regeneration and recovery of ammonia. More particularly, the present invention relates to a process for the regeneration and recovery of ammonia from ammonium sulphate solutions or ammonium sulphate containing wastewater streams.

Background Art

Ammonia is widely used in mineral processing and chemical applications for its mild alkaline characteristics, ease of use, and potential ability to be recycled. For example, in the mineral processing industry, ammonia is used in the ammoniacal leaching, nickel/cobalt hydrogen reduction, and nickel/cobalt solvent extraction processes.

The production of nickel, using downstream nickel solvent extraction technology or hydrogen reduction, can require the use of large quantities of anhydrous ammonia. This requires pressure storage vessels on site and also specialised transportation. The supply of ammonia is further limited by the location of both the metal plant and the ammonia manufacturer. The ammonia added to the process can ultimately be recovered as a by-product fertiliser (ammonium sulphate), but the market for this product is limited.

Presently, ammonia is recovered from solutions of ammonium sulphate by reacting the ammonium sulphate solution with milk of lime (hydrated calcium oxide or hyd rated/slaked lime) in a stirred tank, referred to as a "lime boil". The hydrated lime (Ca(OH)₂) is typically produced by reacting quicklime (CaO) with water in a "slaker" or similar device. The milk of lime is then added to the ammonia recovery system.

The lime utilised in this process must be hydrated prior to addition to the lime boil as gypsum (CaSO .2H₂O) is formed during the process. The gypsum thus formed coats the quicklime and consequently there is very poor utilisation of quicklime in the process.

When ammonium sulphate reacts with the milk of lime, ammonia is liberated by the reaction of the ammonium ion with the hydroxide ions in the milk of lime, and the insoluble gypsum is formed as a reaction product. The ammonia is stripped off the slurry by heating to about 90 to 100°C by the addition of steam. The off- gases from the process contain both ammonia and water vapour. The ammonia is able to be recovered in a relatively pure form as a solution of ammonia in water by cooling the off-gases.

The consumption of lime in this process is often significantly higher than that predicted by the stoichiometry of the chemical reactions taking place, usually 10 to 50% higher. This is a product of the fact that the gypsum formed in the process has a tendency to coat any particles of unreacted lime. This restricts further reactions with any underlying lime particles that may not be fully hydrated. The gypsum further tends to precipitate on the inside of the tank, on the agitator, and any internal pipe work such as the steam injection pipes. These precipitates can be so severe as to interfere with the continuous operation of the process and significant periods can be spent removing precipitate. There have been instances where lime boil plants have been shut down due to the inability to effectively operate this process.

As the process operates at elevated temperatures there is considerable expense incurred in heating the slurry to the required temperature. This problem is exacerbated if the process cannot be run continuously. In hydrometallurgical processes that use ammonia as a reagent, ammonia regeneration/recovery can be a critical process both technically and economically.

US Patent 2182078 describes a process for the recovery of an aqueous solution of ammonia from ammonium sulphate crystals, utilising a rotating mill in which the ammonium sulphate crystals and lime are combined, so as to prevent the build up of insoluble calcium sulphate/gypsum on any unreacted lime. Steam is passed into the mill and vapours containing ammonia withdrawn and passed to a rectifying column prior to condensation. However, the time for ammonia liberation and stripping in the mill is limiting in such an arrangement. Importantly, the process of lime slaking, ammonia liberation and ammonia stripping all occur in the mill. This limits the throughput to the slowest component of the process, being the strip.

In WO 00/41967 the present applicant describes a process for the recovery of ammonia somewhat similar to that described in US Patent 2182078. Again, steam is injected into the reaction slurry within the mill to strip liberated ammonia.

The process for the regeneration and recovery of ammonia from ammonium sulphate solution of the present invention has as one object thereof to overcome at least in part the above problems associated with the prior art, or to provide a useful alternative thereto.

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the present invention there is provided a process for the recovery of ammonia from an ammonium sulphate solution, the process comprising the method steps of:

(i) combining ammonium sulphate solution and quicklime (CaO) in a milling means to provide a reaction slurry;

(ii) running the milling means whereby the milling action thereof acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means so as to liberate ammonia; and (iii) transferring the reaction slurry containing liberated ammonia from the milling means to an independent stripping means for ammonia recovery.

Preferably, the reaction slurry generates heat through the slaking of the quicklime.

Preferably, the milling means is provided in the form of a vibrating mill.

Still further preferably, the vibrating mill has a non-rotary action. The charge in the mill is preferably mobile at all times and the mill's shell does not rotate, thereby allowing the mill to be gas-tight.

The stripping means is preferably a column. The column preferably has provided therein a plurality of baffles.

The process of the present invention preferably avoids substantially the production of gypsum in the stripping means.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:-

Figure 1 is a diagrammatic flow sheet of a process for the regeneration and recovery of ammonia in accordance with the present invention;

Figure 2 is a graphical representation of the equilibrium fractions of ammonia species as a function of solution pH in the reaction slurry of the process of the present invention, showing that complete liberation of ammonia requires a pH>11.4;

Figure 3 is a graphical representation of the % liberation of ammonia for a variety of different temperatures for two different ammonium sulphate containing raffinates when milled in accordance with the present invention; and Figure 4 is a graphical representation of the results of bench scale test work on a sample nickel raffinate designed to investigate the impact of pressure conditions on the speed of stripping.

Best Mode(s) for Carrying Out the Invention

In accordance with the present invention and with reference to Figure 1 , ammonium sulphate solution 10, from which it is desired to recover ammonia, is mixed with quicklime 12 (CaO) at ambient temperature in a milling means, for example a vibrating mill 14 adapted for the purpose, to provide a reaction slurry. The vibrating mill 14 containing a charge of balls as grinding media, with a non- rotary action, is preferred as the charge is mobile at all times and the shell does not rotate, thereby allowing the mill to be gas-tight.

Liberation of ammonia from an ammonium sulphate solution involves the reaction of the ammonium ion with hydroxide ions to form ammonia and water. This reaction is represented by Equation 1. Hydroxide ions (OH^") are supplied via the addition of lime to the liberation vessel and are used to shift the equilibrium towards the right.

NH₄* + OhT o NH₃ + H₂0 Ka = 8.25 x iσ⁶ Equation 1

The action of the charge or balls in the vibrating mill 14 acts to continuously break up any gypsum precipitate (CaSO₄.2H₂O) as it forms during the ammonia recovery process. The breaking up of the gypsum precipitate in this manner exposes fresh quicklime surfaces allowing for further reaction with the ammonium sulphate solution and prevents gypsum scale formation inside the mill 14 and associated pipe work.

The slaking of the quicklime that occurs in the mill 14 within the reaction slurry generates heat that is effectively used to heat the reaction slurry. As such, there is less need for additional process heating when compared with prior art processes. This heat further enhances the liberation of ammonia. Typically, 95% liberation occurs within 1 minute at high temperatures of about 80°C. The lime slaking reaction is represented by Equation 2. This reaction is exothermic and releases 64.5 kJ/mol.

CaO + H₂0 → Ca(OH)₂ Equation 2

Metals present will precipitate as their hydroxide form.

The processes of lime slaking and ammonia liberation are specifically arranged to occur in the mill 14, due to their relatively fast kinetics when compared with stripping.

Figure 2 shows the equilibrium fraction of ammonia species as a function of solution pH derived from Equation 2. As is shown, complete liberation of ammonia requires a pH >11.4.

Table 1 below and Figure 3 show % liberation of ammonia for a variety of different temperatures for two different ammonium sulphate containing raffinates. As indicated previously, 95% liberation occurs within 1 minute at temperatures of about 80°C.

Table 1

Table 1 continued

Slurry from the mill 14 is introduced to a stripping means, for example a column 16, at the base of which steam 17 is introduced. The stripping process is represented by Equation 3.

NH₃ (aq) → NH₃(g) Equation 3

The NH₃ vapour is then recovered by condensation as aqueous ammonia solution as represented by Equation 4.

NH₃ (g) + H₂0 → NH +OH⁰ Equation 4

In Figure 4 there is shown the results of bench scale test work on a sample nickel raffinate designed to investigate the impact of pressure conditions on the speed of stripping. As is clearly evident, neither vacuum or pressure conditions provided any speed advantage over conducting the strip at atmospheric pressure.

The overall equation for the liberation and stripping process is shown by Equation 5.

2NH₄ ⁺+ S0₄ ^2' + Ca(OH)₂ → 2HH₄ + 20hT+ CaS0₄ Equation 5

The column 16 is provided with baffles 18 to prevent substantially fouling or blocking with gypsum. The likelihood of scaling is reduced as no gypsum is produced in the column 16, the production of gypsum having gone to completion in the mill 14. The steam flow rate is controlled to ensure the temperature at the top of the column remains above 90°C, with a target of 95°C. This allows all ammonia in the column 16 to be recovered to the product solution 20.

The present invention may be better understood with reference to the following examples, each taken from a pilot plant operation involving ammonia regeneration for nickel solvent extraction. However, it is to be appreciated that the generality of the invention as described above is not to be limited by the following examples.

Example 1

Nickel raffinate containing ammonium sulphate was fed through a single plate Tranter heat exchanger and the off gas scrubber before entering the Vibra-Drum grinding mill at a rate of 56L/h. Quick lime was simultaneously fed via a screw feeder into the front of the Vibra-Drum grinding mill at a rate of 4.4 kg/h.

The lime is slaked in the Vibra-Drum mill and the slaking reaction produced heat important for the subsequent liberation reaction. No extra water in the form of milk of lime is added, which allows a concentrated ammonia product to be obtained, and low steam consumption.

The mill operated at 64Hz and was charged to 40% volume with steel grinding media. The grinding media comprised 2mm to 20mm diameter balls and cylinders of 6mm dia x 10mm height. The media is fairly evenly distributed within the mill with a slight trend of larger media at the feed end and smaller media at the discharge end.

Due to the particular milling action of the mill and its effective hydration of quicklime the hydration step of prior art processes may be avoided. This produces savings in equipment and operating costs and provides a safer process with fewer alkaline transfers. The ammonium hydroxide and gypsum products discharge the mill at 57.6°C and pH 11.3. This slurry is introduced at a height of 3.7m to a 5.4m high Pyrex™ glass stripping column. Steam is introduced to the base of the column at about 108°C and the ammonia/steam product vapour passes through a Sigma heat exchanger. The cooled ammonia solution was collected in a sealed drum at 4.1 L/hr and samples were sent for quantitative ammonia analysis. The product solution with 164g/L NH₃ should have an SG (Specific Gravity) of 0.926 at the after condenser temperature of 44°C (Perry 1984). This was supported with the recorded average SG of 0.92.

Gypsum was pumped from the base of the column to a settling vessel before being pumped to waste disposal.

Example 2

The general flowsheet and mill operating conditions are the same as Example 1 , but the flow rates, temperatures and product data differ. A nickel raffinate feed rate of 76L/h prompted an elevated quick lime feed rate of 5.6 kg/h. The mill discharge temperature was 47.8°C and pH was 11.3.

The ammonia solution was collected at 11L/h. and assayed 120 g/L NH₃. The product solution with 120g/L NH₃ should have an SG of 0.942 at the after condenser temperature of 40°C (Perry 1984). This was supported with the recorded average SG of 0.942.

Operating data for both Examples 1 and 2 are provided in Table 2 below.

Table 2: Ammonia regeneration operating data.

Example 1 Example 2 Target 9-Feb-04 11-Feb-04

Mill Feed Flowrate L/h 70 56.4 76

Mill Vibrations Hz 63 64 64

Quick Lime Addition Rate kg/h 3.51 4.4 5.6

Mill Discharge Temp. °C 45 57.6 47.8

Mill Discharge pH 11.9 11.3 11.3

Mill Efficiency* NH₃ % 97 97 97.3

Strip Column Temp. (Top) °C 95 94.8 96.5

Ammonia Prod'n Rate L/h 4.1 11

Strip Efficiency** NH₃ % 99 98.4 98J

Ammonia Product SG 0.92 0.943 NH₃ g/L 150 164 120

Strip Column Discharge PH 11.5 11.5 12.1 Ni mg/L 0.1 0.1 0.1

Mill efficiency* = (NHU). SCX. in mill feed - (NH )?SC>4 in mill discharge x 100% (NH₄)₂S0₄ in mill feed

Strip efficiency** = (NH )?SO-. in mill feed ■ (NhUySO.. in column discharge x 100% (NH₄)₂S0₄ in mill feed

It will be apparent to the skilled addressee that the characteristics of the feedstock, the ammonium sulphate solution, and any downstream requirements may have an impact upon the specific manner in which the process of the present invention is carried out, whilst not departing from the scope of the present invention.

As can be seen from the above description, the process of the present invention requires that lime slaking and ammonia liberation take place in the milling means, whilst ammonia stripping occurs outside the milling means in a stripping means. The decoupling of these process steps allows a faster and more efficient ammonia recovery when compared with the prior art.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

1. A process for the recovery of ammonia from an ammonium sulphate solution, the process comprising the method steps of:

(ii) running the milling means whereby the milling action thereof acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means so as to liberate ammonia; and

(iii) transferring the reaction slurry containing liberated ammonia from the milling means to an independent stripping means for ammonia recovery.

2. A process according to claim 1 , wherein the reaction slurry generates heat through the slaking of the quicklime.

3. A process according to claim 1 or 2, wherein the milling means is provided in the form of a vibrating mill.

4. A process according to claim 3, wherein the vibrating mill has a non-rotary action.

5. A process according to any one of the preceding claims, wherein the charge in the mill is mobile at all times.

6. A process according to any one of claims 3 to 5, wherein the mill's shell does not rotate, thereby allowing the mill to be gas-tight.

7. A process according to any one of the preceding claims, wherein the stripping means is a column.

8. A process according to claim 7, wherein the column has provided therein a plurality of baffles.

9. A process according to any one of the preceding claims wherein the production of gypsum in the stripping means is substantially avoided.