PREVENTION OF CALCIUM SULPHATE CRYSTALLISATION IN WATER DESALINATION PLANTS USING SLURRY PRECIPITATION AND RECYCLE REVERSE OSMOSIS (SPARRO)

PREVENTION OF CALCIUM SULPHATE CRYSTALLISATION IN WATER DESALINATION PLANTS USING SLURRY PRECIPITATION AND RECYCLE REVERSE OSMOSIS (SPARRO)
Report No 1372/1/06
February 2006

EXECUTIVE SUMMARY

BACKGROUND

The principal objective of the research project was to investigate the Slurry Precipitation and Recycle Reverse Osmosis (SPARRO) process from a crystallisation perspective. To achieve this objective, the following aims were identified:

To expand both the fundamental and practical understanding of the operation of the Slurry Precipitation and Recycle Reverse Osmosis (SPARRO) system;
To develop design specifications relating to the crystallisation parameters of the system and the control of the desupersaturation reactor based on the critical operating parameters for the SPARRO system;
To develop design specifications relating to membrane selection and treatment in the SPARRO system;
To define the critical operating parameters for the SPARRO system.

OVERVIEW

Tasks relating to the crystallisation aspects of the project are complete and considerable success has been achieved in this area. Detailed information can be found in the MSc thesis of Shilpa Seewoo (Seewoo, 2003). Tasks relating to the membrane aspects of the project are also complete. Two dedicated process systems and associated equipment have been designed and commissioned during the course of this project: the Sparro Membrane Test Rig (SMTR) mini-plant and the Directed Slurry Spraying Technique (DSST).

Considerable progress has been made in addressing the central question of this research project, which is: To what extent do the crystallisation parameters affect membrane damage in the SPARRO system?

Aim 1: To expand both the fundamental and practical understanding of the operation of the Slurry Precipitation and Recycle Reverse Osmosis (SPARRO) system

Fundamental and practical understanding of the SPARRO system has been expanded through the literature review and the experimental program, as well as through interaction with industrial partners in the project. These aspects are presented in more detail in the body of the report.

Aim 2: To develop design specifications relating to the crystallisation parameters of the system and the control of the desupersaturation reactor based on the critical operating parameters for the SPARRO system;

The crystallisation aspects of the process have been thoroughly investigated and an extensive understanding of gypsum crystal behaviour developed. Design specifications relating to the crystallisation parameters of the system and control of the desupersaturation reactor have been developed based on the critical operating parameters for the SPARRO system.

The conclusions from the crystallisation aspects of the work are as follows:

Rate of consumption of supersaturation is significantly increased by the use of seeds as a source of secondary nucleation;
Needle-like seeds were produced under conditions of low supersaturation and were characterised by a long induction time;
Plate-like seeds were formed at high supersaturation, with a significantly reduced induction period;
Digital image processing could be used to quantitatively distinguish between gypsum morphologies;
XRD analysis showed similar traces for all morphologies and analytical grade gypsum;
Sulphate to calcium ratio and pH extremes affected the solubility of gypsum and thus the rate of desupersaturation by the common ion effect and influence on the speciation of SO₄^2- and Ca²⁺;
Seed morphology controlled the morphology at equilibrium under the batch conditions investigated;
At low seed concentrations, the plate-like seeds significantly reduced desupersaturation times due to higher specific surface area;
The kinetic rate constant for growth was affected by seed quantity (due to available surface area);
Increased agitation reduced desupersaturation time and relative increase of seed size, suggesting attrition under conditions of high turbulence;
Statistical analysis confirmed that, of the four factors tested, the seed quantity had the most significant effect on all three response variables;
A set of operating conditions were identified to minimise desupersaturation time, while still maintaining a plate-like precipitate morphology

Aim 3: To develop design specifications relating to membrane selection and treatment in the SPARRO system;

The following progress has been achieved with this task:

In the early stages of the project, it was established that the only locally manufactured membranes are cellulose acetate (CA) and thus it was decided to restrict the membrane investigation to these.

A single membrane module, the SPARRO Membrane Test Rig (SMTR) through which feed slurry with specific crystal characteristics is recycled in a closed loop circuit under specified operating conditions was constructed. Commissioning problems and difficulties in inflicting significant membrane damage hampered progress in membrane testing with the SMTR. Modifications to the test work included operating the system at the highest possible linear velocity (2.44m/s) and with an increased concentration of gypsum crystals in the feed. Based on this method of experimentation, the following conclusions can be drawn:

For intermittent SMTR operation over a total operational time of 200 hours, the needle type morphology had a greater detrimental effect on both salt rejection and membrane flux, with the system taking up to 8 hours to return to the optimal performance levels subsequent to a shut down.
For continuous SMTR operation with a seeds concentration of 2.14g/L, over a total operational time of 160 hours, it is evident that the needle morphology shows a greater degree of variability in salt rejection and membrane flux when compared to the platelets.
For continuous operation, with a seeds concentration of 14g/L, over an operational time of 180 hours, there was no significant variation in the salt rejection over time, when compared to the results of the standard salt test, which would be indicative of membrane damage having occurred for either a needle or platelet type morphology.

The following conclusions can be drawn with regards to the experimentation using the novel Directed Slurry Spraying Technique (DSST) that was developed during the course of the project:

It is clear that, for both morphologies, the extent of damage due to scouring is significantly less at a spraying angle of 45o, which would be more representative of the angle of impact of the crystals with the surface in normal operational mode.
When comparing the membrane surfaces that were sprayed with the two different morphologies, the damage caused by the needle type morphology was significantly more pronounced than that of the platelet type morphology. This compliments the results obtained using the SMTR by demonstrating that the crystal morphology does have the potential to play a significant role in membrane damage. Conclusive evidence was generated that the needle type morphology has a significant impact on membrane damage compared to the platelet type morphology under these testing conditions.
The implications of this work are that, if the platelet morphology of the crystals in the SPARRO process is maintained, membrane damage is expected to be reduced.

Based on the DSST experimental work that has been carried out, it is apparent that, although there is evidence that gypsum crystal morphology of a needle type does have the potential to influence membrane damage through scouring, under typical SPARRO operating conditions, simulated by the SMTR-mini plant, the evidence that the different morphologies actually damage the membranes is inconclusive. This therefore shifts the focus onto membrane fouling as the predominant membrane damaging mechanism and as the reason for the failure in the SPARRO process.

Aim 4: To define the critical operating parameters for the SPARRO system.

This aim depended on the crystallisation and membrane aspects of the work being integrated and was addressed in the latter stages of the project.

Aqueous chemistry modelling was carried out using predictive thermodynamic software in order to ascertain how the complex aqueous chemistry of the real waters under consideration would affect the findings of this research which, of necessity, is based on synthetic waters. It was found that:

For the selected mine waters, a water recovery of approximately 60% is achievable before the supersaturation increases to the scaling level. The Grootvlei phase 1 and phase 2 waters are both supersaturated with respect to Al(OH)₃ which is responsible for the fouling mentioned in Juby’s (Juby, 1994) report. The Grootvlei phase 2 water is also supersaturated with respect to CaSO₄.2H₂O;

The implications of the two modelling exercises for the complex mine waters under consideration are contradictory:

On the basis of water composition, the scaling potential of the complex mine waters is increased compared to the pure solution;
On the basis of ionic strength, the scaling potential of the complex mine waters is decreased compared to the pure solution;

More detailed modelling and experimental studies would need to be carried out in order to establish the net result of these two counteractive effects.

Based on the findings of the membrane aspects of the research, it is clear that membrane fouling as a result of the feed being supersaturated with respect to Al(OH)₃ was the predominant cause for membrane failure in the SPARRO process. However, whilst the evidence is inconclusive with regards to crystal morphology categorically causing membrane damage, there is evidence of morphology having an influence on membrane performance. Consequently, from the SPARRO Membrane Test Rig (SMTR) mini-plant studies and the Directed Slurry Spraying Technique (DSST), it is clear that, besides aqueous chemistry, control of crystal morphology is essential in order to minimise membrane damage in the SPARRO process. In this process there are three parameters that can be used to control morphology:

Supersaturation to control nucleation
Supersaturation to control growth
Seed volume to control morphology

If heterogeneous nucleation in the membrane is to be avoided, the supersaturation must be controlled. This can be manipulated by adjusting the water recovery.
If sufficient seeds are provided, the morphology of the gypsum crystals can be controlled to the platelet shape, even in regions where the needle morphology is dominant. Thus, the membrane damaging needle-like morphology can be avoided.

Model design charts that relate seed volume and supersaturation or sulphate: calcium ratio to gypsum morphology in pure solutions have been developed.

RECOMMENDATIONS

The major recommendation arising from this work is related to the level of understanding of the chemistry of the mine waters being treated using the SPARRO process. It is recommended that full analyses should always be carried out on waters of interest.

It is also recommended that future work focus on the aqueous chemistry of real mine waters, and the implications of that aqueous chemistry for interactions in the aqueous phase as well as for scaling species.