How To Prevent Gypsum Scale

Reverse osmosis (RO), nano-filtration (NF) and ultra-filtration (UF) membranes have long been used in the desalination industry to treat seawater and brackish water. However, as the global population increases[1], making more fresh, clean water available is imperative. That's why Genesys International has turned its expertise to providing antiscalants and cleaners to a less traditional industry – mining.

According to a 2012 article by Paulina Szyplinska, mining is the fourth largest consumer of water worldwide[2], but 90% of minewater is recyclable using technologies such as RO or NF. Although this is still a comparatively new application of water filtration membrane technology, the number of plants associated with mining operations has increased dramatically over the past 10 years.

There is a specific challenge that affects minewater filtration plants – high levels of total dissolved solids (TDS), which leads to a strong tendency for scale to form, especially calcium sulphate (CaSO4) or gypsum.

These conditions fall outside membrane manufacturers’ recommended operating parameters. For that reason, the membrane technology industry has traditionally dismissed the mining sector as a source of business, considering it just too challenging to be worthwhile.

However, despite that, it seems that mining operators are increasingly investing in RO and NF membrane filtration, attracted by the benefit of enhanced metal recovery as well as the ability to clean up wastewater. Through Genesys International's own research, it has identified as many as 389 mines currently using membrane filtration, 301 of them commissioned since 2009.

The company believes that, with the right chemistry and an open mind, it's possible not only to inhibit complex scale but also to clean fouled and scaled membranes effectively to restore their performance. And it seems like others are beginning to catch on. The team at Genesys were recently encouraged to see the following comments in a paper by Darryl Butcher, director of BDB Process, Australia, regarding two membrane plant projects he had been involved with in Africa[3]: "Neither of these applications operated the chosen membranes within the recommended operating windows published by the manufacturers, but both were technically and commercially, very successful and both exceeded the forecast performance used to justify the project capital expenditure… It appears likely that membranes will be applied in a broad range of applications to improve process outcomes."

Unlike other types of scaling, CaSO4 is pH independent. That means that the addition of an acid or alkali will have no effect in preventing scale formation. Genesys has developed two new antiscalants – Genmine AS26 and Genmine AS65 – which are capable of inhibiting CaSO4 at both low and high pH (up to x8-10 saturation levels).

Genmine AS26 is particularly resilient to acidic conditions, which is rare for an antiscalant. Formulated and perfected through rigorous testing, it can inhibit CaSO4 at pH values as low at 1-2 (Fig. 1). This means that no costly pre-treatment is needed to neutralise mining effluent for the antiscalant to work. That should result in CAPEX and OPEX savings, while making it even more feasible to reclaim both metals and clean water from tailings waste.

Of course, not all minewaters are acidic, which is why the firm has also developed Genmine AS65, which is capable of inhibiting CaSO4 scale at saturation levels of x8-10 (Fig. 2) at neutral and alkaline pH values. This is particularly relevant for the alkaline cyanide heap leach processes, where membranes are used to concentrate both pregnant and barren liquors. By providing products specific to both acidic and alkaline minewaters, the company is able to provide the right chemistry for potentially any minewater that is affected by gypsum scale.

If gypsum scale has already built up, perhaps due to using ineffective antiscalants or dosing inappropriately, removing it without damaging the membrane is challenging, but not impossible. This should be done as promptly as possible to prevent the scale itself causing irreversible damage to the membrane surface.

CaSO4 scale initially forms needle-like particles. It can then go on to form platelets and rosettes, which can damage the membrane surface through abrasion. This will ultimately lead to indents and holes in the surface, affecting the membrane's salt rejection properties and increasing its flux (allowing more ions to pass though).

The correct pre-treatment and effective use of our specialist antiscalants will drastically reduce scaling. However, if feed conditions are constantly changing or if, at some point, the saturation limit of the antiscalant is exceeded, scale will still form.

As mentioned above, calcium sulphate scaling is particularly difficult to control, and unlike with calcium carbonate scale, adding acid won't help.

Following extensive research, Genesys has developed three new cleaning products specifically designed to address these issues: Genmine C07 (neutral pH) and Genmine C15 and Genmine C17 (high pH). All three have shown great promise in CaSO4 removal, vastly outperforming traditional cleaners in testing.

A heavily scaled Vexar spacer was weighed before and after cleaning, with the differential between weights giving an insight to the cleaner's performance. After a three-hour clean, Genmine C17 removed 99.3% of scale, Genmine C15 removed 96.5% and Genmine C07 removed 93.3%.

The dosing of membrane chemicals is particularly sensitive in the challenging conditions created by minewater. One tool that can assist here is Membrane Mine Master (MMM) software, developed specifically to predict the appropriate antiscalant dosing for RO, NF or UF membrane plants used to filter minewater. This new software breaks the rules of conventional scale prediction software by functioning across the pH range (1-14), while most are limited to a narrow pH range, typically 5-9.

Based on the plant specifics entered, particularly details of the water chemistry, it will recommend an appropriate antiscalant and dose rate, and gives the option of producing a summary report.

The software is now being trialled using real site data and its accuracy will continue to be improved as more field data on the performance of the antiscalants becomes available.

Mining companies are increasingly adopting membrane technology both to enhance metal recovery and treat complex wastewater, whether for reuse or discharge into the environment.

Genesys recognised the need to provide better support for the mining community and rose to the challenge. Its specialist antiscalants can prevent the formation of CaSO4 scale at saturation levels up to 8-10 times normal solubility, across the whole pH range, without additional pre-treatment and its dedicated cleaning products are capable of completely removing gypsum scale, restoring membrane plant performance, provided they are used soon enough.

The Membrane Mine Master software can help operators work out the appropriate antiscalant and dose rates to keep scale at bay and reduce the costs associated with pre-treatment and disposal of wastewater.

In addition to the range of CaSO4 specific products, the company offers antiscalants to prevent silica, calcium carbonate, barium sulphate and metal deposits commonly encountered in mining. It also offers a range of cleaning products to tackle fouling with clay, iron and other metals and organics, including biofilm.

REFERENCES

1 Global population of eight billion and growing: we can't go on like this, McKie, R (2019)

2 Thirsty world of mining: Harvesting new water solutions, Szyplinska, P (2012)

3 Review of membrane technology as a process tool, Butcher, D (2020)

Gregory Gibson, Stephen Chesters and Max Fazel are with Genesys International