A cost-effective, cleaner route to potassium sulphate

SOP is in demand as a potassium source for chloride-sensitive crops such as sunflowers: PHOTO: FLICKR/KUNIAKILGARASH

Potassium sulphate (SOP, K2SO4) is a premium, chloride-free source of potassium. One of the major production routes, the high-temperature Mannheim process, is energy-intensive and generates hydrochloric acid (HCl) as a by-product. Ballestra has developed a room-temperature, water-based ‘metathesis’ SOP process. This has been validated at bench-scale, avoids generating HCl entirely and yields a second saleable liquid fertilizer alongside crystalline SOP.

This article describes this novel SOP production process, as well as the test campaign behind it, and shows how it compares with the Mannheim route in terms of feedstock consumption, capital cost and realised revenue.

Ballestra and the SOP opportunity

Ballestra SpA is a Milan-headquartered technology licensor and engineering contractor with a leading position in the detergent, surfactant, soap and inorganic chemicals industries – the latter including sulphuric acid, phosphoric acid and fertilizers. The company supports projects from process design through commissioning to after-sales technical assistance. Backed by a worldwide sales network, it operates four offices together with four laboratory and pilot installations globally.

Ballestra recently became part of Nextchem, the sustainable technology company of MAIRE Group. This has broadened the Group’s technology offering, with Ballestra’s potassium, sulphur and phosphate technologies now sitting alongside Nextchem’s industry-leading nitrogen technology portfolio.

Adding to its existing chemical technologies, Ballestra has developed a wet-route process for an important specialty fertilizer product: potassium sulphate – traditionally known as sulphate of potash (SOP). The process offers a practical alternative to the long-established Mannheim route.

The conventional Mannheim route

The Mannheim process is the traditional, technically-proven secondary production route for SOP. It reacts potassium chloride with sulphuric acid in a high-temperature furnace (Figure 1):

2KCl + H2SO4 + heat → K2SO4 + 2HCl

This reaction yields potassium sulphate as the marketable product and gaseous hydrochloric acid as a by-product. The HCl generated must be absorbed and then valorised or disposed of. Ballestra has extensive experience in designing Mannheim plants and can supply the design skills needed to minimise wear-part replacement and maximise plant life.

Despite its maturity, the Mannheim process has four structural drawbacks that increasingly weigh on plant economics and environmental performance:

• High energy consumption – continuous heating at about 600–700°C, with fuel accounting for an important share of operating cost.

• Corrosion – HCl generation requires acid-resistant materials and raises maintenance costs, with metal parts subject to accelerated wear.

• Environmental burden – HCl gas emissions require dedicated scrubbing systems, which add capital cost, operating cost and CO2 emissions.

• By-product management – Originally, the Mannheim process was used to produce HCl. Today the 30–35% HCl solution must be captured and then used elsewhere or sold, a commercial variable that can affect plant profitability.

Typical Mannheim figures show the scale of these constraints. A single furnace produces around 30 tonnes per day (t/d) of K2SO4 (chloride < 1%), together with 1,100–1,200 kg of 30–35% HCl per tonne of SOP. It consumes roughly 560–570 kg of 98.5% H2SO4 , 825–850 kg of KCl, 64–77 kg of natural gas and 100–110 kWh of electrical power per tonne of product.

Ballestra’s wet-route process

Ballestra set out to develop a milder SOP production route that would: eliminate the use of a furnace, avoid HCl generation, and therefore eliminate the by-product disposal issue, by generating a second saleable fertilizer co-product.

The result is a room-temperature, water-based metathesis* process (Figure 2):

2KCl + (NH4)2SO4 → K2SO4 + 2NH4Cl (in solution)

*Chemical double displacement

The process exploits the reciprocal salt-pair equilibrium of the four-component system (NH4)2SO4 + 2KCl = K2SO4 + 2NH4Cl in water, as first characterised in detail by Hill and Loucks (1937). Across the relevant temperature range, potassium sulphate is the least soluble species. It therefore precipitates as a pure crystalline solid, while the remaining nutrients stay in solution as a nitrogen-potassium-sulphur (NKS) liquid, the mother liquor.

Because the reaction proceeds in water at room temperature, this wet route requires no thermal energy input and produces no gaseous emissions. The precipitated K2SO4 is separated, milled and screened to market specification. The mother liquor is recovered as a ready-to-use liquid fertilizer or blending component.

Two products, two markets

The wet route yields two distinct fertilizer streams. The K2SO4 solid is a high-grade and essentially chloride-free form of SOP. The chloride does not disappear. Instead, it passes into the mother liquor, which contains 9–12% chloride as a result. This liquor can be marketed as a liquid fertilizer for chloride-tolerant crops, in contrast to the chloride-sensitive crops served by solid SOP. Typical compositions of both are shown in Table 1.

Production and specific consumptions

Reagent and utility consumption are summarised in Table 2. The wet route yields one tonne of solid K2SO4 (Cl– < 1%) and about 4.4 tonnes of mother liquor.

One point deserves highlighting: the wet route is more feedstock-intensive than the Mannheim route, consuming 1,160 kg of potassium chloride per tonne of SOP versus 825–850 kg/t for the Mannheim process. This issue is discussed further in the economics section.

From the lab to the plant: the R&D test campaign

Ballestra has investigated the wet route repeatedly at bench scale across every variable necessary for a bankable industrial design. That includes:

• Raw-material specification and particle size

• Reaction and precipitation behaviour

• Solid-liquid separation

• Mass-balance closure

• Product quality and crystal habit.

The R&D test campaign ran in two phases:

Firstly, batch operations in glass beakers were carried under controlled dosing – to map solubility behaviour, stoichiometry and achievable yield across a range of raw-material qualities.

Secondly, once the chemistry and separation steps were understood, the work then moved to a bench-scale simulation rig for continuous solid-liquid separation (Figures 3 and 4). This functioned as a scaled-down physical representation of an SOP plant, with dosing pumps, agitated reaction vessels arranged in series, and a centrifuge

In the continuous bench-scale rig, the two feedstocks, ammonium sulphate solution and precisely metered potassium chloride, were introduced in parallel. Transfer pumps moved the reacting suspension between two agitated vessels arranged in series, giving a residence time of roughly 25 minutes per vessel.

After an initial stabilisation period, the system ran continuously. Product was withdrawn by centrifugation at regular intervals over a run of more than six hours. Samples from both vessels were taken to track ion concentrations and confirmed steady-state conditions.

The results validated the wet-route SOP process in every respect:

• Mass-balance closure – the overall solid and liquid balance over the full 410-minute run closed to within 1.3%. The ionic species in both the solid product and the mother liquor were charge-balanced, confirming no unaccounted losses.

• Yield – the experimental SOP yield reached 74–79% by weight, above the 63–66% theoretical yield calculated from SOP solubility at 25°C. This is consistent with the lower operating temperature of about 20°C, at which achievable yield rises. One sample reached 89%.

• On-spec product without washing – even before any washing stage, the centrifuged SOP complied with typical commercial specifications, with chloride below 1% by weight and a K2O/SO3/N profile in line with market-standard SOP.

• Stable mother liquor – the composition of the co-product mother liquor remained essentially constant throughout the run and closely matched the earlier batch campaign, underlining the robustness of the chemistry.

• Particle size – the discharged product was concentrated in the 250–500 µm range, with a mild coarsening trend over the run. The fraction above 500 µm rose from about 8% at one hour to 16% at six hours, consistent with well-formed crystalline SOP.

The tangible outcome of these trials is genuine crystalline SOP generated directly by a continuous rig. Under the microscope, the product appears as well-formed, robust crystalline aggregates (Figure 5) – a free-flowing, storable material of the kind the wet route is designed to deliver at scale.

Taken together, the two-stage campaign – from the initial exploratory batch trials to the continuous simulation with full instrumentation – provides this SOP wet-route with an unusually high degree of experimental grounding for a process at this stage of development. It also provides a robust basis for scale-up to a full industrial plant.

The economics: consumptions, capital cost and revenue

The wet route versus the Mannheim route is best assessed on a like-for-like basis of 10,000 tonnes per annum (t/a) product output with residual chloride content < 1%. For this comparison, the capital costs and principal operating parameters are set out side-by-side in Table 3. (Please note that the investment case for the wet route becomes increasingly attractive at higher production capacities.)

For product value, the realised revenue (not margin) per tonne of SOP for both routes are reported in Table 4. In the Mannheim route, the HCl by-product is credited as a co-product and natural gas is charged as a cost. In the wet route, the second stream is assigned to the NKS liquid fertilizer.

From revenue to margin

The wet route consumes more of the key raw materials per tonne of SOP product output: about 1,160 kg of KCl and 1,140 kg of ammonium sulphate, against roughly 837 kg of KCl and 565 kg of sulphuric acid for the Mannheim route (Table 5).

Any full economic comparison must therefore weigh these feedstock costs, which favour the Mannheim route, against the wet route’s lower energy, maintenance and emissions-abatement costs, as well as the absence of the HCl offtake risk. The net balance is site-specific – being dependent on local prices for KCl, ammonium sulphate, sulphuric acid, energy and the HCl offtake.

A key strategic point is that the wet route’s second stream should be marketable. Being a liquid NKS fertilizer, it should be valued by the same customer base as SOP, whereas Mannheim’s HCl by-product needs to be placed on an unrelated market outside of the operator’s control. That can make it as much a liability as an asset. The wet route also offers materially lower capital cost, no gas consumption, no CO2 emissions from fuel combustion, and lower power demand.

A compact, ready-to-build plant

The full-scale wet-route SOP plant has been engineered into a compact footprint of roughly 60 by 20 by 12 metres (Figure 6). This should make it relatively straightforward to integrate alongside existing potassium chloride and ammonium sulphate handling facilities.

Product end markets

Because it is chloride-free, the solid SOP makes an ideal potassium source for chloride-sensitive crops:

• Grapevine, citrus, potato, tobacco, strawberry

• Beans and peas (legumes)

• Fruit trees – peach, apricot, avocado

• Onion, garlic and other chloride-sensitive vegetables and fruit.

Whereas the mother liquor – a chloride-containing liquid NKS fertilizer – can be directed to chloride-tolerant crops such as:

• Cereals

• Oilseed crops

• Sugar beet, cabbage and other brassicas, celery, chard, asparagus, coconut palms, and many vegetables and fruit.

This liquid fertilizer should be well-suited to fertigation and hydroponics, as well as being valuable for quick corrections of crop nutrient deficits and for targeted nutrient supplementation.

Conclusions

The Mannheim process remains a widely-used and technically-proven industrial method for potassium sulphate (SOP) production. But its high energy consumption, corrosion, harsh operating conditions, environmental footprint and by-product burden are increasingly hard to justify in a market that rewards efficiency and sustainability.

Ballestra’s wet route replaces a process based on high-temperature furnace chemistry with a room-temperature, water-based metathesis reaction instead. It eliminates hydrochloric acid generation and gaseous emissions, removes thermal energy input, and generate a market-ready liquid fertilizer as a secondary product. It produces crystalline SOP with market-standard purity at lower capital cost and with lower energy and maintenance demand, while delivering a higher realised revenue per tonne.

Because the wet route is more feedstock-intensive, any net margin advantage remains site-specific and should be tested against local reagent and energy prices. Even so, the direction of travel on capital cost, emissions and by-product risk is favourable.

These developments reflect Ballestra’s broader strategy of placing efficient, environmentally-responsible process innovation at the centre of its offer to the fertilizer and chemical industries. They also give producers a cost-effective route to more sustainable SOP production.

Reference

Hill, A. and Loucks, C., 1937. The reciprocal salt-pair (NH4 )2SO4 + 2KCl = K2SO4 + 2NH4Cl in water and in ammonia-water at 25°C. Journal of the American Chemical Society.