Sustainable. What is sustainable sand washing and processing and is it the same as being ?green??
There are many parts to the sustainability equation that are not valid when discussing being ?green?. The term ?green? seems more appropriate when discussing environmental issues, whereas sustainability is all about the industry. It is easily argued, however, that ?green? is always part of being sustainable.
Recent attendees at the IQA?s annual meeting in the Hunter Valley listened to industry experts discuss sustainability as a core part of our industry?s future. They spoke on topics including resources, employment and recruitment and the community profile.
But how does washing/processing sand fit into the sustainability equation? If being ?green? is a part of being sustainable, and if environmental issues are key to extending or establishing a new permit, then the subject matter should be front and centre. If the utilisation of resources is getting the most out of the deposit, then it is a critical part of sustainability.
It starts with a simple two-part question: ?What do I have, and need to make??
DIFFERENT VIEWS, SAME APPROACH
From a US perspective, plant designers, producers and manufacturers often come at the problem from different viewpoints but the approach should be the same for all of them. They should start from the same point – ?What do I have?? – a question which is often under-researched and leads to long-term losses.
The next question is ?How do I get from ?have? to ?want???
Washing sand is a series of processes. It is about having the right equipment for the right part of the process. Understanding the relationship between the parts of the processes and their effect upstream and downstream is a key element. Correct sizing of the equipment for the duty is also critical.
Past experience in North America has been one of ?I want a tank and screw like we?ve always had?. The theory is ?no one was ever fired for buying a tank and a screw?, even though a lot of tanks became water scalping devices, not classification tools.
A common problem is companies undersizing the sand washing equipment to make an attractive deal for the producer. So what if equipment efficiently rated for 75 tonnes per hour struggles at the required 100 tph – it?s cheap!
Experiences like this have led to decades of inefficient operation. Cheaper, undersized equipment and processes lead to inefficiency, inadequate processing and wasted useable resources. Factor in the results of inefficient equipment the next time you are looking at the ?cost? of a plant ? what does even a two per cent inefficiency cost over the life of a deposit? Investing in the right equipment for each part of an integrated process will lead to gains in the long term and will be part of the sustainable equation.
The processes of feed preparation, washing, classifying, waste solids and water management enable the sand producer to get the best out of his deposit with good sand processing practice. Feed preparation is often overlooked. When it is addressed, it?s often a ?one stop shop? approach with vendors saying, ?Sure, that attrition cell will handle 50mm material? or ?Yes, that log washer will handle 300mm lumps and sand? or ?That scrubber will handle that plastic clay?. But will they really? When you are looking at a quicker, cheaper solution, it could cost you dearly in the long run.
When it comes to washing and classifying, classification is often not a necessary part of the equation, but when it is, it really is. The removal of contaminants, the retention of fine, useful product and having each particle in the right place are all parts of a successful sustainable process. Efficient hydraulic classifiers, such as hydrosizers (density separators, teetering bed classifiers and flat bottom classifiers), are more common in North America. Coupled with cyclones, separators and dewatering screens, hydraulic classifiers are the benchmark of efficiency. Buying a tank and a screw ? unless it is a valid selection – can cost someone their job.
Finishing up a process with the use of ultra fines recovery systems to recover >40? silt particles and making a product out of them is common in North America. Indeed, the value per tonne of recovered material can be greater than the primary product?s value. A simple system of a sump, pump, cyclone and dewatering screen that recovers as little as five tonnes per hour can be viable over the life of the deposit and fits the green and sustainable profile.
Water management is critical in the USA and Canada for new deposit development. Using a thickener, which recovers 85 per cent of the process water for immediate use and reduces the waste stream to a thickened ?concentrate? of slimes and water, can be cost-effective and green and minimises losses due to leakage and evaporation when compared with settling ponds. The advantages include saving power by not pumping the water to and from the ponds, and no settling ponds covering sand resources.
The final step of filtering the thickener ?waste? to obtain a solid cake and free water is high in capital cost but factored over the life of the deposit can be viable. Belt filters in the USA are being replaced by recessed plate filters due to the lack of additional chemicals needed and near operator-free functions. A solid stockpileable material and recovery of free water versus a pond ? you make the choice. Factor capital cost over the life of the deposit and add the benefits of a green profile of your plant and company.
The lesson is this: all aspects of ?washing? are part of the ?sustainability? equation; as all equipment needs to be reviewed for its appropriateness in a process, an engineered solution is critical; there needs to be an absolute focus on the details of the haves and wants; and, finally, being green and sustainable doesn?t mean you can?t produce at a profit over the life of the deposit.?
John Best is the general manager of the Aggregate Processing Division at McLanahan Corporation, Gallatin, Tennessee, USA.