Deck blinding is a real challenge for many aggregate operations. Soft, powdery limestone is notorious for bad blinding, but this problem can occur in almost any type of aggregate. Dust-sized particles often deposit themselves on flat surfaces inside a shaker, including the tops of the screening wires. Add a little moisture and the deposits become like hard cement. Please refer to the diagram at right.
The blinding process usually starts with a clean wire (#1), and then the powdery deposits stick to the wire (#2). This is often referred to as “crusting” or “icing”. Once crusting sets up, it’s just a matter of time before the deposits actually bridge over the opening and blind it (#3).
The traditional strategies to defeat blinding include the following:
- If possible, increase the top-size particle feed to the deck. Larger particles tend to scour the screening surface.
- Within reason, reduce the size of the wire diameter. The lighter diameter wire will more effectively pick up the resonance of the screen and improve the screening action. The additional gain in open area also aids screening efficiency. When using light diameter wire, care must be taken to ensure adequate support of the wire cloth.
- Consider stainless steel. Carbon steel wire has a pitted, porous surface that allows fine grains to adhere to its surface. Non-rusting stainless steel is smooth by comparison, and it is an aid in relieving blinding. (The combination of steps 2 and 3 is sometimes necessary. Do not expect a miracle cure from the use of stainless wire alone.)
- Consider the use of long-slot openings (slotted openings). The greater the distance between the wires the more difficult it becomes for the material to bridge the opening. It also sets up a more pronounced “humming” action on the wire, creating a cleansing effect. An inherent danger in slotted openings is their tendency to pass elongated particles (flats). Consult with a Hoyt Regional Field Sales Representative before proceeding with a changeover to long slots.
- Switch to Hoyt’s Veno™. Veno™ is a non-blinding/non-plugging screen that does not pass flats and efficiently picks up the resonance of the shaker for good wire cleaning action.
For severe blinding challenges, there’s a new solution — Hoyt’s Serpa™. Hoyt’s Serpa™ offers a unique and unrivaled answer to blinding that has never before been demonstrated in the wire cloth industry. Square opening screens do not have flexible wires and cannot disrupt the bridging over of the opening (#3 in the diagram above), that results in blinding. Serpa™, however, has flexible wire action that breaks up this process. Combine Serpa™ with Hoyt’s full-hard, Type 304 stainless steel wire and you’ll be well on your way to solving the most frustrating blinding problems.
Below are actual before-and-after photos of a blinded deck, defeated with Serpa™.
The blinding on the left is 1/2″ thick throughout the body of the wire cloth. At right, this same panel location now uses Hoyt’s Serpa™. As you can see the panel is now clear. This panel has been in place for eight months.
For extremely severe blinding applications where accurate sizing is a secondary consideration, Hoyt also offers our H-Wave™ and V-1™ weaves. Your Hoyt Regional Manager can size up your specific situation to see if one of these products is right for you.
Blinding – Look At Your Middle Deck
An often overlooked cause of blinding on the bottom deck is an inefficient middle deck. If the screening efficiency of the middle deck is not what it could be, then there is likely to be a high percentage of “carryover.” Re-processing (re-crushing) generates more fines, so the gradations are likely to be “fining-up” (vs. coursing-up). With an increase of re-grind product in your mix, you’re adding more “fine-load” to all your decks, top to bottom. Eventually, your bottom deck can become overburdened with fines, and it will blind over.
What’s the solution?
Often the best solution is to improve the screening efficiency of your middle deck (assuming a three-deck shaker). If you can create a highly efficient middle deck and minimize the need for secondary processing, you’ll go a long way toward eliminating the source of fines in your entire system.
Serpa XLT™ and Hoyt’s revolutionary Hi-Rise™ screens are now available in larger sizes for middle deck applications. Hoyt’s shaped wire technology offers increased open area and throughput combined with enhanced wear-life and load carrying capacity.