Question: We have a chance to expand our business by adding a new production process.
We currently have a 5,000 gallon batch treatment system to remove nickel, lead, copper, and zinc from our plating processes. This new process requires hexavalent chrome, however. We have never used hexavalent chrome before and want to know how we can treat the wastewater produced from this process.
Robin DealA: Hexavalent chrome reduction does not have to be complicated or scary. There are many ways to remove hexavalent chrome from your wastewater.
Let’s look at reducing the hexavalent chrome to its trivalent state first. A reducing agent is needed for this chemical reaction. A reducing agent is a substance that tends to bring about reduction by being oxidized and losing electrons. You can use sulfur dioxide, sodium bisulfite, or ferrous sulfate to name a few. By dropping the pH to 2.5 with sulfuric acid and introducing the reducing agent to an ORP reading of positive 250 the process is usually completed within 15 minutes.
With sulfur dioxide you get: SO2 + H2O -> H2SO3 and 3H2 + 2H2CrO4 -> Cr2(SO4) + 5H20
With sodium bisulfite you will get: 2H2CrO4 + 3NaHSO3 + 3H2SO4 -> Cr2(SO4) + 3NaHSO4 + 5H2O
With ferrous sulfate you will get: 2H2CrO4 + 6FeSO4 + 6H2SO4 -> 3Fe2(SO4) + Cr2(SO4) + 8H20
When hexavalent chrome is reduced to its trivalent state it turns from yellow to blue. It is not always visually possible to see this change. There may be dyes present, other metals, or just not enough light. We want to make sure we give this process the correct amount of time.
Once it has converted, you can introduce the stream into a main treatment system, or you can batch treat it. If you introduce it to a larger treatment system, make sure it goes in prior to coagulation and pH up adjustment. Meter it in so that the chrome does not overtake the entire system. If you are already removing other metals from your wastewater, you should be able to bench test to confirm the amount your system can handle and not get overwhelmed. Bench testing will also confirm if the pH range of your system needs to be adjusted or if there needs to be a change to the coagulant dosing to effectively remove all the contaminants. If you are going to batch treat the stream, bench test to determine the correct amount of coagulant, the correct pH range, and the correct flocculant needed to remove the chrome.
While hexavalent chrome reduction with hydroxide precipitation is the most common treatment method for metal finishers, there is other options. Electro-coagulation, absorption by Activated Carbon or Clays, or ion exchange are some of the ones available. Each method has benefits as well as down sides. No matter which method your facility chooses to go with, notify your local authority of the intent to add a new process that will change your waste stream. Work with the local authority to ensure that you remain compliant with your discharge permits limits. Your chemical supplier should be available to assist with bench testing and to discuss other options.
Robin Deal is a Product Leader for Aquapure at Hubbard-Hall. Please visit www.hubbardhall.com
