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Filtering
Dr. A.K. Williams, Ph.D
San Pedro, Costa Rica
Introduction

Do you know how to filter stuff? Does anyone? Well, if you keep reading this page, you are going to know a great deal about it. Do you use a "coffee Maker"? If so, you already know a little bit about filtering. Here in Central America the standard way to make coffee is to simply put some coffee in a "bolsa", a sort of fuzzy cloth bag and pour hot water through it. I think it is the ultimate coffee maker. It costs about $0.20, and out-performs any other technique that I know of. Just as an aside, do you know what is the worst way to make coffee? A damned percolator. I think my parents just rolled over and screamed at me.

There are so many facets to filtering; it is hard to know where to start. What is filtering, anyway? Well, it’s a little science, a little knowledge, and a little art. That’s right, art! To define the term we could say that it any method that involves separating particles of different sizes by physically screening them through some material. Are you happy with that? I’m not. Why? Because when I said “material” you were thinking of pouring stuff through paper, cloth, a wire screen, or a bed of sand, weren’t you? Yeah, you were! You are right as far as it goes but there are other types of filters. Suppose that you had a mixture of pumice and silica sand. How could you put this mixture through a filter and separate them? We assume that the particles are of the same size. What I would do is to simply dump the mixture into a bucket of water. The water becomes a “filter medium”, not by size, but by density. The low-density pumice floats and the dense silica sinks. We separated it with a density filter. We could make an “electrical” filter. I could make a device that would separate things by the “charge” they carry. Positively charged things end up in this pot and negatively charged stuff ends up in that one. YEAH, YEAH, I know we don’t usually call this type of thing “filtering”, O.K., so it’s all “separation technology” but on this site we are going to consider it as “filtering”. O.K. by you?

We are not going to consider all these strange methods on this page. Maybe on some other page we will get into chromatography, electrophoresis, density gradient separation, etc. Hey guys, we are in MY field now. This is what I did!


Paper Filters

This is probably the most used filtering method for small quantities of slurry. It is also the method that is most used, most convenient, and most practical for most of us prospectors. We know how to do that, right? The first point I want to make is what kind of paper are you going to use? Newspaper? Coffee filter paper? Paper towels? Toilet paper? Or, Real scientific "designer" paper? It doesn't matter which you use as long as it works for the stuff you are trying to separate. This is where the "art" comes in. You must pick the right kind of paper for the job you are trying to do.

There are literally hundreds of types of filter paper sold by scientific supply companies. They all have unique characteristics that make one better than the other for your purpose. There are "hard" papers, "slow" papers, "fine" papers, "open" papers, etc, etc. Hang with me, we are going to try to sort some of it out for you. When I had my labs, I probably had 150 different kinds/sizes of filter paper on hand. It was necessary to get the job done. Correct filtering is not a simple process and if you don't do it correctly you are going to spend lots of time and energy that could be better spent out in the river. A filter that is not right for your purpose is one of the most frustrating things imaginable. On this page I am going to talk about filter paper and I will, for the most part, talk about "Whatman" paper. There are other Manufacturers with products just as good, for example, Schleicher & Schull. It's just that I am more familiar with Whatman.

Before we get into what paper is better for what I would like for you to do a little experiment. It's not complicated, humor me. I want you to find 3 funnels. Take 3 pieces of coffee filter paper (or any other) and fold it in half twice. Open it so that you have a single layer of paper on one side and three layers on the other. Put these cones into the funnels and put the funnels into some container to catch the liquid. Now I want you to take about one tablespoonful of flour in a small pot or frying pan. Add a little cold water and mix well, add more water, repeat until you have the flour in a homogeneous suspension (no lumps). Add water until you have about a quart of suspension. Now measure out one cupful of this stuff and heat it until it just starts to boil and let it cool to room temperature. In your first filter put water, in the second put the non-heated flour suspension, and in the third the heated flour suspension. Now I want you sit back and see how the three filtrations behave. If all is as it should be, the water will drip through pretty fast, the non-heated one will start fast and slow to a slow drip. The heated one will start slow and slow even more to a near dead stop.

The point of this is not just to demonstrate that some things filter faster than others, you already knew that. I want to try to explain why it happened that way so that you can begin to get some insight into what might happen before you try filter something. Sure, you are not going to be able to predict with 100% accuracy what will happen, but with a little understanding, you can increase your chances of picking the right type of filter for the job at hand.

Any filter that is separating things by particle size is composed of a "screen". It can be paper, sintered glass, wire mesh, or other. As long as the holes in the filter remain open, not plugged up, the liquid will run through pretty fast. The larger the holes, the faster the liquid will pass. If I took a frying pan and drilled 3/8 inch holes in the bottom, dumped in a handful of marbles and a gallon of water the water would immediately start to gush through the holes. However, as soon as the marbles had a chance to roll around a bit, they would find themselves a hole to sit in and that hole would be sealed off and no longer available as an exit for the water. As more marbles found a hole the water flow would slow until the last hole had a marble in it and the water could no longer pass through. Everything stops!

One other concept that I would like for you to grasp is similar but not quite the same thing. If in that same frying pan, I dumped in a lot of water that had a paper thin sheet of Jello floating around in it, just about the same thing would happen. As soon as the Jello got layered on the bottom, the water could no longer get through. This is what happens to filters 90% of the time when they are not working properly. You might be thinking, so what, nothing you can do about it! There are lots of things that you can do about it and we will get to them. After all, that's the whole point of this webpage.

In the experiment above, as you would expect, the water having no suspended particles to stop up the pores in the paper, flowed right through. The suspended flour (starch granules) took a little time but just like the marbles in the frying pan, eventually found a pore and closed it off. The heated flour now has a high proportion of soluble starch in it. It's the stuff your grandmother used to starch Granpa' shirts. It's sticky, gelatinous stuff, like Jello. This sort of stuff doesn't really have to find a hole to plug up, it just sort of throws a blanket of sticky goo over the filter paper and shuts it down.

ust a general information note at this point. If you want to buy some general use filter paper to use around the basement or out in the field I am going to make a couple of general recommendations. Without giving all the characteristics of these papers let me say that for general use you will find the following to be very useful.

  • Whatman #1 This is a fairly fast paper that will stop particles of about 11 microns (millionths of a meter)
  • Whatman #4 This is a very fast paper that will retain particles of 20-25 microns.
  • Whatman #41 This is a fast paper that retains particles of 20-25 microns, It is particularly suited for filtration of gelatinous materials.
  • Whatman #42 This is a very slow paper. It will stop particles of about 2.5 microns. Almost nothing gets through it.
  • Whatman #541 This paper is very fast, stops particles of 20-25 microns. Its difference from #4 is that it can be used with very strong solutions of acid and base.

Always buy filter paper in circles larger than you think you might need. It’s easy to cut them down but awfully hard to stretch them. Also, don’t buy “fluted” paper. You can flute it yourself simply by folding it half as many times as you can and when you open it, you have fluted paper. That makes filtering go quite a bit faster due to the increased surface area exposed to the liquid.

Did you know that you can't fold a piece of paper in half more than 7 times? No matter how big the paper or how thick or thin it is. Try it!


Filter Aids

There are a number of ways to filter things that just don’t want to be filtered. Once a sticky material or any other that tends to stop up the paper gets itself into the pores of the paper, everything stops. You can try to suck it through or you can put as much pressure on it as you like but it will only aggravate the situation. This treatment simply drives the particles or other tighter into the pores. So, what can a feller do? He can prevent those particles from ever getting into the pores to start with. There are a number of ways to do this but the simplest and most common is to employ materials called “Filter Aids”.

What is a "filter aid?" Well, there are many materials used for this purpose. You can often use "pulped paper". That is, filter paper that has been ground or shredded. Sometimes you could use something like clean silica sand. The most commonly used filter aid is probably stuff called "diatomaceous earth". So what is that, you say? It is the skeletons of microscopic marine animals called Diatomes. As these animals die they settle to the bottom and collect in layers where all the organic body parts decay leaving the skeletons that contain billions of cavities, cracks, pores, etc. This, of course, results in extremely high surface area on irregularly shaped particles that cannot pack or compress.

There are two basic ways to use filter aids. The right way, that is, of course, our way, and there is a method used by other less knowledgeable folks. We will get to that but lets just say that filter aids, when mixed with a liquid sample tends to provide surface for particles, crystals, gooy stuff, whatever, to get hung up in. It keeps the particles from getting together at the paper surface and stopping up the pores. Sort of keeps all the nasty stuff at arms-length from each other.

suppose that I should explain here that these aids are usually used in vacuum filters. That is a filter that has an evacuated flask attached to the stem of the funnel so that the liquid is being sucked down through the filter paper. This not completely necessary. You can do it by gravity but will probably find that you must use a little more filter aid than you would with vacuum and it will be slower.

BTW -- Does anyone know of any other word in the English language that contains two U's in sequence??

O.K., how to do it. I have seen Ph.D.s in the lab use filter aid by slurrying it with some water, pour it into the filter, suck out excess water, and then pour the sample to be filtered on top of it. He has just successfully defeated the whole purpose of using the filter aid. That ain't the way to do it. The way we Basement Chemists are going to do it is to put the filter aid directly into the liquid sample, slurry it around and then pour it into the funnel. How much do we use for any given volume? I don't know! You have to experiment. It depends on what sort of nasty stuff is in your sample. Just guess. If you have 300 ml try about a teaspoonful of aid. You will know if it works or not. If the goo still accumulates on top of the filter aid you will have to use a bit more. Just recover all of the filter aid and re-slurry it. Add another teaspoonful of filter aid and try it again. I told you that there was a little art involved. One word of caution, once you pour the slurry into the filter, DON'T STIR IT. Don't even scratch around on the surface of the packed filter aid. It almost never helps and it can cause things to slow even more.


Multiple Filters

I must comment on a human phenomenon that in the case of filtering is self-defeating. Here in Central America we have many Diesel vehicles. The fuel here is probably not quite as clean as that you are accustomed to in the USA. It is the custom here to put extra in-line fuel filters on all cars etc. Gentlemen, this is dumb. If you put two identical filters in line you do NOT get twice as good filtering. If a particle passes the first filter, it can pass the second. I once bought a used Land Cruiser here upon which I eventually found five in-line filters. Actually what they do here is when you tell the guy to change your filter he doesn't remove the old one. He simply puts another one before it. The same suspect filter is still in the line and still capable of slowing fuel flow. Never will understand that line of thinking. Same thing with paper filters. The only reason to double-up is to provide physical strength. You will never need it with professional paper.

Vacuum Filters

If you anticipate doing very much filtering I highly recommend that you consider the advantages of using a vacuum filter. The most common, and, most useful of these is the Buchner Funnel. It is usually made of porcelain but you can get them in plastic too. It is simply a funnel with a flat plate or bed on which you can lay a piece of filter paper and suck the liquid through the paper with a vacuum. You will find this devise will make life much more pleasant if you are doing much filtering. Actually, there are two pieces to this apparatus. You must have the Buchner funnel and you must also have a Filter Flask. The latter is the catch vessel and the vessel that is attached to your vacuum source. The other thing that you will need is a rubber stopper with a hole in it to seal the joint between the funnel and the flask. Alternatively, you can buy a set of “adapters”, just little rings of rubber that will serve very nicely. None of this stuff is very expensive. The following drawing will, I hope, illustrate the device that we are talking about.

Now if you decide you want to invest in one of these things, you must decide what size do you want. Well, for average use, I would buy a filter funnel of about 15cm diameter and a flask of 1liter capacity. That is up to you and your needs.

O.K., If you buy a vacuum filter you will still have to have a way to produce a vacuum. There is a device that I highly recommend that is called a Water Entrainment Pump. It is a simple-minded device that you can attach to any water faucet to produce a suprisingly high vacuum. They are of metal or plastic and they are really useful. They are also inexpensive. This is the vacuum source that the Buchner system was designed to work with.

Now, for those of you who would like a vacuum source for less or maybe free, there is another option. Go down to some place that has used or worn out refrigerators. Some that they about to junk. Find one that still runs but is nevertheless ready to be scrapped. Get the compressor from it. Hook it up and you will find that frequently it will still produce a credible vacuum. You can use it!


Sintered Glass Filters

I think that I must mention that there are filters that are made exactly like the Buchner Funnel but instead of having discrete holes for the liquid to flow through, have a filter bed of porous glass. If you should end up with one of these you will find that after a time the filter will no longer allow even water to pass through it. It will laugh at all your attempts to clean it. However, we Basement Chemists are not to be intimidated by such trifles, are we?

When this happens we must resort to our above average knowledge of redox reactions. The first thing to try is to simply dissolve some Nitrate in water and try your best to suck some of it into the sintered glass. Usually you can. Now take the filter (assuming that it is NOT plastic) and put it in the oven. Get it as hot as possible for an hour. This will probably remove all the carbonaceous material that usually causes the problem. You might even have to repeat it.

If that don't work we go to a more extreme method to save our filter. For this you will need a little Sodium Dichromate and some concentrated Sulfuric Acid. To the acid add, with stirring, the Dichromate crystals until you get a nice dark red solution. DON'T GET THIS ON YOU!! It will clean your bones too. Just soak the offending filter for a few hours or days, wash it and see if that got it.

There is another general cleaning solution that you can use to clean this sort of thing. As in the above, I can't remember the exact proportions but just make up a solution of Lye (caustic). About 30 grams/liter, add strong Potassium Permanganate solution, with stirring, until you get a dark blue solution. Like the Dichromate, DON'T GET IT ON YOU!! It will do a job on you too. As above, just dunk the filter in it and forget it for awhile.

Both of these solutions are used extensively to clean glassware in laboratories. They can be used on pipettes, general glassware, ceramic, porcelain, sintered glass, gas spargers, etc.

Ain’t All This Fascinating??

This document maintained by A.K. Williams. Material Copyright © 2003 A.K. Williams

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