Well there is Einstein's result for a dilute suspension.  Denser mixtures like slurries are harder.  As you get more granular the ability to write such equations gets worse and is an open problem.  

so momentarily the slurry viscosity law can be a solution , took as  limit condition withe no grains (0%) and after the viscosity will change with the quantity of grains?

In air things can get very tricky because its viscosity is so low.  You can get air pockets forming flows that create large inhomogeneities in the grain density.  Can you specify the geometry of your problem?  Is there any possibility of jamming in the flow? 

I have an open channel flow , I don't have any possibility of jamming ,  the max value of grains density in the mixed ( air + solid) is about 10 % to 20% .

do u think that's better to take the air viscosity for the mixed (air + sand) 

Is it falling vertically or being blown?  I'd be interested in if the sand is being suspended in the air from turbulence or if it could be moving at different speeds than the net air flow.  It sounds like this could be a very turbulent system.  

yes, the sand is blown by wind , and suspended in the air by turbulence , for the velocity I think that they can have the same values for : air and sand . But grains can also fall down under the effect of their weight .

thank you very much 

For turbulent flow it might be well described by Darcy-Weisbach.  Viscosity might be a naive choice of parameters in that you may need a more complicated constitutive law.  On the other hand, if it is turbulent enough you don't even need the viscosity.  If the mass flux moves at a different rate and direction than the flow it becomes more interesting and harder.  Otherwise you may be able to just modify the density and get a good answer.  

Thank you very much 

Hi there ^^ , Thank you for raising this question but I in addition to that I want to know if the solids also have a dynamic viscosity and if yes I looked for the dynamic viscosity of iron but in vain I found no subject Treating it in the solid state and at ambient temperature, I would appreciate your help ^^

There is a theory of what is called "solid viscosity."  The pioneering work was done by Zener.  You can read about it in Landau and Lifshitz's book on elasticity.  Also https://en.wikipedia.org/wiki/Standard_linear_solid_model 

What 's about the mixture of gypsum+water+papers (plasterboard+water)Any ideas?

It is well described by Darcy-Weisbach.